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| =The Cartilaginous Skull Of A Human Embryo Twenty-One Millimeters In Length=
| | #REDIRECT [[Paper - The cartilaginous skull of a human embryo twenty-one millimeters in length]] |
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| by Warren H. Lewis.
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| :'''Links:''' [[Book_-_Contributions_to_Embryology|Carnegie Institution of Washington - Contributions to Embryology]]
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| -----Content to be added----
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| {{Template:Historic Disclaimer}}
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| ==Introduction==
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| The human skull has for generations attracted the attention of anatomists.
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| From the imaginative comparative anatomists, imbued with the doctrine of the
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| segmentation theory, we have inherited a bulky Uterature, gradually passing into
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| oblivion, on the segments of an unsegmented skull. Its eflfects, however, are still
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| to be noticed in the attempts to imagine a segmentation in the unsegmented brain.
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| In the slowly accumulating hterature on the cartilaginous skull of man and other
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| vertebrates another phase is manifested. It was anticipated that in the so-called
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| primordial or cartilaginous skull there would be found many indications of the
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| phylogenetic relationships, and by a comparison of the cartilaginous skulls of the
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| various vertebrates Gaupp and lus school expected to show this even more clearly.
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| The theory expressed in the terse phrase, "ontogeny repeats phylogeny," formed the
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| basis for such expectations. We have, however, gradually come to reaUze that
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| there is more untruth than truth in it. For example, how is it possible to fit the
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| conditions found in embryo No. 460 (Carnegie Collection), which are shown in
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| figures 7 and 8, to the theory that "ontogeny repeats ph\^logeny?" Who would
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| claim that our primitive ancestors had more brains than skull, except perhaps the
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| few who believe in the downfall of man? And yet in this embryo the brain is enor-
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| mous in size as compared with the cartilaginous skull or "primordial cranium."
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| Even with the maximum development of the cartilaginous skull the conditions are
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| essentially the same.
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| Comparative anatomy shows that in lower mammals and vertebrates the skull
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| is relatively large as compared vdth. the brain, while human ontogeny shows exactly
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| the reverse. The whole assumption that "ontogeny repeats phylogeny" was based
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| upon the erroneous notions concerning evolution that were prevalent before the
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| present-day conceptions of the germ-plasm were introduced. If the various steps
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| in evolution have come about primarily through the modification of the germ-
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| plasm, then we should expect changes to appear in the egg and in the subsequent
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| stages of ontogeny, and the entire development would thus be modified as much
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| as the adult. There undoubtedly are fleeting indications of our primitive ancestors
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| in the development of the embryo, but they are not very numerous and are usually
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| extremely difficult of interpretation.
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| It is probable that in the phjdogenetic history some sort of a membranous
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| skull preceded the cartilaginous skull, and the latter preceded the osseous; but it
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| is apparent from recent studies on vertebrate cartilaginous skulls that they no more
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| form a phylogenetic series than do the adult skulls of the san\e species. The series of cartilaginous human skulls with which we are now familiar, modeled from
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| embryos varying in length from 13 to 80 mm., has failed to add much or any addi-
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| tional phylogonotic evidence regarding the form of the skulls of our remf)te ancestors.
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| In fact, these cartilaginous skulls are as characteristically human as the adult skull
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| is human. It is becoming more and more clear, as our knowledge of the anatomy of
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| the human embr>'o increases, that both it and its various organs are at all stages as
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| characteristicalh' human as are the adult body and its organs. One can distinguisli
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| with ease between the cartilaginous skull of man and that of the ape, the pig, the
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| cat, the rabbit, or the mole, as each is as characterLsticallj^ formed as are the adult
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| skulls of the same species. Homologies and similarities are to be found in the
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| cartilaginous skulls just as in the adult skulls, but it is doubtful if much additional
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| evidence of phylogenetic relationships will be revealed by a comparison of carti-
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| laginous skulls.
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| The great need in the embrj^ology of the human skull, as of other vertebrates,
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| is a more complete and detailed series of the various stages, showing the gradual
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| development from the primitive membranous stages to the adult. The present
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| communication is concerned primarily with a particular stage that helps to fill in
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| one of the many gaps still existing in the literature. The studj' of each stage is
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| necessarily laborious, since it is practically impossible to picture such complicated
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| forms without resort to models. The various structures in the head and neck,
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| including the cartilaginous skull of this embryo, 21 mm. in length, were modeled
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| with the plaster of paris technique.
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| Table 1 includes all of the human chondrocrania that have been reconstructed and described within recent years.
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| ==THE CHONDROCRANIUM AS A WHOLE==
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| The chondrocranium at this stage forms a continuous mass of cartilage and
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| precartilage. It constitutes but a small jiart of the brain covering, as does that part
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| of the adult cranium which is o.ssified in cartilage. One can best understand the
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| cartilaginous skull by comparing it with that portion of the human adult skull
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| which is ossified in cartilage rather than with the cartilaginous skulls of the lower
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| vertebrates. To do this with accuracy and precision an extensive series of stages
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| intervening between the stage to be comi^ired and the adult should be at hand.
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| Unfortunately, but few stages are known, although in man more has been done than
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| with any other vertebrate. We have in hunuui anatomy a large literature on the
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| ossification centers of both the cartilaginous and the membranous bones. This is of course, a great help towards understanding the cartilaginous skull. Such a
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| literature for any of the lower vertebrates is scarcely e.xistent, and therefore very
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| little is now to be gained through a comparison of the cartilaginous skulls of different species.
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| It is well to bear in mind the great difference in size between the cartilaginous skull of the embryo and that part of the adult skull ossified in cartilage. Table 2
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| gives a few comparative measurements. Unfortunately, the embryonic measurements are all from the one cartilaginous skull under consideration; it would be most
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| desirable to have several such specimens, since the size of the skull varies considerably in embryos of corresponding ages. To realize the extent of this variability
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| one has but to compare the length of the basal plate, from the foramen magnum
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| to the hypophysial canal, in embryos of approximately the same age. Similar
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| differences exist in the adult, and the adult measurements here used are the averages for six skulls.
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| Table 2.
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| These figures serve to indicate in a general way the greater immaturity of
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| the anterior end of the cartilaginous skull at this stage. The adult basioccipital
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| is about 13 times as long as the cartilaginous basioccipital; the adult nasal septum
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| about 30 times as long as the embr^^onic nasal septum. In agreement with this
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| we find that the basioccipital consists of more advanced cartilage than the nasal
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| septum. In one of the following paragraphs I have stated reasons for beUeving
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| that the alar process of the sphenoid at a later stage becomes incorporated into
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| the body of the sphenoid; this is partlj^ substantiated by comparing the ratios for
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| the distances between the alar processes of the embrj^o and between the Ungulse
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| of the adult with the width between the outer edges of the optic foraminae or the
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| distance between the foramina^ rotundte (the latter formed in precartilage in the
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| embryo) in both embryo and adult. These ratios are the same in each case, namely,
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| 1:14, thus indicating that if the rate of growth in each of these regions of the
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| sphenoid is the same the alar process must be looked upon as ultimately forming the
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| lateral part of the body of the sphenoid. If, however, we compare the width of
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| the basisphenoid in the embryo (not including the alar processes) with that of the adult (including the lingulse) it will he seen that the ratio is 1 :25, which corresponds
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| exactly to the ratio for the width of the basioccipital between the otic capsules in
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| the embryo and in the adult. One would naturally expect the two to grow at about
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| the same rate and, if this is true, then the alar process should not be looked upon as
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| ultimately forming part of the basisphenoid. Factors such as the possible pressure
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| of the otic capsules on this region of the basioccipital may, however, account for
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| the discrepancy, since the ratio for the distance between the hyi)oglossal fora-
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| mina? (1 :15) is about the same as that for the basisphenoid, including in the embrj'o
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| the alar processes and in the adult the Ungulae.
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| ==DORSAL ASPECT==
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| The most familiar aspect of the chondrocranium is the upper surface of the
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| base, shown in figures 1 and 2. These figures should be compared with a some-
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| what similar view of the upper surface of the base of the adult skull shown in figure
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| 4, where the bone, ossified in cartilage, is colored blue. One is surprised, somewhat
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| startled in fact, bj' the general similarity between the cartilaginous skull of the
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| embryo and that part of the adult skull which is ossified in cartilage. The general
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| resemblance is even more marked in later stages when the chondrocranium is more
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| completely de^'eloped, as shown in the figures by Macklin and Hertwig of skulls
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| from embryos 40 and 80 mm. in length. Again, if the dorsal aspect of the base
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| of the entire skull, as shown in figure 3, is compared with that of the adult, new
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| resemblances will appear. In figure 3 is shown not only the cartilage, but also
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| the precartilage, blastema, and part of the dorsal membrane which enters into the
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| formation of the brain capsule. In comparing the adult skull with figures 1 and
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| 3, it must be borne in mind that these figures are drawn with the basioccipital
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| horizontal and not inclined, as in the usual view of the adult skull or in the view of
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| the cartilaginous skull shown in figure 2. At the caudal end of the cartilaginous
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| skull is the basioccipital with the two lateral parts, or exoccipitals, one on each side.
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| The exoccipitals are continued into the occipital squamse or nuchal plates which are
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| not yet united in the middorsal line, so that the foramen magnum is incomplete.
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| The basioccipital is continuous in front into the basisphenoid, and from it the two
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| wings project on each side. The mesethmoid is continuous from the basisphenoid,
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| without Une of demarcation, to the anterior end of the chondrocranium, and con-
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| nected with it are the nasal capsules. Lateral to the anterior half of the basi-
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| occijntal and fused with it are the large otic capsules corresponding to the i:)etrous
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| bones of the adult, each being separated from the exoccipital by the large jugular
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| foramen and the mastoid plate.
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| The basioccipital consists of a flattened plate extending from the foramen
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| magnum to the basisphenoid. The caudal edge bordering the foramen magnum
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| is more deeply notched than in the adult. This notch is continued into a deep
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| groove on the cranial side of the cartilage. At the apex of the groove is the oi)en-
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| ing where the notochord, after passing through the condensed mesenchyme which
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| fills the groove, enters the body of the occipital.
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| Both in the embrj^o and in the adult each exoccipital arises from the side of
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| the posterior half of the basioccipital and is perforated at its origin by the hypo-
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| glossal canal. In tliis embryo the right hypoglossal canal is divided on the cranial
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| side into two parts by a cartilaginous bar; on the pharyngeal side there is but a
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| single orifice, a condition not uncommon in the adult. The a.\is of the hypo-
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| glossal canal is nearly dorso-ventral in the chondrocranium, while in the adult
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| it passes almost laterally.
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| The lateral borders of the foramen magnum are formed by the lateral parts
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| of the occipital or exoccipitals and the squamae. There is a general similarity in
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| direction, in position, and in form of the exoccipital in the embryo and adult.
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| Included in the lateral occipital and constituting much of it, as seen in this view,
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| is the occipital neural arch, which is partially separated near its dorsal end from
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| the exoccipital and squama by a fissure (the occipital fissure) which is filled with
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| condensed mesenchjine.
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| That part of the exoccipital (the alar lamina) which continues upward from the
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| jugular process is concave on its inner surface for the large transverse sinus. The
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| sinus continues upward and outward, as in the adult, across the exoccipital and
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| mastoid portion of the temporal, but turns forward over the dorsal margin of the
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| otic capsule instead of backward over the squama of the occipital, as in the adult.
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| At the bottom of the transverse sulcus the exoccipital joins the mastoid portion of
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| the temporal. Along the upper part of this junction j'oung cartilage is present, but
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| towards the jugular foramen the two are joined by precartilage and at the jugular
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| foramen by dense mesenchjone or blastema, which in older embrj'os changes into
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| cartilage. There is no line of demarcation between the exoccipital and the squama.
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| The latter continues upward from the exoccipital as a broad, curved plate of cartilage.
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| Above the region of the exoccipital the occipital squama is continuous with
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| the mastoid portion of the temporal. There is no Une of demarcation between the
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| squama and mastoid, and therefore much confusion exists in the literature regard-
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| ing this region. This has come about through the use of the term parietal plate
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| for the plate of cartilage lying above the outer edge of the otic capsule and partly
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| continuous with it. We naturalh^ associate the word parietal with the membrane
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| bone of the same name, and the assumption has been that the parietal plate dis-
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| appeared at a later stage and was replaced by the membrane bone. Its fate, how-
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| ever, has never been carefullj' traced, and it is probable that this so-called parietal
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| plate, which extends backward into the nuchal plate, is part of the mastoid, for in
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| the adult the mastoid is continued upward, around the posterior and even the dorsal
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| edge of the petrous bone.
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| The foramen magnum is incomplete dorsally except for a thin membrane of
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| connective tissue, the dorsal membrane. In later stages (as shown bj^ Le\T[,
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| Hertwig, and Mackhn) the foramen is completed by a bridge of cartilage formed
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| by the junction of the occipital squamae, which gradually' extends around the
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| brain. This cartilaginous bridge or band, usually called the tectum posterius
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| or tectum syyioticum, in older stages extends around the brain from the lateral
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| occipital and mastoid plate of one side to the lateral occipital and mastoid plate of the other, and corresponds to tlio nuchal j^hite of the adult occijMtal squama,
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| which, as is well known, ossifies in cartilage. It seems quite unnecessary to intro-
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| duce into human embryology a term appUed to a somewhat similar bridge of
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| cartilage, arching over the brain from one auditory capsule to the other, in the
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| cartilaginous skulls of ami)liibia and reptiles. The nuchal plate in the human
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| embryo is not homologous to the tectum synoticum of the reptiles, since in man it
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| is jirimarily continuous with the lateral parts of the occijiital, and in reptiles with
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| the auditory capsules only. It would be better to avoid entirely the term tectum
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| posterius in human embryology, and to use the term nuchal plate, or squatna, as
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| Levi has done, which at once gives to it its true significance in the liigher vertebrates,
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| in man at least.
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| The relations about the foramen magnum can be appreciated best by com-
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| paring figures 2 and 4. Figure 2 is drawn in the same position as the adult skull
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| and the squama is intact. In its growth around the central nervous S3\stem it
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| precedes that of the occipital neural arch and (as has been pointed out by MackUn)
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| fuses with its fellow of the opposite side to form the primitive foramen before the
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| occipital neural arches meet or nearly meet.
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| The large jugular foramen hes between the exoccipital and the otic capsule,
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| as in the adult, extending laterally to the junction of the exoccii^ital with the
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| mastoid cartilage, and medially to the junction of the exoccipital with the otic
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| capsule. In the embryo and in the adult the same structures pass through the
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| foramen, but both the nerves and the vein are proportionally much larger in the
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| embryo. These structures have, however, approximately the same relations to
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| each other; the transverse sinus occupies the posterior and lateral part, the vagus
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| and the accessory nerves are just anterior and medial to it, and the glossopharyngeal
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| is still more anterior and medial and lies in the notch on the posterior edge of the
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| otic capsule.
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| The temporal cartilage, as in the adult, has an otic capsule or petrous part
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| and a mastoid plate or mastoid part. The otic capsule forms one of the most
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| conspicuous features of the cartilaginous skull, especiallj' at this stage, for at this
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| time the relatively small size of the occipital squama behind and of the orbital
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| wing of the sphenoid and the ethmoid cartilage in front gives e\en greater j^romi-
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| nence to the otic capsules than at a later stage when the squama and anterior end
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| of the chondrocranium are fully developed in cartilage.
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| The otic capsule consists of a medial cochlear part fused with the lateral side of
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| the anterior half of the basioccipital, and a lateral canalicular portion that forms part
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| of the lateral wall of the chondrocranium. The otic cai)sule is broader in j^ropor-
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| tion to its length than the ))etr()us bone. The internal acoustic meatus, the oiHMiing
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| of the aqueductus vestibuli, and the fo.ssa subarcuata are all relatively enormous at
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| this stage. In the infant the fossa subarcuata also forms a relatively large depres.sion.
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| The direction of the axis of the otic capsule or petrous portion of the temporal
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| is very similar to that of the adult. The upper surface of the otic capsule, as
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| alreadj' noted, contains the internal acoustic meatus, the opening of the aque-
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| ductus vestibuli, and the large fossa subarcuata under the superior semicircular canal. In the adult these openings are on the posterior surface of the petrous
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| bone. If the adult skull is rotated so that the cranial surface of the basioccipital
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| is horizontal, as in figure 1, of the embryonic chondrocranium, it will be seen that
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| the posterior surface of the petrous bone is directed upward and corresponds in
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| position to the upper surface of the otic capsule as seen in figure 1. In the embrj^o
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| one leaf of the tentorium cerebelU is attached to the anterior margin of the otic
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| capsule, and in the adult the tentorium is likewise attached to the corresponding
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| margin of the petrous bone, so that the upper surface of the otic capsule faces the
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| posterior cranial fossa, as does the corresponding surface in the adult. All of the
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| mastoid plate (except its most anterior tip) and the squama of the occipital are pos-
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| terior to the tentorium and face the posterior cranial fossa, as do the mastoid
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| and nuchal plates of the adult.
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| Surrounding the caudal and dorsal edges of the canaUcular part, and con-
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| tinuous with it, is the mastoid plate or cartilage. This cartilage forms part of the
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| outer wall of the chondrocranium and is interrupted by a large mastoid foramen.
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| We have already' noted the fusion of the mastoid plate with the exoccipital and occip-
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| ital squama. It is also grooved for the transverse sinus, and the latter covers
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| practically all of the inner surface of the cartilage. That portion of the mastoid
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| cartDage above the outer edge of the otic capsule corresponds, therefore, more
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| nearly to the upper part of the mastoid bone lying caudal to the upper part of the
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| outer end of the petrous bone. This becomes the more apparent when we consider
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| that the upper surface of the otic capsule, as seen in this aspect of the chondro-
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| cranium, presents both the internal acoustic meatus and the opening of the aque-
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| ductus vestibuli, and thus corresponds to the posterior surface of the petrous bone
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| and faces the posterior cranial fossa.
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| The basisphenoid, which continues forward from the basioccipital without line
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| of demarcation, occupies about the same relative position as in the adult. The
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| dorsum sellse, although well marked, is but imperfectlj' developed and consists
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| mostly of precartilage and blastema, not indicated in figures 1 and 2 but shown
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| in figure 3. To this blastema is attached the medial part of the tentorium. Be-
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| tween the dorsum sellse and the tip of the otic capsule the abducens nerve passes
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| forward towards the orbit. In front of the dorsum sellse is the large shallow sella
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| turcica. The hypophysial canal is seen in the center of the sella and contains
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| remnants of the h\i)ophysial stalk. Connected with the basisphenoid are the
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| temporal and orbital wings. The greater size of the orbital wings in the cartil-
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| aginous skull is exactly what we would expect when it is taken into consideration
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| that the part of the adult temporal wing which is ossified in cartilage is smaller
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| than the adult orbital wing ossified in cartilage.
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| The temporal wings project laterally from the sides of the basisphenoid, just in
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| front of dorsum sellse and at a lower level than the floor of the sella turcica. These
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| wings form a small part of the very incomplete floor of the- lateral joarts of what
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| will later be the middle cranial fossa. Each lateral part of the middle cranial
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| fossa, although indicated here by the depressed area between the orbital wings
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| and the otic capsules, is entirely filled by the lower end of the embryonic tentorium cerebelli. The boundaries of the middle cranial fossa are more clear in figure 3,
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| where the lateral blastemal walls are shown. The temporal wing extends into
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| tliis lateral blastema and thus completes the medial portion of the floor of the
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| middle fossa. Between the temporal wing and the otic capsule is the large middle
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| lacerated foramen, through wliich pass the internal carotid arter_y and the mandib-
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| ular nerve. The facial nerve, with the geniculate ganglion and the greater super-
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| ficial petrosal nerve, also passes through the foramen. The large otic ganglion lies
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| across the foramen (figs. 12 and 13). The precartilaginous tissue attached to the
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| anterior aspect of the temporal wing completes the foramen ovale and separates it
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| from the supraorbital fissure. Between the temporal wing and the orbital wing,
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| with its precartilage, is the large supraorbital fissure, and through it the oculomotor,
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| trochlear, abducens, and ophthalmic nerves pass into the orbital cavity (fig. 15).
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| The embrj'onic tentorium cerebelh consists of two lateral prismatic masses of
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| mesenchjTne connected across the midline by a band of condensed mesenchjone
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| that extends upward from the whole breadth of the dorsum sellae. The bases
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| of the prisms are against the lateral walls of the membranous skull and the apical
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| edges at the membranous band that extends upward from the dorsum sellae. The
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| lower end of each prism fills one of the lateral depressed areas of the potential
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| middle cranial fossa, between the otic capsule in back, the orbital wing in front,
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| and the lateral wall of the membranous skull. The posterior wall of each pris-
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| matic mass of the tentorium is composed of a thin layer of condensed mesenchjone,
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| and its lower edge is attached along the anterior edge of the otic capsule and
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| extends upward. The anterior wall of each prismatic mass, hkewise composed of
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| a thin layer of condensed mesenchyme, is attached to the posterior border of the
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| orbital wing and its precartilage and extends upward. These two membranes
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| come together along the medial apical edge where they are continued into the
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| medial connecting band. Laterally, the two membranes fuse with the lateral
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| membranous wall of the skull. The interior of each prism is filled with loose
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| mesenchyme in which are imbedded the semilunar gangUon, large blood-vessels,
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| and nerves. The trochlear and oculomotor nerves pass from above downward
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| through the entire length of the prism. The tentorium obUterates entireh^ the
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| potential middle cranial fossa. Later, when this fossa develops, the anterior wall
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| of the tentorium must be pushed down and back into the floor and against the
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| posterior wall of the fossa.
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| Each ala temporaUs consists of two distinct parts, a medial alar process joined
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| to the basisphenoid by young cartilage, and a lateral part wliich is, more strictly
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| speaking, the cartilaginous temporal wing. It is attached to the under surface of
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| the alar jprocess by condensed mesenchyme and Ues at a lower level (figs. 1, 2, 3,
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| 5, 6, 10, 11, and 14). Each alar process is usually regarded as forming the hngula,
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| but since the internal carotid artery, which enters the cranial cavity between it
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| and the apical end of the otic capsule, passes over its cranial surface, it seems
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| prol^ablc that it gives rise as well to that part of the basisphenoid wliich forms the
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| carotid sulcus. The relation of the greater superficial i)etr()sal nerve to the alar
| |
| process also indicates that the latter is incorporated into the body of the sphenoid since, in the passage of the nerve from the geniculate ganglion to the spheno-
| |
| palatine ganglion, part of its course is just beneath the alar process medial to the
| |
| medial end of the temporal wing proper. It is here that the junction of the body
| |
| with the temporal wing and the pterygoid process occurs and where later develops
| |
| the pterygoid canal (fig. 13). Only the lateral part of the ala temporalis of the
| |
| embryo, then, corresponds to that part of the temporal wing of the adult which
| |
| ossifies in cartilage. In the adult the carotid sulcus and the lingula are at a higher
| |
| level than the temporal wing, a condition which e.xists in the embryo if we consider
| |
| the alar process as forming a part of the body of the sphenoid.
| |
| | |
| Since most of the temporal wing and both plates of the pterj-goid process are
| |
| ossified in membrane, the cartilaginous wing, even when fully developed, repre-
| |
| sents but a small part of the temporal or greater wing and jjterygoid process of the
| |
| adult. The maxillary nerve lies in front and the mandibular nerve behind this
| |
| cartilage. In later stages the cartilage grows around the maxillary nerve and
| |
| separates it from the supraorbital fissure, thus forming the foramen rotundum
| |
| (figs. 3 and 10). Cartilage does not grow around the mandibular nerve, and the
| |
| foramen ovale is said to be formed by membrane bone. At this stage, therefore,
| |
| the large cleft between the temporal wing and the orbital wing represents more
| |
| than the supraorbital fissure.
| |
| | |
| The alar process and the temporal wing proper each has its own center of
| |
| chondrification, as pointed out by Bardeen and Fawcett. The alar process chondri-
| |
| fication unites with the basisphenoid before the temporal center unites with the
| |
| alar process.
| |
| | |
| The cartilaginous orbital wing, Uke the temi;)oral wing, is very incomjjletely
| |
| developed, as will be seen by comparing it with the older stages of Levi, ]\Iacklin,
| |
| and Hertwig. It consists of two parts, a proximal or basal part, and a lateral,
| |
| sickle-shaped part, which springs upward, outward, and forward around the optic
| |
| nerve and has much the same general position as that part of the orbital wing
| |
| immediately about the optic nerve of the adult. The basal part of the orbital
| |
| wing is connected to the basisphenoid by young cartilage and to the lateral part
| |
| by cartilage not quite so far advanced as that in either the basal part or the lateral
| |
| part. It seems probable that both parts of the orbital wing maj- arise from inde-
| |
| pendent centers of chondrification. From the general position of this basal part
| |
| and its relation to the optic nerve and to the general mesench^-me of this region,
| |
| and from the fact that it gives origin to all of the muscles of the orbit except the
| |
| superior obUque, I think it must ultimately become incorporated into the body of
| |
| the sphenoid. The sickle-shaped lateral part of the orbital wing is intimately
| |
| related to a larger precartilaginous part indicated by the green-colored structure
| |
| in figure 3. This precartilaginous part shades off into the frontal blastema and
| |
| there is in realitj^ no sharp line between the two. The cartilaginous and pre-
| |
| cartilaginous parts together have somewhat the same form as the orbital wing of
| |
| the 40 mm. embrj'o described by ^lacklin.
| |
| | |
| | |
| Ill the cartilaginous skull the optic foramen is incomplete but, as shown in
| |
| figure 3, is completed by the precartilage extending from the tip of the orbital wing
| |
| to the anterior part of the body of the sphenoid.
| |
| | |
| In figure 1 it can be seen how the anterior end of the basisphenoid rapidly
| |
| narrows and continues into the mesethmoid. Both in figures 1 and 2 the dorsal
| |
| thin edge of the mesethmoid can be seen; it corresponds in the adult to that part
| |
| of the mesethmoid which bisects the posterior part of the cribriform plate. The
| |
| mesethmoid extends into the large crista galli, which consists mostly of precarti-
| |
| lage. In figure 3 the position of the future cribriform plate is indicated by the large
| |
| olfactorj' foramina, one on either side of the short dorsal edge of the mesethmoid.
| |
| On either side of the mesethmoid, and connected with the anterior edge in front of
| |
| the crista galli, are the lateral wings of the nasal capsules. As seen in figures 1
| |
| and 2, the posterior edge of each capsule is presented.
| |
| | |
| The anterior cranial fossa is not evident in the cartilaginous skull. In the
| |
| combined cartilaginous and membranous skull shown in figure 3 its hmits are
| |
| clearly indicated. The floor of the fossa is formed by the precartilage of the orbital
| |
| wing, frontal blastema, and dorsal membrane, and in the center are the mesethmoid
| |
| and crista galli and the olfactory and ethmoid foramina. Figure 3 shows not
| |
| only the floor but also a considerable extent of the anterior wall formed by the
| |
| dorsal membrane.
| |
| | |
| MEDIAN SAGITTAL ASPECT.
| |
| | |
| In order to compare the view from the median sagittal plane shown in figure
| |
| 5 with a similar view of the adult skull, the latter should be rotated so that the
| |
| cranial surface of the basioccipital is horizontal. In the median sagittal section
| |
| of the adult skull before me the pharyngeal edge of the basioccipital measures 26
| |
| mm., the basisphenoid 24 mm., and the upper edge of the vomer (from the basis-
| |
| phenoid to the anterior nasal spine of the maxilla) 54 mm. The latter distance
| |
| corresponds to the ventral edge of the mesethmoid. In the cartilaginous skull the
| |
| corresponding distances are 2 mm., 1.6 mm., and 1.8 mm. The ratios, then, are
| |
| respectively 1:13, 1:15 and 1:30. The basisphenoid is thus proportionally shorter
| |
| in the embryo at this stage than the basioccipital, and the mesethmoid much more
| |
| so. This is what one would expect to find in the less differentiated regions at the
| |
| anterior end of the .skull.
| |
| | |
| The angles which the pharyngeal surface of the basioccipital make with the
| |
| same surface of the basisphenoid, and the latter with the mesethmoid, arc prac-
| |
| tically identical w^ith those found in the adult. Since the edge of the basisphenoid
| |
| is concave, a straight line connecting the two ends was used. It is interesting
| |
| to note in this connection that the angle made by the cranial surface of the basi-
| |
| occipital, projected sagittally on to a line parallel with the upper edge of the zygo-
| |
| matic arch, is almost exactly the same in tliis embryo as in the adult. Such
| |
| measurements serve to indicate that the cartilaginous skull is laid down from the
| |
| very beginning on much the same plan as that part of the adult skull ossified in
| |
| cartilage.
| |
| | |
| | |
| The basioccipital shows in its median sagittal section two parts — a thin caudal
| |
| part hang beneath the notochord connecting the posterior part of the two lateral
| |
| thickened masses of the body from which the exoccipitals arise, and a long wedge-
| |
| shaped part in front of and above the notochord; this Ls thickest where it joins the
| |
| basisphenoid. The diagonal course of the notochord through the basioccipital
| |
| and its reentrance into the basal plate at or near the junction of the basioccipital
| |
| with the basiphenoid is also clearly shown. The three roots of the occipital neural
| |
| arch arise from the posterior half of the basioccipital and unite lateral to the
| |
| hypoglossal foramina into a tapering, rod-Uke cartilage. The serial relationship
| |
| of the occipital neural arch with the cervical neural arches is very apparent in
| |
| figure 5. The dorsal tip is in line with the dorsal tips of the cervical neural arches.
| |
| The occipital squama is very prominent in this view; it appears to continue
| |
| upward from both the exoccipital and the otic capsule, and even to project in
| |
| front of the otic capsule. There is no hne of demarcation between it and the
| |
| mastoid plate which encircles the posterior and upper edges of the canalicular
| |
| part of the otic capsule and thus no definite limits can be given to either cartil-
| |
| aginous area. The anterior part of this plate is usually called the parietal plate,
| |
| a term which we have already discussed and discarded, since it is not apparent
| |
| that it includes anji,hing more than what might be embodied under the term
| |
| mastoid plate. In considering this region it should be borne in mind that figure 5
| |
| is drawn with the basioccipital horizontal. The large mastoid foramen (capsulo-
| |
| parietal fissure) Ues at the bottom of the transverse sulcus and interrupts the
| |
| continuity of the mastoid plate. Above the region of the otic capsule the masto-
| |
| squamal plate is grooved for the endoljonphatic sac. Whether this groove Ues
| |
| on the mastoid or squamal part is uncertain. The fate of the various regions of
| |
| the masto-squamal plate will remain obscure until we have a more complete series
| |
| of stages.
| |
| | |
| The surface of the otic capsule presented in this A-iew corresponds to the
| |
| posterior surface of the petrous bone and faces the posterior cranial fossa. Its
| |
| diagonal position also corresponds to the adult when the latter is seen from the
| |
| same angle. The cochlear part is mostly above the level of the basioccipital, pro-
| |
| jecting but shghtly below the level of the latter. The canaUcular part shows the
| |
| position of part of the superior semicircular canal in front and of the posterior
| |
| canal in back. The endolymphatic sulcus serves to mark the position of the
| |
| common duct.
| |
| | |
| The medial section of the basisphenoid extends only to the hypophj'sial canal
| |
| and a diagonal cut in front of this cuts off the orbital wing and extends to the edge
| |
| of the cartilage. It is impossible to determine the anterior and posterior hmits
| |
| of the basisphenoid. The posterior Umit is probably at or near the notochord
| |
| and the anterior limit somewhere near the line (*). The body is biconcave in the
| |
| region of the sella turcica. Anteriorly, it narrows from side to side, as shown in
| |
| figure 1, and increases in its sagittal thickness, as shown in figure 5. The tem-
| |
| poral wing projects below the body and the orbital wdng above. The mesethmoid is shown intact after removal of the nasal capsule. The large crista galli and
| |
| anterior end consist largely of young cartilage and precartilage.
| |
| | |
| INFERIOR ASPECT.
| |
| | |
| In comparing the inferior surface of the chondrocranium with that of the adult,
| |
| only the occipital and otic regions lend themselves readily to our purpose, unless we
| |
| imagine as eliminated the membranous bones, the maxillse, the palatines, the vomer,
| |
| the jitorygoid jiroeess, most of the great wing of the sphenoid, and the tjinpanic part
| |
| of the temporal. Figure (i is drawn with the pharyngeal surface of the basioccipital
| |
| parallel to the plane of the pai)er. This is not the usual view of the adult skull, and
| |
| the latter must be rotated into the corresponding jilane for comi)arison.
| |
| | |
| The basioccipital and the basisphenoid present no new features of especial
| |
| interest not already considered, except the openings of the canal for the notochord.
| |
| The anterior opening where the notochord reenters the basal plate is probably
| |
| near or just back of the junction of the basioccipital and basisphenoid. The
| |
| exoccipital shows the large hypoglossal foramina; these are not covered by the
| |
| large condj'les, as is the case in the adult. The condyles have scarcely begun to
| |
| develop, and the occipital is united to the atlas by dense mesenchjTne without
| |
| articular surface. The tip of the occipital neural arch is quite distinct and sepa-
| |
| rated from the squama. The prominent transverse or jugular process Ues just
| |
| caudal to the jugular foramen and, as in the adult, extends shghtly lateral to the
| |
| outer border of the foramen. The process is more prominent than in the adult
| |
| and has more the character of the vertebral transverse processes.
| |
| | |
| The immature condition of the inferior surface of the otic capsule and mastoid
| |
| region is accentuated by the fact that such structures as the carotid artery and
| |
| the facial nerve are not inclosed by cartilage, but Ue partly in sulci beneath the
| |
| capsule. The internal carotid artery passes forward and medially in the mesen-
| |
| chyme beneath the cochlear part, with only the beginning of a groove formed by
| |
| precartilage, not indicated in this figure. It enters the cranial cavity between the
| |
| apex of the cochlear part and the alar process. The facial nerve, which enters the
| |
| otic capsule through the internal acoustic meatus, soon emerges again from
| |
| the facial foramen located in the anterior region of the capsular sulcus between
| |
| the cochlear and canalicular parts. The geniculate ganglion hes just outside this
| |
| foramen (figs. 12 and 13). The greater part of the facial nerve, which in the adult
| |
| is inclosed in bone, is extracapsular and passes backward and outward in a groove
| |
| on the under surface of the canalicular part. The groove lies ])etween the fossa
| |
| incudis and the fenestra vestibuli or ovalis, and medial to the root of the styloid.
| |
| The cartilaginous auditory ossicles Ue beneath the canalicular jiart (fig. 14).
| |
| | |
| The serial relationshij:) of the jugular process, the mastoid jirocess, the upper
| |
| end of the styloid, and the incus is clearly indicated in figure G, in which the
| |
| jugular, mastoid, root of the styloid, and fossa incudis are .shown. It is impossible
| |
| to determine at present whether this has any jjhylogenetic significance or not.
| |
| In some of the embryos of al)out this stage there is a small sejjarate cartilage in the blastema of the mastoid process to which are attached the digastric and stapedius
| |
| muscles. The serial relationship shown in the embryo is quite different from that
| |
| of the adult, where the mastoid process which gives attachment to the digastric
| |
| is some little distance lateral to the jugular process, and the styloid process is im-
| |
| mediately in front of the latter. Just in front of the jugular foramen is the opening
| |
| for the aquaeductus cochlese.
| |
| | |
| The sphenoid shows the basisphenoid with the opening of the hypophysial
| |
| canal near its center and the two wings on each side. Part of the orbital wing is
| |
| obscured by the lateral part of the nasal capsule, as it would be in the adult from
| |
| this point of view.
| |
| | |
| The line of fusion of the nasal capsule with the mesethmoid is shown, and the
| |
| anterior nares directly face the observer as they do in the adult. The junction
| |
| of the nasal capsules with the mesethmoid is effected along the middle tliird of the
| |
| anterior edge of the mesethmoid by precartilage. The cut edge of this junction
| |
| is shown in figure 5.
| |
| | |
| LATERAL ASPECT.
| |
| | |
| The cartilaginous skull covers but a small area of the lateral surface of the
| |
| brain, namely, part of the medulla, part of the cerebellum, and a small area in the
| |
| region of the optic nerve (figs. 7 and 8). The cartilaginous skull, even in its
| |
| completed form, is a very inadequate protector for the brain, never covering more
| |
| than a small fraction of its surface. The condensed mesenchyme or blastema
| |
| covers a much larger surface, but even the cartilage and blastema together form
| |
| at this stage a very incomplete brain capsule (fig. 9). The capsule is completed
| |
| by the thin dorsal membrane. The inclosing of the central nervous system by the
| |
| gradual spreading of the blastema and cartilage, wliich invade and replace the dorsal
| |
| membrane, is similar to the well-known development of the thoracic and abdominal
| |
| walls and the disappearance of the ventral membrane.
| |
| | |
| The blastema covers almost all of the lateral surface of the cartilaginous skull. | |
| A small part of the occipital cartilage, including the transverse process, part of the
| |
| squama and occipital neural arch, part of the orbital wing of the sphenoid, and
| |
| part of the lateral surface of the nasal capsule, are uncovered (figs. 9 and 15).
| |
| Into the blastema covering the squamal cartilage, rather than into the cartilage
| |
| itself, are inserted the various occipital muscles (figs. 14 and 15). The blastema
| |
| covering the squama and the lateral surface of the otic capsule probably fases
| |
| later with the perichondrium, but at this stage it seems to be continuous with the
| |
| rest of the blastemal wall which later gives rise to membrane bones. It is in the
| |
| sphenoidal and frontal regions that the blastema greatly predominates over the
| |
| cartilage. All of the lateral wall of the middle cranial fossa consists of blastema
| |
| and the greater part of the floor (as well as all of the lateral wall of the anterior fossa)
| |
| is Ukewise formed by blastema. The orbital walls are mostly of blastema; cartilage
| |
| of the orbital wing of the sphenoid takes part in the formation of the apical region
| |
| of the orbit about the optic foramen, and a portion of the medial wall of the orbit
| |
| is formed by part of the lateral wall of the nasal capsule. Connected with the cartilage is considerable precartilage extending along the inner wall towards the
| |
| apex (fig. 15). Part of the outer wall of the orbit, consisting of the zygomatic
| |
| blastema and the zygomatic process of the frontal blastema, and also the zygo-
| |
| matic arch, are shown in figure 9. In lateral views of the adult skull the upper
| |
| border of the zj'gomatic arch is horizontal, and it is therefore easier to compare
| |
| figure 9 and the other lateral views with the adult if the figure.s are rotated so as
| |
| to bring the zygomatic arch into a horizontal position. Attention has already
| |
| been called to the fact that the angle made by projecting the hne of the upper
| |
| edge of the zygomatic arch on to the basioccipital is almost exactly the same in
| |
| the embryo and in the adult.
| |
| | |
| By rotating figure 7 to correspond to the usual adult position it will be seen
| |
| that the occipital squama projects backward from the exoccipital and mastoid,
| |
| and that, since it covers only part of the medulla and cerebellum, it is concerned
| |
| with the wall of the posterior cranial fossa. It is from this lateral view that one
| |
| gets the impression that the squama extends into the jugular process, or more
| |
| strictly, into that part of the exoccipital which continues into the lateral part of
| |
| the jugular process, rather than into the occipital neural arch wliich lies in a deeper
| |
| plane. The occipital neural arch has attained only a small proportion of the growth
| |
| necessary to inclose the medulla. The tip of the squama, on the other hand, has
| |
| grown farther around. The condition here is not as far advanced as that found by
| |
| Levi in a supposedly younger embryo.
| |
| | |
| The lateral aspect of the jugular process is quite prominent, and to it are
| |
| attached two muscles, the rectus capitis lateralis (serially related to the inter-
| |
| transversarii) and the occipitomastoid muscle (fig. 14). The former needs
| |
| no special comment. The latter muscle is not found in the adult and is infre-
| |
| quent in embryos. Its presence would seem to indicate either that the mastoid
| |
| process was separate or movable, or that the temporal and occipital cartilages
| |
| were at one time movable on each other. We do find that the occipital and
| |
| mastoid cartilages are separated by young cartilage or precartilage in the region
| |
| below the mastoid foramen. Above this, however, no fine of separation can be
| |
| found, but tliis does not necessarily mean that the two cartilages were articulated
| |
| by a movable joint. The significance of the mastoid process is described elsewhere.
| |
| | |
| The canalicular part of the otic capsule forms a conspicuous part of the lateral
| |
| wall of the chondrocranium. A slight bulging on the lower part of its lateral
| |
| surface indicates the position of the lateral semicircular canal. The styloid process
| |
| and incus he beneath the lateral edge of the otic capsule almost flush with the
| |
| lateral surface.
| |
| | |
| Between the canalicular part and the occipital cartilage is the mastoid cartilage.
| |
| It is fairly clearly defined in the region between the otic capsule and the exoccip-
| |
| ital below the mastoid foramen. In the region above the otic capsule there is
| |
| no line of demarcation between the squama and mastoid. The mastoid process
| |
| projects from the lower edge of the mastoid cartilage. In this embrj'^o it consists
| |
| of blastema; in other embryos, as will be stated farther on, a small cartilaginous | |
| nodule is sometimes eml)edded in the blastema. To the mastoid process are attached several muscles, namely, the occipitomastoid, the atlantomastoid, the
| |
| digastric, the stapedius, and the longissimus capitis (figs. 14 and 15).
| |
| | |
| The anterior part of the cartilaginous skull, the sphenoid and the ethmoid,
| |
| bears very little resemblance to anything ordinarily seen in the lateral view of the
| |
| adult skull, partly because most of the bone ossified in cartilage becomes covered
| |
| or shut out from the lateral view by membrane bone. If we can imagine all of
| |
| this membrane bone stripped away, the resemblance between the two would be
| |
| clearly seen.
| |
| | |
| The lateral surface of th3 body of the sphenoid, with the shallow sella turcica
| |
| odging the hypophysis, is to be seen in front of the apex of the cochlear part of the
| |
| otic capsule. The prominent dorsum sella projects towards the great mid-brain
| |
| fissure, and from it (but not shown) the thin medial membranous part of the ten-
| |
| torium projects into this fissure. The temporal wing hes for the most part below
| |
| the level of the basisphenoid and is attached to the under surface of the alar
| |
| process, while the orbital wing Ues, for the most part, above the level of the basis-
| |
| phenoid and is curved around the optic nerve. The two parts of the orbital
| |
| wing are clearly indicated in figures 7 and 14. In the description of the middle
| |
| cranial fossa and the embryonic tentorium cerebelli it was noted that the lateral
| |
| prismatic mass was so placed that the apical edge extended upward from the
| |
| dorsum sellse in connection with the thin medial membranous part of the tentorium
| |
| lying in the great mid-brain fissure. The lower end of the embryonic tentorium,
| |
| as previously noted, occupies the space between the otic capsule and the orbital
| |
| wing of the sphenoid, the semilunar gangUon filling up most of the gap (fig. 8).
| |
| It hes outside of the cartilaginous skull but within the membranous skull. The
| |
| geniculate gangUon and adjacent sections of the facial nerve are not incased in
| |
| cartilage, and they also he in this middle cranial fossa or gap (figs. 12 and 13).
| |
| | |
| In the lateral aspect of the chondrocranium most of the mesethmoid is shut
| |
| out from view by the nasal capsule. Between the orbital wing and the nasal capsule
| |
| the anterior end of the basisphenoid and a bit of the mesethmoid appear. If only
| |
| the cartilaginous skull were considered, this region of the basisphenoid might be
| |
| regarded as interorbital and the term inter orhital septum could be apphed. But
| |
| this so-called interorbital septum exists only in the cartilaginous skull; in the
| |
| more complete skull, with all the blastema and precartilage, the medial wall of
| |
| the orbit is complete enough to shut it out from the orbit. The term inter orhital
| |
| septum probably has no significance whatever in human embrj^ology. The
| |
| prominent crista galU projects between the cerebral hemispheres. The lateral
| |
| surface of the nasal capsule forms a large proportion of the inner wall of the orbit
| |
| and probably ossifies into the lamina papyracea of the ethmoid.
| |
| | |
| THE OCCIPITAL CARTILAGE.
| |
| | |
| The occipital cartilage consists of an elongated, flattened body (or basioc-
| |
| cipital), the lateral parts (or exoccipitals), and the two nuchal plates (or squamae).
| |
| | |
| The body forms the greater part of the so-called basal plate, a term much used
| |
| in descriptions of the cartilaginous skull. It continues without line of demarcation into the basisphenoid, and one can only approximate the future hne of junction
| |
| between the two bones which ossify in the cartilage by the relation to the apex of
| |
| the otic capsule and the entrance of the notochord. The basioccipital Ues in a
| |
| plane almost exactly horizontal to the long axis of the embryo. This elongated
| |
| quadrilateral jilate is thinner at the center, slightly concave on the cranial surface,
| |
| and slightly convex on the pharj-ngeal surface. It becomes thicker where it joins
| |
| the sphenoid. The rounded caudal end is bent downward and notched in the
| |
| midline at the foramen magnum. The caudal part of the basioccipital gives much
| |
| more of a clue to its vertebral origin than the flattened cranial part. The caudal
| |
| part is thickened and on the cranial surface is incompletely divided into two lateral
| |
| masses by a deep median groove. These correspond, perhaps, to the bilateral
| |
| masses which fuse to form the bodies of the cervical vertebrae. Levi finds in his
| |
| 13 mm. embryo two bilateral cartilaginous centers medial to the hypoglossal roots.
| |
| They probably represent, as Macklin suggests, the bilateral centers for the occipital
| |
| vertebrse. Their position in the 13 mm. embryo corresponds to the position of the
| |
| lateral masses in this 21 mm. embryo. These masses are united across the midline
| |
| beneath the notochord, and anteriorly are continued into the broad flat plate of
| |
| the basioccipital. The center of each mass contains the most highly differentiated
| |
| cartilage of the entire chondrocranium.
| |
| | |
| In the 14 mm. embryo described by Levi the otic capsule and the basioccipital
| |
| are apparently separated by a wide gap. Levi probably did not include all the
| |
| precartilaginous tissue and blastema of the otic capsule, since in embryo No. 109
| |
| (Carnegie Collection), 11.5 mm. in length, the blastema of the otic capsule is in con-
| |
| tact with the basioccipital. It is only in younger stages, 9 mm. length, that I find
| |
| the blastema of the otic capsule separated from the basioccipital. The union of the
| |
| blastema, then, precedes that of the cartilage in this region. In embryo No. 460
| |
| the anterior half of the lateral border of the basioccipital is fused with the cochlear
| |
| part of the otic capsule along a crescentic line. The line of fusion can still be recog-
| |
| nized by the differences in the degree of differentiation of the two cartilages. The
| |
| cochlear part consists of young cartilage and the nuclei are closer together than in
| |
| the basioccipital. The border of the basioccipital is, however, not so far advanced
| |
| as the central part. The two cartilages were so completely fused in the 17 mm.
| |
| embryo described by Levi that he was unable to recognize any histological border
| |
| between them. Embryo No. 128 (Carnegie Collection), 20 mm. in length, although
| |
| supposedly smaller and younger than the 21 mm. embryo (No. 4G0), shows a more
| |
| advanced conchtion in the cartilaginous differentiation of this region, and it is
| |
| impossible to find a histological border between the basioccipital and the cochlear
| |
| part except at either end of the line of fusion. In another 20 mm. embryo (No. 22,
| |
| Carnegie Collection) there is only the very sUghtest indication of the line of fusion .
| |
| There is probably some variation in the rate of differentiation of the cartilages of
| |
| this region in embryos of the same age and size.
| |
| | |
| | |
| OCCIPITAL VERTEBRA.
| |
| | |
| Each exoccipital corresponds for the most part to a vertebral neural arch or
| |
| hemiarch, which is so distinct in this embryo that we may properly apply the term
| |
| occipital hemiarch. The roots, laminae, and transverse processes can be readily
| |
| recognized. The roots are broad and thick and correspond to the roots or pedicles
| |
| of the vertebral neural arches. On the right side, as already noted, there are three
| |
| roots dividing the cranial end of the hypoglossal canal into two parts, a condition
| |
| not uncommon in the adult. The caudal root on each side is the largest and con-
| |
| sists of more advanced cartilage, and it is questionable whether the anterior roots
| |
| should be looked upon as forming part of the occipital vertebra. It has often been
| |
| suggested that they are part of an anterior, still more rudimentary vertebra. The
| |
| laminae extend outward and upward from the roots, forming part of the lateral
| |
| border of the foramen magnum. They are much thickened, rounded, tapering
| |
| cartilages, quite distinct, and in the dorsal part entirely separate in this particular
| |
| embryo from the squamal cartilage. The cartilage of the occipital arch is more
| |
| differentiated than the squamal. During the further course of development, as
| |
| Macklin has pointed out, the occipital neural hemiarches grow around the central
| |
| nervous system and approach each other in the midUne. It is not clear whether
| |
| they actually meet and fuse, or whether a small part of the squamal cartilage may
| |
| not intervene. The squamal cartilages, which likewise grow around the central
| |
| nervous system, meet and fuse before the occipital hemiarches. The ossicle of Kerck-
| |
| ring, which develops in this region, may represent a separate ossification center
| |
| of the nuchal plate intervening between the tips of the occipital neural hemiarches,
| |
| or this center may possibly be looked upon as the ossification of the fused occipital
| |
| spinal epiph3'ses. Schultz described a skull with, two bilateral ossicles in this region,
| |
| and suggests that they may correspond to the epiphyses of the spinous processes.
| |
| | |
| In this embryo the separation of the dorsal part of the lamina of the occipital
| |
| vertebra from the squama of the occipital, and the more intimate relation of the
| |
| latter with the transverse process, would seem to indicate that the squama is an
| |
| extension upward from the transverse process rather than an outgrowth from the
| |
| occipital neural lamina. The relation of the squama to the transverse process
| |
| and to the occipital neural lamina is clearly shown in figures 1, 7, and 16. The
| |
| occipital transverse process forms part of the caudal and lateral margins of the
| |
| jugular foramen and continues up into the squama and alar lamina without
| |
| line of demarcation. The alar lamina, as will be seen in figure 16, constitutes the
| |
| ventral and more medial part of the squama adjoining the lamina of the occipital
| |
| and the medial part of the transverse process. There is also a more gradual transi-
| |
| tion as regards the degree of differentiation from the cartilage of the transverse
| |
| process into the squama than from that of the lamina.
| |
| | |
| In embryo No. 22 (Carnegie Collection), 20 mm. in length, the squama of the
| |
| occipital is fused wath the lamina throughout the entire length of the latter on
| |
| both sides of the embryo, although throughout most of this length the younger
| |
| cartilage of the squama can easily be distinguished from the thickened, older
| |
| cartilage of the lamina. Towards the apex of the arch, however, the two cartilages are of the same degree of differentiation and no distinction between them can be
| |
| seen.
| |
| | |
| In embrj'o No. 128 (Carnegie Collection), 20 mm. in length, the resemblance
| |
| which the occipital neural arch and its transverse process bear to those in the
| |
| cervical region is very striking. The cartilages of the occipital and cervical neural
| |
| arches show much more advanced differentiation than the thin transverse processes.
| |
| The muscle intertransversarius, between the transverse processes of the second
| |
| and the first cervical vertebra?, is in line with and serially related to the rectus capitis
| |
| laterahs, which passes between the transverse process of the first cervical vertebra
| |
| and that of the occipital. In this embryo (No. 128) it can be seen also how the
| |
| squama of the occipital is a continuation upward from the transverse process
| |
| rather than from the lamina. The laminae of the occipital vertebra are also fused
| |
| along the entire length to the squamae, but are readily distinguishable from the
| |
| latter by their much more advanced cartilaginous differentiation. The tips of
| |
| these laminse are connected with the tips of the laminae of the atlas by the inter-
| |
| spinous hgament, as in embrj^o No. 460.
| |
| | |
| In embryo No. 240 (Carnegie Collection), 20 mm. in length, the cartilage of
| |
| the occipital neural arch on each side is Ukewise fused with the squama throughout
| |
| its entire length. The lamina also shows more advanced differentiation than
| |
| the squama. In embryo No. 431 (Carnegie Collection), 19 mm. in length, the
| |
| tips of the occipital neural arches are separated from the squama on either side
| |
| by the condensed mesenchyme forming the perichondrium. The e.xtent of the
| |
| fusion of the laminae and the squamae is, however, more extensive than in embryo
| |
| No. 460. These observations indicate that the occipital neural arch is more marked
| |
| in this stage than in the adult, and that probably an extreme degree of accentua-
| |
| tion in the embryo would precede any marked manifestation of an occipital vertebra
| |
| in the adult. The manifestation of an occipital vertebra in adult skulls has been
| |
| described by Gladstone, Kollman, and others. Such variations remind one of the
| |
| conditions found in embryos at this stage, and we shall probably find that embryos
| |
| vary as much, if not more, than adults and that such embryonic variations always
| |
| precede the adult variations.
| |
| | |
| The relation and attachment of the dorsal membrane to the tips of the occipital
| |
| hemiarches are similar to its relation and attachment to the tips of the neural
| |
| arches of the cervical vertebrae, in that in both cases the dorsal membrane is con-
| |
| tinuous with the perichondrium on the medial side of the tip of the neural arch.
| |
| Its attachment to the upper border of the squama is somewhat different, since here
| |
| it is merely continuous with the thin edge. The tip of the occijjital hemiarch is
| |
| connected with the tips of the vertebral hemiarches by a distinct band of condensed
| |
| membrane, the interspinous ligament described by Bardeen (fig. 9).
| |
| | |
| The transverse or jugular j^rocess springs from the occipital hemiarch at the
| |
| junction of the roots and lamina and projects laterally back of the jugular foramen.
| |
| We have already noted its serial rclationsliip with the vertebral transverse processes.
| |
| The lateral extremity of the jugular process has a knob-like enlargement, and into
| |
| this are inserted the rectus capitis lateralis muscle and the occipito-mastoid muscle
| |
| (fig. 24).
| |
| | |
| | |
| | |
| The occipital squama or nuchal plate springs from the upper border of the
| |
| lateral part of the jugular process. Near its origin it is narrow in the sagittal plane
| |
| and wide m the frontal plane. It is continuous with the basal part of the lamina
| |
| by a curved plate, the alar lamina (figs. 1 and 16), and rapidly widens into a broad,
| |
| thin plate, which continues upward to a thin edge that runs into the dorsal mem-
| |
| brane. Both the jugular process and the squama extend into the mastoid cartilage.
| |
| At the jugular foramen these two cartilages are separated by blastema and precar-
| |
| tilage; above this they are joined by young cartilage, which gradually changes into
| |
| cartilage indistinguishable from that of the squama or mastoid. The lateral surface
| |
| of the squama is covered by condensed mesench>Tne into which the edges of the
| |
| cartilage merge. It is into this mesench3'me that the occipital muscles— namely,
| |
| the trapezius, splenius capitis, semispinahs, rectus capitis posticus major and minor,'
| |
| and the obUquus capitis superior, appear to be inserted (figs. 14 and 15).
| |
| | |
| ==THE SPHENOID CARTILAGE==
| |
| | |
| The sphenoid cartilage consists of a body with two lateral wings or processes
| |
| on each side, the temporal and orbital wangs (figs. 1, 2, 3, and 6). The cartilage of
| |
| the body is continuous with the body of the occipital, and in front narrows and
| |
| thickens as it passes into the nasal septum. The cartilaginous center from which
| |
| the body develops is described as being caudal to the hypophysial canal. The
| |
| chondrification spreads caudally to meet the basioccipital, upward into the dorsum
| |
| sellae, and forward around the hypophysial canal where the cartilage fuses around
| |
| the canal and extends forward to form the anterior part of the sphenoid and the
| |
| nasal septum. Fawcett, after an examination of a 21 and a 19 mm. embryo, con-
| |
| cluded that the dorsum sellae was an independent formation, since he found in each
| |
| embryo a separate cartilaginous bar above the region of the body and separated
| |
| from it by mesenchj^ne. Twenty embryos of the Carnegie Collection, ranging in
| |
| length from 15.5 mm. to 24 mm., were examined, and such a cartilaginous bar was | |
| found in but one (No. 229), 19 mm. long. Twelve of the embryos were between
| |
| 18 and 23 mm. Great ^■ariation in the form of the dorsum sellse was observed, but
| |
| the rather rare presence of a separate center of chondrification would indicate that
| |
| as a general rule the dorsum sellse develops from the basisphenoid, as described by
| |
| Levi. In front of the dorsum sellse the body is biconcave and perforated near the
| |
| center by the hj^jophj^sial canal. There is practically no indication in front of the
| |
| sella turcica of the tuberculum sellse. The anterior part of the bodj^ is very incom-
| |
| pletely developed, and not until the anterior tip of the orbital wing has grown
| |
| medialward to meet its fellow of the opposite side and fuses with, the body is the
| |
| anterior border of the chiasmatic groove apparent (fig. 3).
| |
| | |
| The temporal wing consists of two parts : the alar process, which springs from
| |
| the body just in front of the level of the dorsum sellse and projects laterally and
| |
| sKghtly caudally toward the apex of the cochlear part of the otic capsule, and the
| |
| lateral part, attached to the under surface of the alar process and projecting later-
| |
| ally. Each part is supposed to have an independent center of chondrification. In
| |
| this embryo the alar process is joined to the basisphenoid by j'oung cartilage and to the lateral part by blastema. We have already given sufficient consideration to
| |
| the fate of the alar process and its probable incorporation into the body of the
| |
| sphenoid, and also the lateral part which corresponds more strictly to that part of
| |
| the temporal wing that ossifies in cartilage. This part of the cartilage is still incom-
| |
| j)letely developed; as in later stages the cartilage extends around the maxillary nerve.
| |
| The orbital wing is even more immature than the temporal, as will be seen by
| |
| comparing it with the older stages of Macklin and Hertwig. In figure 3 is shown the
| |
| precartilaginous part, which is more extensive than the cartilaginous part. The
| |
| basal part of the orbital wing lies at the apex of the orbit and gives attachment to
| |
| all the muscles of the orbit except the superior oblique (figs. 10 and 15).
| |
| | |
| THE TEMPORAL CARTILAGE.
| |
| | |
| This cartilage includes the otic capsule and the mastoid cartilage, which are
| |
| intimately fused together.
| |
| | |
| OTIC CAPSULES.
| |
| | |
| The otic capsules form the cartilaginous basis for the petrous bones of the
| |
| adult. They are very prominent, being striking features of the chondrocranium,
| |
| and extend on each side in a caudo-dorso-lateral direction to the lateral surfaces
| |
| of the skull. Each capsule consists of two broadly united and continuous parts
| |
| (a medial cochlear part and a lateral canalicular part) inclosing the cochlear and
| |
| semicircular canals respectively. The two parts are more or less set off from
| |
| each other by a broad shallow groove, the capsular sulcus.
| |
| | |
| The cochlear part is somewhat egg-shaped and fuses with the basioccipital.
| |
| The cranial surface is rounded and presents the large, rounded, internal acoustic
| |
| meatus. The ventral surface is also rounded and projects below the level of the
| |
| basioccipital, forming with it a distinct groove. The cochlear part consists of
| |
| somewhat younger cartilage than the canalicular part. Its relation to the cochlear
| |
| duct is shown in figure 16.
| |
| | |
| The capsular sulcus extends entirely around the otic capsule. On its anterior
| |
| surface is a largo foramen for the exit of the facial nerve. This foramen is separated
| |
| by a narrow bar of cartilage from the internal auditory meatus. The facial nerve
| |
| has but a very short course within the otic capsule; the geniculate gangUon and the
| |
| nerve distal to it are outside of the capsule, but close against it (figs. 12 and 13).
| |
| The capsular sulcus is broad and shallow on the inferior surface and near its center
| |
| hes the fossa vestibularis or ovalis, in which is imbedded the base of the stapes.
| |
| Immediately about the fossa is an area of young cartilage; the floor of the fossa
| |
| is also covered with young cartilage. The posterior part of the sulcus, which
| |
| borders the jugular foramen, contains the opening for the aqueductus cochloaris.
| |
| At this stage only a small plexus of veins pass through it, but later it contains the
| |
| perihTnphatic duct.
| |
| | |
| The canalicular part of the otic capsule constitutes the lateral half and forms
| |
| part of the outer wall of the chondrocranium. It is more or less oval in form
| |
| and flattened in a mcdio-latoral direction. It consists for the most part of the
| |
| thickened cartilaginous covering for the semicircular ducts. The locations of the latter are more or less apparent by the configuration of the cartilage. The
| |
| position of the superior semicircular duct is quite clearly indicated; it lies within
| |
| the prominent, rounded semicircular mass constituting the antero-dorsal portion
| |
| of the canalicular half of the capsule (figs. 1, 2, 3, 5 and 15). The deep fossa sub-
| |
| arcuata Ues at the center of its curve. The lateral semicircular canal produces a
| |
| distinct bulge on the lower part of the lateral surface of the capsule, while the
| |
| position of the posterior duct is indicated by the configuration of the posterior
| |
| portion of the canaUcular part. The cranial surface shows a well marked sulcus
| |
| for the endoljTnphatic duct which leads into the opening for the aqueductus
| |
| vestibuU. The upper edge of the cartilage, where it joins the mastoid, is deeply
| |
| grooved for the transverse sinus. The lower edge of this posterior portion projects
| |
| ventrally in the form of a ridge, the crista parotica, to the caudal end of which is
| |
| attached the styloid process. In front of the crista there is a slight depression, the
| |
| fossa incudis, in which is lodged the short crus of the incus. The facial nerve lies
| |
| in a groove just medial to the crista and styloid process (figs. 6, 12, and 13).
| |
| | |
| MASTOID CARTILAGE.
| |
| | |
| The mastoid cartilage is continuous with the caudal and upper border of the
| |
| canalicular part. We have already described its fusion with the exoccipital and
| |
| squama of the occipital. Its inner surface is hoUowed out for the great transverse
| |
| sinus, and in the upper part of this sulcus is the large mastoid foramen. A dis-
| |
| tinct mass of blastema or precartilage projects ventrally from the inferior edge of
| |
| the mastoid just back of the root of the styloid process. Since to this mass are
| |
| attached the digastric, splenius capitis, longissimus capitis, and sternomastoid
| |
| muscles, it probably represents the mastoid process. In addition to the above-
| |
| mentioned muscles the stapedius also appears to arise from the inner side of the
| |
| mass. At this stage the stapedius is relatively large and entirely extracapsular.
| |
| Two interesting rudimentary muscles, not found in the adult, are likewise attached
| |
| to this mastoid process. One, short and thick, arises from the occipital transverse
| |
| process; the other, longer and more slender, extends from the transverse process of
| |
| the atlas (figs. 14 and 15). I have named these muscles the occipito-masloid and the
| |
| atlanto-mastoid. The occipito-mastoid is serially related to the intertransversarii.
| |
| | |
| This mastoid blastema, or mastoid-process blastema, presents interesting
| |
| variations. In embryo No. 382 (Carnegie Collection), 20 mm. in length, there is
| |
| a small nodule of cartilage in this region — the mastoid-process cartilage. This is
| |
| attached to the occipital transverse process by a short ligament. Both the digas-
| |
| tric and stapedius muscles are attached to it, but no trace of the occipito-mastoid
| |
| or atlanto-mastoid muscles were found. In embryo No. 431 (Carnegie Collec-
| |
| tion), 19 mm. in length, a similar nodule of young cartilage was found attached
| |
| in the same manner to the transverse process of the occipital by a Ugament. The
| |
| stapedius muscle is attached to its inner surface and the digastric to its lower
| |
| surface. In both embryos the mastoid-process cartilage is closely associated with
| |
| the mastoid blastema and the Ugament or band of condensed mesenchj-me, connect-
| |
| ing it wdth the jugular process, occupies the same position as the occipito-mastoid
| |
| muscle found in embryo No. 460. The attachment of the stapedius and digastric suggests that the mastoid process may be regarded as a remnant of the proximal
| |
| end of a branchial cartilage, while its serial relation with the transverse process of
| |
| the occipital and cervical vertebrae, and its connection with the former by muscle,
| |
| as in embryo No. 460, or by ligament, as in embryos Nos. 431 and 382, suggest that
| |
| it may be a rudimentary^ portion of the transverse process of an occipital or temporal
| |
| vertebra. A still more speculative idea would be to consider the branchial bars as
| |
| serially related to the vertebral transverse process, and the occijiito-mastoid muscle
| |
| could be looked upon as serially related to the intertransversarii muscles on the one
| |
| hand and to the stapedius on the other.
| |
| | |
| THE ETHMOID CARTILAGE.
| |
| | |
| The ethmoid cartilage consists of the mesethmoid and the lateral nasal cap-
| |
| sules joined to the former by precartilage. The ethmoid is very poorly developed
| |
| at this stage and all the figures include more or less precartilage. The gradual
| |
| transition from cartilage to young cartilage and precartilage makes it impossible
| |
| to draw sharp Unes between the cartilage and precartilage. The mesethmoid
| |
| consists of fairly well-differentiated cartilage near its junction with the sphenoid.
| |
| In this region the pharyngeal edge is much thicker than the dorsal edge. It gradu-
| |
| ally tapers towards the apex, the entire septum attaining an even thickness. The
| |
| crista galli is relatively large when both cartilage and precartilage are included,
| |
| as shown in the various figures.
| |
| | |
| The nasal capsules consist mostly of young cartilage and precartilage. They
| |
| are relatively simple and show only slight indications of the turbinate processes.
| |
| Part of the lateral surface enters into the formation of the medial wall of the orbit
| |
| and part is covered by maxillary and nasal blastema, the precartilaginous tips
| |
| extending beyond (fig. 15). The inner wall of the orbit is completed by frontal
| |
| blastema and precartilage of the orbital wing of the sphenoid. This blastema
| |
| and precartilage, as already noted, form a continuous sheet (fig. 3). The orbital
| |
| surface of the sheet is more or less closely united to the upper edge of the nasal
| |
| capsule (fig. 15) . On the Une of junction between the cartilage and frontal blastema
| |
| are found, as in the adult, the anterior and posterior ethmoidal foramina, through
| |
| which pass the nasociliary nerve and the ethmoidal branches.
| |
| | |
| | |
| | |
| ==Bibliography==
| |
| | |
| | |
| | |
| Bakdeen, C. R., 1910. MorphogcncsLs of the skeletal system. Keibel and Mall, Manual of Human Embryology.
| |
| | |
| Fawcett, 1910. Notes on the development of the human sphenoid. Jour. Anat. :iii<l Phys., vol. ) 1, p. 207.
| |
| | |
| Fisher, E., 1903. Zur EntwicklunK.sne.seliii-hte des Affen.sehiidels. Zeit.f.Morph.u.Anthropol.,Bd.5.
| |
| | |
| Gladstone, 1915. Manife.station of occipital vertebra; and fusion of the atlas with the occipital. Jour. Anat. and Phj\s., vol. 49, p. 190.
| |
| | |
| Jacoby, M., 1895. Ein Beitrag zur Kcnntnis des menschlichen Primordialcraniums. Arch. f. mikr. Anat., Bd. 44.
| |
| | |
| KoLLMAN, J., 1905. Varianten am Os occipitale, besonders in der Umbegung des Foramen occipitale magnum. Anat. Anz., Bd. 27. Verb. d. anat. Gesellsch., p. 231-2.36.
| |
| | |
| Levi, G., 1900. Beitrag Zum Studium der Entwieklung des knorpeUgen Primordialcraniums des Menschen. Arch. f. mikr. Anat. u. Entwick, Bd. .55.
| |
| | |
| Lewis, VV. II., 1915. The use of guide planes and plaster of paris for reconstructions from serial sections, etc. Anat. Rec, vol. 9.
| |
| | |
| Macklin, C. C, 1914. The skull of a human fetus of 40 mm. Am. Jour. Anat., vol. 16.
| |
| | |
| Mead, C. S., 1909. The chondrocranium of an embryo pig, Sus scrofa. Am. Jour. Anat., vol. 9.
| |
| | |
| ScHULTZ, A. IL, 1917. Ein Paariger Knocken am Unterrand der Squama occipitalis. Anat. Rec., vol. 12.
| |
| | |
| Terry, R. J., 1917. The primordial cranium of the cat. Jour. Morph., vol. 29.
| |
| | |
| Van Noorden, W., 1887. Beitrag zur An.>»tomie der knorpeligen Schiidelbasis bci menschlichen Embryonen. .Arch. f. Anat. u. Physiol, Anat. Abth.
| |
| | |
| VoiT, Max, 1909. Das Primordialcranium des Kaninchens unter Berucksichtigimg der Deckknocken. Ein Beitrag zur MorphologiedesSiiugeticrschadels. Anat. Hefte, Bd. 38, Heft. 116.
| |
| | |
| ==Explanation of Figures==
| |
| | |
| :[[Book_-_Contributions_to_Embryology_Carnegie_Institution_No.39|'''Lewis Links''']]: [[:File:Lewis1920_Plate_1.jpg|Plate 1]] | [[:File:Lewis1920_Plate_2.jpg|Plate 2]] | [[:File:Lewis1920_Plate_3.jpg|Plate 3]] | [[:File:Lewis1920_Plate_4.jpg|Plate 4]] | [[:File:Lewis1920_Plate_5.jpg|Plate 5]] | [[Book_-_Contributions_to_Embryology_Carnegie_Institution_No.39|Contribution No.39]] | [[Book_-_Contributions_to_Embryology#Volume_IX|Volume IX]] | [[Book_-_Contributions_to_Embryology|Contributions to Embryology]]
| |
| | |
| <gallery>
| |
| File:Lewis1920_Plate_1.jpg|Plate 1
| |
| File:Lewis1920_Plate_2.jpg|Plate 2
| |
| File:Lewis1920_Plate_3.jpg|Plate 3
| |
| File:Lewis1920_Plate_4.jpg|Plate 4
| |
| File:Lewis1920_Plate_5.jpg|Plate 5
| |
| </gallery>
| |
| | |
| Dorsal aspect of base of the cartilaginous skull with the basioccijjital in the horizontal plane. The cervical vertebra; are included. At the anterior end some precartilage is included with the crista galli and the nasal capsule. X 10 diameters.
| |
| | |
| Right half of dorsal aspect of the base of the cartilaginous skull. The adult position, basioccipital inclined at an angle with the horizontal plane. X 10 diameters.
| |
| | |
| Dorsal aspect of cartilaginous and membranous skuU. Same view as shown in figure 1. X 10 diameters.
| |
| | |
| Dorsal aspect of base of the adult skull. Bone ossified in cartilage stained blue. Modified from Spalteholz Atlas.
| |
| | |
| Median sagittal aspect of the cartilaginous skuU. Part of crista galli and anterior end of raesethmoid are precartilage. XIO iliameters.
| |
| | |
| Ventral aspect of base of the cartilaginous skull. XIO diameters.
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| Lateral aspect of cartilaginous skull and cervical vertebrsE, with the brain and cervical cord and hypophysis in position. Merkel's cartilage, the styloid, hyoid, and laryngeal cartilages are also shown.
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| XIO diameters.
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| Lateral aspect of cartilaginous skuU and cervical vertebra; with the brain, cervical cord, and nerves. XIO diameters.
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| Lateral view of cartilaginous skull and cervical vertebrie with the overlying membranous skull and the dorsal membrane. X 10 diameters.
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| Dorsal aspect of sphenoid cartilage, showing attachment of the orbital muscles to the basal part of the orbital wing. X20 diameters.
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| Dorsal aspect of sphenoid cartilage. X20 diameters.
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| Lateral view of the right otic region. Part of the malleus and incus cut away showing course of facial nerve and position of otic ganglion. X20 diameters.
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| Lateral view of right otic region showing relations of facial nerve. X20 diameters.
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| Lateral view of base of cartilaginous skull with deeper muscles of occipital region and of the mouth and pharynx. X20 diameters.
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| Lateral view of part of cartilaginous and membranous skull. The inner wall of the orbit and part of occipito-temporal region exposed by cutting away part of the membranous skull. X20 diameters.
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| Dorsal view of temporal and occipital cartilages, showing the relation of the inner ear to the otic capsule. X20 diameters.
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| All of the figures except No. 4 are from models or combinations of models and graphic reconstructions of one embryo No. 460, Carnegie Collection.
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| Cartilage is colored blue, except figure 4, precartilage green, nerves yellow, muscles red; the blastema remains uncolored.
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| The illustrations are mainly the work of Mr. C. W. Shepard and Mr. J. F. Didusch.
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| 324
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| EXPLANATION OF FIGURES.
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| ABBREVIATIONS.
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| alar, lam.,
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| alar, pro.,
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| amp. lat. tl.,
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| amp. p. d.,
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| amp. s. (1.,
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| ansa, hy.,
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| ant. arch. I. C,
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| ant. nar.,
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| ant. rt.,
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| aq. vcs.,
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| atl. mas.,
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| atl. occ. art.,
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| basiocc,
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| bas. pt.,
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| biventcr,
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| can. pt.,
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| chr. pi.,
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| ch. ty.,
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| cLl. g.,
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| coc. d.,
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| coc. pt.,
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| com. d.,
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| oris, gal.,
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| digas.,
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| dor. mem.,
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| dor. sel.,
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| ed. bl.,
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| end. d.,
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| end. sul.,
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| epiph.,
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| eth. for.,
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| fac. for.,
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| [ac. sul.,
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| fen. ves.,
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| for. rot.,
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| fos. inc.,
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| fos. sub.,
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| fron. bl.,
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| gang. I. C,
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| gen. g.,
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| gen. glo.,
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| gen. hy.,
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| gr. sup. pet. n.
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| hy. for.,
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| hyo. glo.,
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| hyp. c.,
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| hyp.
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| inf. alv.,
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| inf. col.,
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| inf. orb.,
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| in. sp. lig.,
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| int. ac. m.,
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| Jac. cart.,
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| jug. for.,
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| jug. pro.,
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| jug. v.,
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| lat. rec,
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| lig. fla..
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| Ion. cjip.,
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| It. ma«.,
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| mand.,
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| mas. for.,
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| mas. pro..
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| alar laminsi
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| alar process.
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| ampulla lateral semicircular duct.
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| ampulla |>osterior semicircular duct.
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| ampulla superior semicircular duct.
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| ansa hypoglossi.
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| anterior arch atlas.
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| anterior nares.
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| anterior root occipital neural arch.
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| aqueduct vcstibuli.
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| atlanto-mastoid muscle.
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| atlanto-occipital articulation.
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| basioccipital.
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| basal part orbital wing sphenoid.
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| biventer ccrvicis muscle.
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| canalicular part otic capsule.
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| choroid plexus.
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| chordatympani.
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| ciliary ganglion.
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| cochlear duct.
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| cochlear part otic capsule.
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| common duct.
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| crista galli.
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| digastric muscle.
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| dorsal membrane.
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| dorsum sellae.
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| edge cranial blastema.
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| endolymphatic duct.
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| endolymphatic sulcus.
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| epiphysis.
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| ethmoid foramen.
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| facial foramen.
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| sulcus for facial nerve.
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| fenestra vestibuli.
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| foramen rotundum.
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| fossa incudis.
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| fossa subarcuata.
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| frontal bl;i.stema.
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| ganglion first cervical nerve.
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| geniculate ganglion.
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| genioglossus muscle.
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| geniohyoid muscle.
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| greater superficial petrosal nerve.
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| hypoglossal foramen.
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| hyoglossus muscle.
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| h>'pophyscal canal.
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| hypophysis.
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| inferior alveolar nerve.
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| inferior colliculus.
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| infraorbital nerve.
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| interspinous ligament.
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| internal acoustic meatus.
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| Jacobson's cartilage.
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| jugular foramen.
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| jugular process.
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| jugular vein.
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| lateral rectus muscle.
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| ligamenta subflava.
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| longLssimus capitis muscle.
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| lateral mass basioccipital.
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| mandible.
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| mastoid foramen.
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| mastoid process.
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| max..
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| maxillary blastema.
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| med. lac. for..
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| metlian lacerated foramen.
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| med. rt..
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| median root occipital neural arch.
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| Meek.,
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| Meckel's cartilage.
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| meseth..
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| mesethmoid.
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| nas. bl.,
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| iia-sal blastema.
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| nas. cap.,
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| nasal capsule.
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| neu. arch.
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| neural arch.
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| obi. cap. inf..
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| obliquus capitis inferior muscle.
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| obi. cap. sup..
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| obliquus capitis superior muscle.
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| occ. arch,
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| occipital neural arch.
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| occ. fis.,
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| occipital fissure.
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| occ. mas..
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| occipitomastoid muscle.
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| occ. con.,
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| occi|)ilal condyle.
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| olf. for..
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| olfactory foramen.
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| opt. for.,
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| optic foramen.
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| opt. n.,
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| optic nerve.
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| orb. wing,
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| orbital wing.
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| ot. cap.,
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| otic capsule.
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| otic gang.,
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| otic ganghon.
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| pal. n..
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| palatine nerve.
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| para, gl..
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| parathyroid gland.
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| post, rt..
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| posteiior root occipital neural arch.
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| rec. cap. ant..
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| rectus capitis anterior muscle.
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| rec. cap. lat.,
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| rectus capitis lateralis muscle.
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| rec. cap. post..
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| rectus capitis posterior muscle.
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| sel. tur..
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| sella turcica.
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| semispi. cap.,
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| semLspinalis capitis muscle.
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| sem. g..
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| semiluiKir ganglion.
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| squama,
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| squaiiKi occipital cartilage.
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| sph. pal. g.,
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| si>hcni)palatiiic ganglion.
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| sph. pal. n..
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| .sphcndpakitiiic nerve.
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| ster. thy..
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| sternothyroid muscle.
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| sty. glo..
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| styloglossus muscle.
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| sty. hy.,
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| stylohyoid muscle.
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| sty. ph.,
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| stylopharyngcus muscle.
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| submax. cap..
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| subina\illary gland capsule.
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| sup. can..
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| superior semicircular canal.
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| sup. ling.,
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| superior lingual muscle.
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| sup. orb. fis.,
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| suiierior orbital fissure.
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| sup. rec.,
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| superior rectus muscle.
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| temp, wing,
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| temporal wing.
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| thy. hy..
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| thjTohyoid muscle.
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| thym. gl.,
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| thymus gland.
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| thyr.,
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| thyroid cartilage.
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| thyr. gl..
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| thyroid gland.
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| tr. pro.,
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| transverse process atlas.
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| tr. sul.,
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| transverse sulcus.
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| zyg-.
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| zygoma blastema.
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| zyg. arc,
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| zygomatic arch.
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| zyg. pro.,
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| zygomatic process of frontal b'astema.
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| III.
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| III. cranial nerve.
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| IV.
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| IV. "
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| V.
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| V.
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| VI.
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| VI.
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| Vll.
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| VII. "
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| IX.
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| IX. "
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| x'S;}
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| X.I .. ..
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| XL/
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| XII.
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| XII. "
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| I. C. N.
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| I. cer\'ical "
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| II. C. N.
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| 11.
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| line between si)henoid and mesethmoid
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| {{Glossary}}
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| {{Footer}}
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| [[Category:Historic Embryology]] [[Category:Human]]
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