The Anatomic Museum David E. Marshall, DDS, MS Syracuse, N.Y.
O n e of the most fascinating of all studies is the study of man. In the Anatomic Museum, we are reviewing man, not as a creature of today but as one passing through a vast series of ages. It is through the study of the skull that we can see the process of evolution of the human race, not only in the past and present but also in the future. Just as an astronomer, by his observations of the sky can calculate a whole orbit as a unit and predict when a phenomenon will recur, perhaps hundreds of thousands of years hence, we can, by observing man's evolutionary past, predict the anatomic changes of future man. The fifties were known as the atomic age, the sixties as the missile age, the seventies as the electronic age, the eighties as the computer age, and I believe the nineties will be known as the biologic age. In the past 45 years of research, and from my publications on various aspects of the skull, I have assembled my materials and opened an anatomic museum of the skull (see photographs). I have housed the collection in the second story of my office building, which I renovated especially for this project, and I am sharing my work with members of related professions and with the public (Figs. 1 through 4). Of all the parts of man's frame, the most interesting is the skull itself. It represents and identifies the individual. Man's shape, especially his skull, has changed with his environment. The shape of his skull is such that no other shape could function more effectively in man's surroundings. For at least 10,000 years there has been no notable progress in the evolution of man, but 10,000 years is entirely too brief a term in which to look for marked evolutionary advances. Actually, the size of the human brain has not increased since the time of the CroMagnon race. In fact, cranial capacity of the Neanderthal race of Homo sapiens was, on the average, equal to or even greater than that of modem man. However, cranial capacity and brain size are not reliable indicators of intelligence or intellectual abilities. In human evolution so far, there has been a tendency toward a simpler and more generalized organism, as can be seen in the
811112100
simplication of many organs and systems. The progressive degeneration of certain parts of the organism and resulting simplifications are validated by the presence of many rudimentary structures. When we observe the present, we see it, notwithstanding its great complexities, merely as a continuation of the past. Man is still straggling with the environment. Although he is controlling it more and more, each day he is still changing. In general, man's past and present show that he is not yet perceptibly near the end of his evolution, and, according to all indications, he will, for a long time, continue to progress in adaptation, refinement, and differentiation. In the changes that have already occurred, we find less need for the use of the jaws and muscles of mastication because of better prepared foods. Reduction in the size of the face, with reduction in the breadth and length of the jaws, reduces the number of teeth. The head in general is becoming slightly broader and larger; the skull and facial bones thinner, and the physiognomy more lively and expressive. Hair, especially in men, is being lost prematurely. Some of the dental units tend to disappear. We see that the cranium as a whole is becoming shorter, broader, and higher. The forehead is becoming more erect and vaulted. The frontal bone fully participates in the direct formation of the anterior wall of the brain case. The occipital bone is becoming more globular; its greatest breadth is being shifted from the base upward to the parietal bones with the development of typical tubera parietale. The walls of the cranium are becoming thinner as a result of the greater expansion of the brain, and this causes the parietal sutures to close at a later period in life. The face, particularly in the area of the upper jaw, is becoming reduced in height and length, and the long axis of the cranium seems to be moving to a position below the frontal portion of the cranium. The palate and the dental arch are becoming shorter and wider. The general reduction of the jaw also affects the dentition in that crowns and roots are smaller and lose, to some degree, such characteristic structures as the cingula, cusps, and crests. Originally the palate and the dental arch were long and narrow, and premolarmolar rows ran almost parallel to each other; however, in modem man the palate and the dental arch form a 5
6
Am. J.
Marshall
Orthod. Dentofac. Orlhop. July 1990
Fig. 1.
shallow parabolic figure, and premolar-molar rows diverge. The curtailment of the masticatory apparatus, as well as the increased cranial capacity, results in diminution or disappearance of cranial superstructures. Superorbital ridges lose prominence or vanish entirely. The same is true for sagittal and nuchal crests. The temporal lines shift downward and cross the parietal bone at one half its height or lower. Frontal sinuses are lost or have become considerably smaller in size and extension. The phylogenetic evolution of the human skull, as revealed in the continuous enlargement of the brain and in the skull's consequent transformation, is certainly related to early man's adoption of erect posture. We have seen that brain enlargement produces changes in the face, palate, dental arch, and teeth. Reduction of the entire masticatory apparatus, together with expansion of the brain case, results in a fundamental imbalance of the mechanical connections. If the brain case is small but the masticatory apparatus is large, the cranial surface is not large enough to provide the necessary space for the attachment of
massive musculature; nor is it strong enough to sustain the chewing force. Because of this disproportion, the cranial surface is enlarged and strengthened by superstructures, such as the sagittal and nuchal crests and the postorbital processes. In contrast, if the brain case is large but the masticatory apparatus is small, the cranial surface is large enough to accommodate the weaker muscles and serve as a buttress for the diminished chewing force. With the ensuing inequality among parts of the skull, the air sinuses (those regions left vacant between the essential pillars of the face and the cranium) dwindle in size and disappear. Speculation about the meaning and outcome of evolution has no place in science, but it does occupy a permanent and legitimate place in every mind. The past course of evolution, together with the evidence for teleology, is a strong argument for a purpose in evolution. Man is the highest product of evolution, a process in which we are so vitally interested. The past is the prologue to the future. What changes can we predict for future man. As previously noted, the head is becoming slightly
Volume 98 Number 1
Special article
7
J
I
Fig. 2.
larger and broader, the skull and facial bones, thinner, and the physiognomy, more lively and expressive. The general features of the head will become even more refined. The sensory organs and centers, particularly those of sight, hearing, and taste, will become more efficient and more adaptive to the environment. The teeth will be fewer in number and will therefore cause less trouble with eruption. The jaws will become smaller because of lack of function. As man evolves, hair will become less and less necessary. As the size of the brain increases, the bones of the skull will become thinner, partly because of the increase in brain size but mainly because of the still further expected diminution of the stress on the muscles of mastication. Because of lateral and anteroposterior growth, the skull will develop and become larger in the path of least resistance, and the face will become more refined. The skull is designed to offer maximum protection
with minimum additional weight. All bones in the skull, as well as those in the rest of the body, have internally trabecular arrangements concemed with mechanical stresses. The bony tissue of the skull, with its struts and levers, is evidently adapted with exquisite precision and a suitable degree of resilience to resist all forms of physical stress. The development of the maxillary sinus and other pneumatic cavities of the skull is analogous to the development of the large marrow spaces of the skull, which is analogous to the development of the large marrow spaces of tubular bones. Both owe their existence to the fact that the maintenance of bone substance is largely dependent on its mechanical function. The development and growth of these air-filled cavities of the skull can be understood only if their function is clarified. These spaces develop as evaginations of the nasal cavity or the cavity of the middle ear into the adjacent bone. They remain in communication with the cavity from which they originate. A solid bone would
8
Am. J. Orthod. Dentofac. Orthop. July 1990
Marshall
Fig. 3.
carry only a slightly greater load, but it would be heavier and would need stronger muscles for its movement, and thus the level of functional adaptation would be lowered. Mechanical adaptations of the same type are at work in certain regions of the skull. The mechanical adaptations of this type are present in many sections of the skull, such as the sphenoid sinuses, frontal sinuses, and the mastoid process. Here the structure of bone is found to resist maximum force with a minimum of material. Wherever the spongy core of a bone of the skull is not under mechanical stress, the bony tissue is lost. Marrow is not deposited in its place; rather, the bone is made hollow by a diverticulum of the adjacent air-conduction passages. The frontal sinus, the maxillary sinus, and the sphenoid sinus are evaginations of the nasal cavity. The cells of the mastoid process originate from the cavity of the middle ear. The frontal sinus enlarges at the time the supraorbital ridge arises as a buttress for the masticatory forces. The cancellous architecture of most bones is generally regarded as conducive to lightness without loss of strength, though perhaps it is associated with economy in the use of material. The fact that some sinuses, particularly the small air cells, appear to de-
velop from the diploic or cancellous tissue of certain cranial bones serves to illustrate the above principle. The completely formed adult skull is an extremely complicated structure; some of the individual parts are united in such a way that it is quite difficult to recognize them. Some bones, indeed, are scarcely visible in a perfect skull, since they are, to a great extent, covered or overlapped by the other cranial bones. In the growth and development of the skull, the sphenoid and ethmoid complex plays an important part. The skull is a masterpiece of precise planning through genetics, a delicate and complex apparatus within which various components work as a unit to complete their natural engineering project. The cranial and facial bones are connected by the anterior cranial base. The sphenoid and ethmoid bones, which are part of the cranial base, articulate with all the cranial and facial bones of the skull, with the exception of the mandible. The growth of the cranial and facial bones (except the mandible) is primarily sutural, initiated by the proliferation of the sutural connective tissue. In the mandible, however, the main growth center is the hYaline cartilage in the mandibular condyle. The sphenoethmoid articulation is fixed and reaches adult dimensions by the age of about
Volume 98 Number 1
Special article
9
Fig. 4.
7 years. There are various types of growth: dimensional growth, proportional growth, and growth by adjustment. The main functions of sutures are to serve as sites of active dimensional bone growth and to provide space for movement, so that growth can take place as the bones adjust to each other. The adjustment growth, as well as the dimensional growth, is caused by internal and external forces. The internal forces include the growth of organs, such as the brain, the eyeballs, and the tongue, as well as the cartilage of the chondrocranium and the chondrofacial skeleton. The external forces are the development of the musculature and the air sinuses and the intake of oxygen. Since the sphenoid-ethmoid articulation is fixed at an early age, the rest of the bones, except the mandible, must make a great number of adjustments to this complex during growth; the obliteration of most of the sutures begins at a much later time in life. The adult skull has a dome-shaped braincase composed of the frontal bone over the forehead and the
parietal bones at the top and upper sides. The bones of the cranial vault are joined to each other at the sutures. The serrated sutures, combined with the convex shape of these bones, enable the cranium to withstand blows of considerable force. The human forehead is smooth and almost vertical. The orbits, or eye sockets, are roughly quadrilateral in shape, and they face forward and slightly outward. This position of the eyes in man is of great importance because it permits an overlap of the visual field that makes true stereoscopic vision possible. The orbital cavities are conical in shape and are found to taper as they are traced deep into the skull. The bones forming the inner wails of the orbit are thin and delicate; those of the roof and floor are heavier. In the center of the facial skeleton are the external orifices of the nasal cavities. The bridge of the nose is bounded by the maxillary bones. The greatest part of the nasal cavities lies deeply buried within the skull. On the outer wall of each nasal cavity are two scrolls of b o n e s - - t h e turbinate bones (conchae). In the midline between the nasal cavities is a bony nasal septum made up of the
10
Marshall
projection of the perpendicular plate of the ethmoid bone and the vomer. In the adult the maxillary bones (right and left) make up the main mass of the upper jaw and bear 16 teeth. Internally the maxillary bones are hollowed out to form the maxillary sinuses. The mandible is a large single bone that makes up the lower jaw. Like the maxilla, it bears 16 teeth. The shape of the mandible can be recognized by its horizontal body, which carries the teeth on its upper surface, and by its posterior ascending portion, the ramus. At the upper margin of the ramus are two prominences--one in front, called the coronoid process, and one farther back, called the condylus. Between the coronoid process and the condylus is the coronoid notch. The condylus is rounded and coated with cartilage. It fits into a depression in the undersurface of the temporal bone (the glenoid fossa) to form a movable joint. A vertical section shows the bones of the vault of the skull overlying the brain. These bones consist of three layers: an inner and an outer table of compact bone with a middle layer of bone marrow, or diplo~. This section shows the large air sinuses within the frontal bone above the nasal cavity and within the body of the sphenoid bone. These sinuses are continuous with the nasal cavity. Behind the frontal bone is the ethmoid bone which forms a perforated plate in the floor of the skull. It also has an upward projection, the crista galli, and a downward projection that forms the upper part of the nasal septum. The rest of the septum consists of the vomer, the maxillary bones, and the palatine bones. The upper surface of the sphenoid bone is hollowed out to form the sella turcica, in which the pituitary gland lies. A massive portion of the temporal bone in the floor of the skull is called the petrous bone or the petrous portion of the temporal bone. It is the hardest bone of the skull, and it houses the internal mechanical structures of the ear. The perpendicular plate of the ethmoid bone takes part in the formation of the nasal septum. The rest of the septum consists of the vomer and maxillary bones. The hard palate in the roof of the mouth is made up of the maxillary bones and the palatine bones. The interdependence of structure and function is one of the fundamental laws of biology. In functional construction, the mandibular joint demonstrates with clarity its interdependence with the other parts of the masticatory system. The shape of the condyloid processes varies according to occlusion and articulation. This fact is of great importance in mandibular movement. The form of the condyle changes considerably according to the type of occlusion; its configuration is steeper where
Am. J. Orthod. Dentofac. Orthop. July 1990
there is a pronounced anterior overbite. High buccal cusps result in a similar condition. The condyloid process is flatter where there is only a slight anterior overbite or an edge-to-edge bite and also where the buccal cusps are flat or have been lost. The mandibular joint also undergoes gradual changes to compensate for loss of teeth. The changes of shape and function in one part are followed by corresponding changes in another. The shape and structure of bones, combined with their mechanical function, enable them to resist maximum force with a minimum of material. The structure of bone depends on extrinsic factors, such as pressure of adjacent organs, which may influence the modeling of the bone. The final shape of the bone and the final elaboration of its internal structure are the end results of the influence of normal function. Thus there has been an attempt to show how biologically sound the concept of interdependence of structure and function is by showing how nature has created and shaped the bones for their maximum potential use. As previously noted, one of the most fascinating studies in life is the study of life itself. Within this study, growth is the most exciting. It is everywhere and is a continual wonder to behold. There are many questions that still mystify scientists. However, growth is oniy one aspect of the larger process of development. From the moment of conception, aging proceeds to the eventual death of the organism. Human tissues, designed for a cycle length of seventy-odd years, experience some 20 years of growth, 30 years of maturity, and 20 years of degeneration. At every period, some portion of the body is getting old and making way for something else. Even in the fetal stage, certain parts of the body atrophy and yield to other growing structures. Physical growth comprises all the morphologic modifications that characterize the life span of an individual. Modifications known to take place during human ontogeny include changes in kind, number, size, shape, position, pigmentation, and texture of body parts. Age is used to denote the stages of human life. As a measure of developmental capacity, it may be expressed by the chronologic, dental, or skeletal age. These ages are indices of maturity. All are present in biologic organisms, but the timing of each may vary. Aging involves a series of changes, not only the addition of material to produce size increase but also the differentiation of body parts for functional purposes. It represents alteration in the form of the body as a whole, as well as in the form of certain organs and systems. Substitution may occur, as when cartilage is convened into bone or when permanent teeth replace de-
Voh~me 98 Number 1
ciduous ones. Changes in number of structural units may occur throughout life. There are, for instance, periods of increase and decrease in the number of bones and teeth and in the amount of hair. At 5 years, the child has 48 to 52 teeth in varying stages o f development; a decade later, the loss of 20 deciduous teeth reduces the number to 32, and, with the passage of a few more decades, there are still fewer. Development of the skeleton, the basic structural unit of the body, involves ongoing bone formation and bone coalescence. During the adolescent years, the amount of bone mass decreases from approximately 350 to less than 220. Bone may be termed a dynamic substance because formation and destruction proceed continuously. In youth ossification predominates; in old age reabsorption increases. Skeletal age has become a measure of bodily maturation. In the embryo, bone is derived from the mesoderm germinal layer, and teeth are derived from the ectoderm and the mesoderm. Enamel originates from the ectoderm, and in this respect, is closely related to hair, nails, and other ectodermal derivatives. The dentin, cementum, and pulp are formed by the mesoderm; dentin and cementum resemble bone, and the pulp is a form of modified connective tissue. In all studies of skeletal maturation, the hand-wrist roentgenogram has been the most widely used, but the aging process can be observed as well in the development and eruption of the dentition. The sequence of these changes is roughly similar for a given bone or tooth in every person, but timing may differ according to skeletal and dental development: it may be advanced, average, or delayed. There are exceptions to uniformity, and occasionally some of the centers of eruption may appear in unusual sequence. In general, however, the course o f events is reproducible enough to permit comparisons between individuals. Radiologic examination allows us to determine how far the skeleton has progressed toward adulthood. The wrists, hands, and teeth are most commonly used for this purpose because there are many centers of ossification in these regions. We know that every center of ossification passes through a series o f morphologic stages. These changes in the outline of the centers can be identified with reasonable certainty and compared with key diagrams that show the norm. Interpretations based on key diagrams should not be relied on uncritically. Quite apart from the fact that children of diverse racial and socioeconomic backgrounds mature at different rates, there is considerable genetic variation in the order in which centers o f os-
Special article
11
sification appear. The fact that a given center has not appeared at its expected time is thus not an indication that skeletal maturation is delayed. In checking the skeletal age against the norm, we find that it is in no sense constant. Maturation does not proceed at a steady rate and has significantly different meanings at different stages of development. Individual variation in maturation of skeletal development, like individual variation in growth, is so extensive that it is quite possible for a child at one stage of growth to be far removed from the median maturity picture without the implication of a deleterious effect. Hence, radiologic (skeletal) age is only one factor to be considered in a general examination of the patient, and it should be correlated with the dental age. Skeletal maturation proceeds roughly parallel to skull and dental growth, and both are terminated when the epiphyses close at the time of the eruption of the third molar. Skeletal age is closely related to dental age; a severe disturbance in one usually results in a reaction in the other.
REFERENCES
1. Marshall D. Cleft lip and palate deformities. Dent RadiogrPhot~r 1973;46:!-19. 2. Marshall D. Radiographic correlation of hand, wrist and tooth development. Dent Radiogr Photogr 1976;49:51-72. 3. Marshall D. Interpretation of the posteroanterior skull radiograph. Dent Radiogr Photogr 1969;42:27-36. 4. Marshall D. Embryology of the skull. Dent Radiogr Photogr 1974;47(1):1-14. 5. Marshall D. Interpretation of the lateral skull radiograph. Dent Radi~r Photogr 1970;43:71-82. 6. Marshall D. Engineering of Human Skull. Part I. Engineering and biologic similarities. Dent Radiogr Phot~r 1978;51(3):4155. 7. Marshall D. Engineering of human skull. Part II. Relationship of parts in formation of skull. Dent Radi~r Photogr 1979; 52(3):49-61. 8. Marshall D. Changes in the skull--past, present and future-because of evolution. J Am Dent Assoc 1975;91:938-42. 9. Marshall D. Sphenoid-ethmoid complex. Oral Surg Oral Med Oral Pathol 1964;17(supp. 3):1-26. 10. Marshall D. Dimensional growth. Am J Ortho 1950;44:99-11I. 1I. Marshall D. Orthodontists, be kind to future generations. Part I. Dent Digest 1972;78:236-41. 12. Marshall D. Orthodontists, be kind to future generations. Part I1. Dent Digest 1972;78:302-6. 13. Marshall D. Skull bones in plastic. Dent Digest 1967;73:21821. Reprint requests: Dr. David Marshall 1124 E. Genese St. Syracuse, NY 13210
O n e of the most fascinating of all studies is the study of man. In the Anatomic Museum, we are reviewing man, not as a creature of today but as one passing through a vast series of ages. It is through the study of the skull that we can see the process of evolution of the human race, not only in the past and present but also in the future. Just as an astronomer, by his observations of the sky can calculate a whole orbit as a unit and predict when a phenomenon will recur, perhaps hundreds of thousands of years hence, we can, by observing man's evolutionary past, predict the anatomic changes of future man. The fifties were known as the atomic age, the sixties as the missile age, the seventies as the electronic age, the eighties as the computer age, and I believe the nineties will be known as the biologic age. In the past 45 years of research, and from my publications on various aspects of the skull, I have assembled my materials and opened an anatomic museum of the skull (see photographs). I have housed the collection in the second story of my office building, which I renovated especially for this project, and I am sharing my work with members of related professions and with the public (Figs. 1 through 4). Of all the parts of man's frame, the most interesting is the skull itself. It represents and identifies the individual. Man's shape, especially his skull, has changed with his environment. The shape of his skull is such that no other shape could function more effectively in man's surroundings. For at least 10,000 years there has been no notable progress in the evolution of man, but 10,000 years is entirely too brief a term in which to look for marked evolutionary advances. Actually, the size of the human brain has not increased since the time of the CroMagnon race. In fact, cranial capacity of the Neanderthal race of Homo sapiens was, on the average, equal to or even greater than that of modem man. However, cranial capacity and brain size are not reliable indicators of intelligence or intellectual abilities. In human evolution so far, there has been a tendency toward a simpler and more generalized organism, as can be seen in the
811112100
simplication of many organs and systems. The progressive degeneration of certain parts of the organism and resulting simplifications are validated by the presence of many rudimentary structures. When we observe the present, we see it, notwithstanding its great complexities, merely as a continuation of the past. Man is still straggling with the environment. Although he is controlling it more and more, each day he is still changing. In general, man's past and present show that he is not yet perceptibly near the end of his evolution, and, according to all indications, he will, for a long time, continue to progress in adaptation, refinement, and differentiation. In the changes that have already occurred, we find less need for the use of the jaws and muscles of mastication because of better prepared foods. Reduction in the size of the face, with reduction in the breadth and length of the jaws, reduces the number of teeth. The head in general is becoming slightly broader and larger; the skull and facial bones thinner, and the physiognomy more lively and expressive. Hair, especially in men, is being lost prematurely. Some of the dental units tend to disappear. We see that the cranium as a whole is becoming shorter, broader, and higher. The forehead is becoming more erect and vaulted. The frontal bone fully participates in the direct formation of the anterior wall of the brain case. The occipital bone is becoming more globular; its greatest breadth is being shifted from the base upward to the parietal bones with the development of typical tubera parietale. The walls of the cranium are becoming thinner as a result of the greater expansion of the brain, and this causes the parietal sutures to close at a later period in life. The face, particularly in the area of the upper jaw, is becoming reduced in height and length, and the long axis of the cranium seems to be moving to a position below the frontal portion of the cranium. The palate and the dental arch are becoming shorter and wider. The general reduction of the jaw also affects the dentition in that crowns and roots are smaller and lose, to some degree, such characteristic structures as the cingula, cusps, and crests. Originally the palate and the dental arch were long and narrow, and premolarmolar rows ran almost parallel to each other; however, in modem man the palate and the dental arch form a 5
6
Am. J.
Marshall
Orthod. Dentofac. Orlhop. July 1990
Fig. 1.
shallow parabolic figure, and premolar-molar rows diverge. The curtailment of the masticatory apparatus, as well as the increased cranial capacity, results in diminution or disappearance of cranial superstructures. Superorbital ridges lose prominence or vanish entirely. The same is true for sagittal and nuchal crests. The temporal lines shift downward and cross the parietal bone at one half its height or lower. Frontal sinuses are lost or have become considerably smaller in size and extension. The phylogenetic evolution of the human skull, as revealed in the continuous enlargement of the brain and in the skull's consequent transformation, is certainly related to early man's adoption of erect posture. We have seen that brain enlargement produces changes in the face, palate, dental arch, and teeth. Reduction of the entire masticatory apparatus, together with expansion of the brain case, results in a fundamental imbalance of the mechanical connections. If the brain case is small but the masticatory apparatus is large, the cranial surface is not large enough to provide the necessary space for the attachment of
massive musculature; nor is it strong enough to sustain the chewing force. Because of this disproportion, the cranial surface is enlarged and strengthened by superstructures, such as the sagittal and nuchal crests and the postorbital processes. In contrast, if the brain case is large but the masticatory apparatus is small, the cranial surface is large enough to accommodate the weaker muscles and serve as a buttress for the diminished chewing force. With the ensuing inequality among parts of the skull, the air sinuses (those regions left vacant between the essential pillars of the face and the cranium) dwindle in size and disappear. Speculation about the meaning and outcome of evolution has no place in science, but it does occupy a permanent and legitimate place in every mind. The past course of evolution, together with the evidence for teleology, is a strong argument for a purpose in evolution. Man is the highest product of evolution, a process in which we are so vitally interested. The past is the prologue to the future. What changes can we predict for future man. As previously noted, the head is becoming slightly
Volume 98 Number 1
Special article
7
J
I
Fig. 2.
larger and broader, the skull and facial bones, thinner, and the physiognomy, more lively and expressive. The general features of the head will become even more refined. The sensory organs and centers, particularly those of sight, hearing, and taste, will become more efficient and more adaptive to the environment. The teeth will be fewer in number and will therefore cause less trouble with eruption. The jaws will become smaller because of lack of function. As man evolves, hair will become less and less necessary. As the size of the brain increases, the bones of the skull will become thinner, partly because of the increase in brain size but mainly because of the still further expected diminution of the stress on the muscles of mastication. Because of lateral and anteroposterior growth, the skull will develop and become larger in the path of least resistance, and the face will become more refined. The skull is designed to offer maximum protection
with minimum additional weight. All bones in the skull, as well as those in the rest of the body, have internally trabecular arrangements concemed with mechanical stresses. The bony tissue of the skull, with its struts and levers, is evidently adapted with exquisite precision and a suitable degree of resilience to resist all forms of physical stress. The development of the maxillary sinus and other pneumatic cavities of the skull is analogous to the development of the large marrow spaces of the skull, which is analogous to the development of the large marrow spaces of tubular bones. Both owe their existence to the fact that the maintenance of bone substance is largely dependent on its mechanical function. The development and growth of these air-filled cavities of the skull can be understood only if their function is clarified. These spaces develop as evaginations of the nasal cavity or the cavity of the middle ear into the adjacent bone. They remain in communication with the cavity from which they originate. A solid bone would
8
Am. J. Orthod. Dentofac. Orthop. July 1990
Marshall
Fig. 3.
carry only a slightly greater load, but it would be heavier and would need stronger muscles for its movement, and thus the level of functional adaptation would be lowered. Mechanical adaptations of the same type are at work in certain regions of the skull. The mechanical adaptations of this type are present in many sections of the skull, such as the sphenoid sinuses, frontal sinuses, and the mastoid process. Here the structure of bone is found to resist maximum force with a minimum of material. Wherever the spongy core of a bone of the skull is not under mechanical stress, the bony tissue is lost. Marrow is not deposited in its place; rather, the bone is made hollow by a diverticulum of the adjacent air-conduction passages. The frontal sinus, the maxillary sinus, and the sphenoid sinus are evaginations of the nasal cavity. The cells of the mastoid process originate from the cavity of the middle ear. The frontal sinus enlarges at the time the supraorbital ridge arises as a buttress for the masticatory forces. The cancellous architecture of most bones is generally regarded as conducive to lightness without loss of strength, though perhaps it is associated with economy in the use of material. The fact that some sinuses, particularly the small air cells, appear to de-
velop from the diploic or cancellous tissue of certain cranial bones serves to illustrate the above principle. The completely formed adult skull is an extremely complicated structure; some of the individual parts are united in such a way that it is quite difficult to recognize them. Some bones, indeed, are scarcely visible in a perfect skull, since they are, to a great extent, covered or overlapped by the other cranial bones. In the growth and development of the skull, the sphenoid and ethmoid complex plays an important part. The skull is a masterpiece of precise planning through genetics, a delicate and complex apparatus within which various components work as a unit to complete their natural engineering project. The cranial and facial bones are connected by the anterior cranial base. The sphenoid and ethmoid bones, which are part of the cranial base, articulate with all the cranial and facial bones of the skull, with the exception of the mandible. The growth of the cranial and facial bones (except the mandible) is primarily sutural, initiated by the proliferation of the sutural connective tissue. In the mandible, however, the main growth center is the hYaline cartilage in the mandibular condyle. The sphenoethmoid articulation is fixed and reaches adult dimensions by the age of about
Volume 98 Number 1
Special article
9
Fig. 4.
7 years. There are various types of growth: dimensional growth, proportional growth, and growth by adjustment. The main functions of sutures are to serve as sites of active dimensional bone growth and to provide space for movement, so that growth can take place as the bones adjust to each other. The adjustment growth, as well as the dimensional growth, is caused by internal and external forces. The internal forces include the growth of organs, such as the brain, the eyeballs, and the tongue, as well as the cartilage of the chondrocranium and the chondrofacial skeleton. The external forces are the development of the musculature and the air sinuses and the intake of oxygen. Since the sphenoid-ethmoid articulation is fixed at an early age, the rest of the bones, except the mandible, must make a great number of adjustments to this complex during growth; the obliteration of most of the sutures begins at a much later time in life. The adult skull has a dome-shaped braincase composed of the frontal bone over the forehead and the
parietal bones at the top and upper sides. The bones of the cranial vault are joined to each other at the sutures. The serrated sutures, combined with the convex shape of these bones, enable the cranium to withstand blows of considerable force. The human forehead is smooth and almost vertical. The orbits, or eye sockets, are roughly quadrilateral in shape, and they face forward and slightly outward. This position of the eyes in man is of great importance because it permits an overlap of the visual field that makes true stereoscopic vision possible. The orbital cavities are conical in shape and are found to taper as they are traced deep into the skull. The bones forming the inner wails of the orbit are thin and delicate; those of the roof and floor are heavier. In the center of the facial skeleton are the external orifices of the nasal cavities. The bridge of the nose is bounded by the maxillary bones. The greatest part of the nasal cavities lies deeply buried within the skull. On the outer wall of each nasal cavity are two scrolls of b o n e s - - t h e turbinate bones (conchae). In the midline between the nasal cavities is a bony nasal septum made up of the
10
Marshall
projection of the perpendicular plate of the ethmoid bone and the vomer. In the adult the maxillary bones (right and left) make up the main mass of the upper jaw and bear 16 teeth. Internally the maxillary bones are hollowed out to form the maxillary sinuses. The mandible is a large single bone that makes up the lower jaw. Like the maxilla, it bears 16 teeth. The shape of the mandible can be recognized by its horizontal body, which carries the teeth on its upper surface, and by its posterior ascending portion, the ramus. At the upper margin of the ramus are two prominences--one in front, called the coronoid process, and one farther back, called the condylus. Between the coronoid process and the condylus is the coronoid notch. The condylus is rounded and coated with cartilage. It fits into a depression in the undersurface of the temporal bone (the glenoid fossa) to form a movable joint. A vertical section shows the bones of the vault of the skull overlying the brain. These bones consist of three layers: an inner and an outer table of compact bone with a middle layer of bone marrow, or diplo~. This section shows the large air sinuses within the frontal bone above the nasal cavity and within the body of the sphenoid bone. These sinuses are continuous with the nasal cavity. Behind the frontal bone is the ethmoid bone which forms a perforated plate in the floor of the skull. It also has an upward projection, the crista galli, and a downward projection that forms the upper part of the nasal septum. The rest of the septum consists of the vomer, the maxillary bones, and the palatine bones. The upper surface of the sphenoid bone is hollowed out to form the sella turcica, in which the pituitary gland lies. A massive portion of the temporal bone in the floor of the skull is called the petrous bone or the petrous portion of the temporal bone. It is the hardest bone of the skull, and it houses the internal mechanical structures of the ear. The perpendicular plate of the ethmoid bone takes part in the formation of the nasal septum. The rest of the septum consists of the vomer and maxillary bones. The hard palate in the roof of the mouth is made up of the maxillary bones and the palatine bones. The interdependence of structure and function is one of the fundamental laws of biology. In functional construction, the mandibular joint demonstrates with clarity its interdependence with the other parts of the masticatory system. The shape of the condyloid processes varies according to occlusion and articulation. This fact is of great importance in mandibular movement. The form of the condyle changes considerably according to the type of occlusion; its configuration is steeper where
Am. J. Orthod. Dentofac. Orthop. July 1990
there is a pronounced anterior overbite. High buccal cusps result in a similar condition. The condyloid process is flatter where there is only a slight anterior overbite or an edge-to-edge bite and also where the buccal cusps are flat or have been lost. The mandibular joint also undergoes gradual changes to compensate for loss of teeth. The changes of shape and function in one part are followed by corresponding changes in another. The shape and structure of bones, combined with their mechanical function, enable them to resist maximum force with a minimum of material. The structure of bone depends on extrinsic factors, such as pressure of adjacent organs, which may influence the modeling of the bone. The final shape of the bone and the final elaboration of its internal structure are the end results of the influence of normal function. Thus there has been an attempt to show how biologically sound the concept of interdependence of structure and function is by showing how nature has created and shaped the bones for their maximum potential use. As previously noted, one of the most fascinating studies in life is the study of life itself. Within this study, growth is the most exciting. It is everywhere and is a continual wonder to behold. There are many questions that still mystify scientists. However, growth is oniy one aspect of the larger process of development. From the moment of conception, aging proceeds to the eventual death of the organism. Human tissues, designed for a cycle length of seventy-odd years, experience some 20 years of growth, 30 years of maturity, and 20 years of degeneration. At every period, some portion of the body is getting old and making way for something else. Even in the fetal stage, certain parts of the body atrophy and yield to other growing structures. Physical growth comprises all the morphologic modifications that characterize the life span of an individual. Modifications known to take place during human ontogeny include changes in kind, number, size, shape, position, pigmentation, and texture of body parts. Age is used to denote the stages of human life. As a measure of developmental capacity, it may be expressed by the chronologic, dental, or skeletal age. These ages are indices of maturity. All are present in biologic organisms, but the timing of each may vary. Aging involves a series of changes, not only the addition of material to produce size increase but also the differentiation of body parts for functional purposes. It represents alteration in the form of the body as a whole, as well as in the form of certain organs and systems. Substitution may occur, as when cartilage is convened into bone or when permanent teeth replace de-
Voh~me 98 Number 1
ciduous ones. Changes in number of structural units may occur throughout life. There are, for instance, periods of increase and decrease in the number of bones and teeth and in the amount of hair. At 5 years, the child has 48 to 52 teeth in varying stages o f development; a decade later, the loss of 20 deciduous teeth reduces the number to 32, and, with the passage of a few more decades, there are still fewer. Development of the skeleton, the basic structural unit of the body, involves ongoing bone formation and bone coalescence. During the adolescent years, the amount of bone mass decreases from approximately 350 to less than 220. Bone may be termed a dynamic substance because formation and destruction proceed continuously. In youth ossification predominates; in old age reabsorption increases. Skeletal age has become a measure of bodily maturation. In the embryo, bone is derived from the mesoderm germinal layer, and teeth are derived from the ectoderm and the mesoderm. Enamel originates from the ectoderm, and in this respect, is closely related to hair, nails, and other ectodermal derivatives. The dentin, cementum, and pulp are formed by the mesoderm; dentin and cementum resemble bone, and the pulp is a form of modified connective tissue. In all studies of skeletal maturation, the hand-wrist roentgenogram has been the most widely used, but the aging process can be observed as well in the development and eruption of the dentition. The sequence of these changes is roughly similar for a given bone or tooth in every person, but timing may differ according to skeletal and dental development: it may be advanced, average, or delayed. There are exceptions to uniformity, and occasionally some of the centers of eruption may appear in unusual sequence. In general, however, the course o f events is reproducible enough to permit comparisons between individuals. Radiologic examination allows us to determine how far the skeleton has progressed toward adulthood. The wrists, hands, and teeth are most commonly used for this purpose because there are many centers of ossification in these regions. We know that every center of ossification passes through a series o f morphologic stages. These changes in the outline of the centers can be identified with reasonable certainty and compared with key diagrams that show the norm. Interpretations based on key diagrams should not be relied on uncritically. Quite apart from the fact that children of diverse racial and socioeconomic backgrounds mature at different rates, there is considerable genetic variation in the order in which centers o f os-
Special article
11
sification appear. The fact that a given center has not appeared at its expected time is thus not an indication that skeletal maturation is delayed. In checking the skeletal age against the norm, we find that it is in no sense constant. Maturation does not proceed at a steady rate and has significantly different meanings at different stages of development. Individual variation in maturation of skeletal development, like individual variation in growth, is so extensive that it is quite possible for a child at one stage of growth to be far removed from the median maturity picture without the implication of a deleterious effect. Hence, radiologic (skeletal) age is only one factor to be considered in a general examination of the patient, and it should be correlated with the dental age. Skeletal maturation proceeds roughly parallel to skull and dental growth, and both are terminated when the epiphyses close at the time of the eruption of the third molar. Skeletal age is closely related to dental age; a severe disturbance in one usually results in a reaction in the other.
REFERENCES
1. Marshall D. Cleft lip and palate deformities. Dent RadiogrPhot~r 1973;46:!-19. 2. Marshall D. Radiographic correlation of hand, wrist and tooth development. Dent Radiogr Photogr 1976;49:51-72. 3. Marshall D. Interpretation of the posteroanterior skull radiograph. Dent Radiogr Photogr 1969;42:27-36. 4. Marshall D. Embryology of the skull. Dent Radiogr Photogr 1974;47(1):1-14. 5. Marshall D. Interpretation of the lateral skull radiograph. Dent Radi~r Photogr 1970;43:71-82. 6. Marshall D. Engineering of Human Skull. Part I. Engineering and biologic similarities. Dent Radiogr Phot~r 1978;51(3):4155. 7. Marshall D. Engineering of human skull. Part II. Relationship of parts in formation of skull. Dent Radi~r Photogr 1979; 52(3):49-61. 8. Marshall D. Changes in the skull--past, present and future-because of evolution. J Am Dent Assoc 1975;91:938-42. 9. Marshall D. Sphenoid-ethmoid complex. Oral Surg Oral Med Oral Pathol 1964;17(supp. 3):1-26. 10. Marshall D. Dimensional growth. Am J Ortho 1950;44:99-11I. 1I. Marshall D. Orthodontists, be kind to future generations. Part I. Dent Digest 1972;78:236-41. 12. Marshall D. Orthodontists, be kind to future generations. Part I1. Dent Digest 1972;78:302-6. 13. Marshall D. Skull bones in plastic. Dent Digest 1967;73:21821. Reprint requests: Dr. David Marshall 1124 E. Genese St. Syracuse, NY 13210
