Book - Contributions to Embryology Carnegie Institution No.48
The skull of a human fetus of 43 millimeters greatest length
By Charles C. Macklin. (1921) 5 plates containing 47 figures
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|Embryo No. 460 is Carnegie stage 20. The following pages also relate to this topic.|
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Since the publication of my work on the skull of a human fetus of 40 mm. (Macklin, 1914) I have had the opportunity of studying and modeling a very satisfactory specimen of a somewhat younger stage. This is human fetus No. 886 of the collection of the Carnegie Institution of Washington. The sections were cut in paraffin in the frontal plane at a thickness of 100 micra and stained with alum cochineal. The technical work was all excellently done, and the series is practically perfect.
The models, of which there are 28, were made by the plaster-of-paris method of Lewis (1915), which reproduces the structures with almost absolute accuracy. The skull as a whole was first reconstructed at a magnification of 10 diameters and the details were then worked out in separate models, most of which were made at a magnification of 20 diameters. In the few instances where small details were lost, these were made good by making new models. Each model was very carefully checked by comparing each separate plate fine, as the model was being painted, with the bromide photographs of the sections. The models are, I believe, as nearly exact reproductions of the original structures as it is possible to obtain. The bones were modeled on the right side only.
The parietal and frontal bones were too delicate to be modeled except in outline, and in drawing their texture the method of profile reconstruction was employed. The same method was used to check up the general outline of the skull and to obtain the relation of the external form and of the brain to the skull.
The drawings were accurately made by Mr. J. F. Didusch. The method of geometric projection was used, which insures an accurate representation of the original.
The study of No. 886 gave an opportunity for comparison of this specimen with the skull of a 40 mm. fetus from Professor McMurrich's collection, known as "I" Toronto," and hereinafter referred to as "la," which I previously modeled. In la the measurement was crown-rump, and in No. 886 it was greatest length, and this accounts for the fact that No. 886 is considerably younger though of greater linear dimension. An opportunity was also afforded to compare the skull of No. 886 with that of No. 460 of the Carnegie collection, which has recently been modeled by Lewis (1920) and which shows the condition of the skull in a human fetus of 21 mm.
Young cartilage and precartilage, although not abundant at this stage, were included, and are mentioned where they occur.
Terms of direction in the following description are all related to the basal plate in a horizontal position. Accordingly there are, at times, at variance with the terms of orientation applying to the adult skull, particularly in the ethmoidal region, since this is usually described with the basal plate almost vertical. For the same reason the terms are often different from those used in the description of la.
Description of the Skull as a Whole
The skull of No. 886, although considerably younger than that of la, resembles it quite closely, so that I have not been put to the task of writing such a detailed account as would have been necessary had my former article not been available. I have endeavored to avoid a repetition of my former description, and to make this article largely a comparison of No. 886 with 1a.
Figure 1 presents the most favorable view from above. The basal plate is not exactly horizontal, the cranial end being a little closer to the eye of the observer than the caudal. The nasal capsules are seen from an oblique direction, and hence a true concept of their length is not obtained, as will be realized when the parts are regarded from the side.
By imagining the frontal and parietal bones as having been inserted on the left side, as well as on the right, and by filling the gaps between the elements of the outer border, it is seen that the contour of the cranial cavity, from this point of view, is oval, and a little wider anteriorly, in the region of the frontal bones, than posteriorly, at the widest part of the parietal plates and otic capsules. Posteriorly this contour is made up of the cartilaginous walls of the future posterior cranial fossa; anteriorly, however, it is composed of membrane and membrane bone. The chondrocranium is divided into two unequal portions in the region of the body of the sphenoid by the superior orbital fissure - an extension of the sphenoparietal fissure. Projecting into this fissure from the body of the sphenoid is the short temporal wing.
If this figure be compared with figure 1 of 1a we note obvious signs of advancement in the latter, perhaps the most marked being the more developed state of the anterior end of the skull. Lewis (1920) has commented upon the relatively more rapid growth which must take place in the prechordal, as contrasted with the chordal, portion of the skull, following the stage of 21 mm. which he studied, and others have expressed themselves similarly. From a comparison with No. 460, on the one hand, and with la on the other, it is very apparent that in No. 886 development of the anterior end of the skull is going on more rapidly than that of the posterior end.
Certain features of la, such as the sharp bending of the otic capsule, suggest that the specimen from which the model was made was laterally compressed and somewhat shrunken.
In figure 2 we have the most favorable aspect of the skull base. In it the basal plate is almost horizontal, the caudal end being a little closer to the eye of the observer. The view is thus not directly antipodal to figure 1, and hence the outer contours of the entire skull, and of the chondrocranium, are slightly different. The ethmoidal region is viewed from the anterior end, making impossible an adequate appreciation of its length and that of the fissura basalis, which separates the ectethmoid from the mesethmoid. The lower jaw has been entuely cut away.
A frank view of the face is seen in figure 3, which also includes partial views of the larynx and cervical vertebrae. The gaping mouth and orbits are conspicuous features.
The skull is so placed in figure 4 that the eye looks squarely into the foramen occipitale magnum. The vertebral column is tilted a httle so that its lower end is slightly nearer to the eye than the upper. The skull is not absolutely symmetrical and there is a slight deflection of the axis of the basal plate to the left, as the figure shows.
Side views of the skull are afforded by figures 5 and 6, the former including the membrane bones. The depth of the posterior cranial fossa is appreciated by looking at the dorsal portion of the chondrocranium. The hyoid, thyroid, cricoid, and four upper tracheal rings are seen. The contours of the external form, brain, frontal and parietal bones, and chondrocranium are seen from the right side in figure 7 in their normal relationship. This figure was made by profile reconstruction. The other figures show various details of the chondrocranium, and will be referred to in the description.
In general form the skull of No. 886 resembles closely the 28 mm. stage of Levi (1900), which seems to be a little farther advanced than the 30 mm. stage of Jacoby (1895).
The central stem of the chondrocranium is seen from the side in figure 10, with the cut surfaces of its adnexa indicated. Its chordal and prechordal limbs meet in the region of the hypophyseal fossa, where the stem appears to have been twisted through an angle of 90Â°, as well as strongly bent. The chordal limb represents the basal plate of the skull, while the prechordal limb contains the body of the sphenoid, with the interorbital and nasal septa. The lower contour lines of the two limbs, as seen in figure 10, meet at an angle of 115Â°, as in la. The prechordal limb in No. 460 is relatively shorter, and the corresponding angle, as measured by me on Lewis's figure 5, is 125Â°.
The basioccipital and basisphenoidal elements of the basal plate are united by continuity of cartilage at the undefined spheno-occipital commissure. Here the plate is thiimest, as figure 11, of the midsagittal section, shows. In this figure, too, it is apparent that the anterior end is much thicker than the posterior and that the upper surface presents a deep antero-posterior concavity, located in advance of the center.
At the junction of its anterior and middle thirds the plate is narrowed, due to the encroachment of the cochleae. The caudal end of the curved basicochlear commissure projects outward beyond the cranial. The cut edge of this union is seen on the basal plate in figure 10, and on the cochlea in figure 15. The dorsal and ventral basicochlear grooves are not so deep as those of 1a.
The dorsal surface of the basal plate (figure 1) is shallowly concave from side to side throughout almost its entire extent. Anteriorly, however, this concavity becomes very narrow and indistinct as the surface rises upon the basisphenoid, and disappears at the root of the dorsum sellae. The ventral surface presents a corresponding low transverse convexity, which falls away laterally into the ventral basicochlear grooves, flanked by the protruding cochlear portions of the otic capsules. The ventral suiiaces of the cochleae and of the basal plate he practically in the same level and thus combine to give to the ectal sm-face of the base of the skull a flattened appearance in contrast to the roomy concavity of the corresponding ental surface. Posteriorly, the diverging limbs pass uninterruptedly into the primordia of the exoccipitals.
The notochord was modeled in relief upon the midsagittal section of the basal plate and epistropheus and is illustrated in figure 11. Commencing below, it proceeds through the body of the epistropheus and dens, emerging from the apex of the latter. The cartilage of the dens projects a little farther forward than the point of exit of the chorda and upon this cartilaginous tip the chorda rests; it then springs across the very narrow gap between the dens and the adjoining caudal edge of the basioccipital and here shows some thickening. It now proceeds along the dorsal surface of the basioccipital, in the midline, for 900 micra, being buried in perichondrium. The anterior end of this part is compressed dorsoventrally. It then traverses the plate as shown in the figure, here perforating the anterior end of the beginning ossification center for the basioccipital.
Within the cartilage it is a very slender thread and at its point of exit it loses its continuity, there being here a break of something less than 100 micra. Shortly after emerging below the basal plate it comes into contact with the long, attenuated pharyngeal bursa and follows a course upon the dorsal edge of this. Especially at the extremities of the bm'sa it is contorted and varicose. Leaving the pharyngeal bursa, the chorda lies immediately above the epithelial roof of the pharynx for a considerable distance. It then turns sharply, as an attenuated and somewhat contorted cord, to approach the bodj' of the sphenoid, which it enters almost at right angles to the surface. Finally, it curves forward to terminate in the body of the sphenoid near its dorsal surface and some distance short of the crista transversa. This basisphenoidal portion of the chorda is a httle wider than that just outside the cartilage and its terminal end is somewhat nodular and irregular in duection.
In its course the notochord of No. 886 thus resembles closely that in Ruber's (1912) human embryo J, No. 47, 32 mm. long, shown in his figure 10.
The occipital region is a homogeneous mass of cartilage whose caudal boundary is formed by the margin of the primitive foramen magnum. The cranial boundary is marked by cartilaginous unions with the otic region and the foramina separating these. Proceeding from the front late ally and backward, we note the following commissures: spheno-occipital, basicochlear, capsule- occipital, and occipitoparietal. Behind the basicochlear commissure a section of the boundary is formed by the posterior margin of the jugular foramen. The lateral half of this, which is thin and forms the concave lip of the sigmoid sulcus, runs almost directly outward and so makes an angle with the medial thicker and more rounded half. It is at this angle that we find the anterior end of the jugular tubercle. The boundary is then continued as the capsulo-occipital commissure, which curves upward and forward around the canaHcular part of the otic capsule, to become directly continuous with the capsulo-parietal commissure. It is interrupted by the conspicuous capsulo-occipital foramen (figs. 5 and 6). Figure 14 shows the surface where the commissure has been severed.
The next section of the boundary is formed by the occipito-parietal commissure, which meets the preceding at an acute angle. It joins together the squama occipitalis and parietal plate. The latter bends inward a little upon the former to make the shallow occipito-parietal groove (fig. 14), which marks the position of the commissure upon the central surface. The sections show that the cartilage is very much thinner here, especially at the dorsal end, though not materially different in quality as compared with that above and below. As in 7a, there is no trace of a corresponding groove upon the ectal surface.
The ventral end of this groove is not very well defined, but may be placed just above the capsulo-occipital foramen. On the right side there is here a very small foramen. The dorsal hmit of the groove is marked by the conspicuous occipito-parietal notch which separates the pointed dorsomedial termination of the parietal plate from the underlying supraoccipital element (fig. 14). Between these extremities the groove pursues a course almost directly backward, upward, and inward.
The occipito-parietal groove has been noted by Kernan (1916) as early as the 20 mm. stage. In 1a it was well marked and presented two foramina upon the right side and one upon the left. These foramina are not represented in No. 886, nor are they noted in No. 460 or in Kernan's specimen. Furthermore, in 1a the occipito-parietal notch was considerably deeper and, indeed, terminated as a narrow slit. What appeared to be degenerating cartilage cells were found in la on the right side (the sections did not include the corresponding region on the left side), uniting the parietal plate with the squama in the region of this cleft. From this it would seem that there is a gradual disunion of these two plates in progress at the posterior end of the occipito-parietal commissure during this developmental period.
The groove for the endolymphatic sac, which Lewis mentions having found upon the "mastoscjuamal plate," corresponds to the anterior end of the occipito-parietal groove. In No. 886 the attenuated prolongation from the endolymphatic sac lies immediately medial to the anterior end of the groove, but separated from it by 0.5 mm. In 1a it was similarly situated. It is not impossible that this groove may be related historically to the endolymphatic sac, for it leads backward and inward to a point quite close to the processes ascendens; and in the lizard, as has been pointed out by Gaupp (1900) and others, the endolymphatic sac lies just lateral to the processus ascendens. The region between the otic capsule and the tectum posterius has undergone an enormous amount of extension in the mammals, and particularly in man, and the endolymphatic sac has accordingly become removed from its original position above the tectum posterius, because of its connection with the otic capsule.
Content to be added----
Explanation of Plates
All drawings were made by Mr. James F. Didusch according to geometric projection. With the exception of figure 7, which was made from a profile reconstruction, all figures were drawn from the original plaster-of-paris models made from human fetus No. 886 of the collection of the Carnegie Laboratory of Embryology. The number of the model from which each figure was drawn is given, together with the magnification.
Note - the magnifications refer to the original print versions, not the online images.
Note - the magnifications refer to the original print versions, not the online images.
Fig. 1. Chondrocranium from above with frontal and parietal bones on right side. The densest part of the frontal bone is inclosed by a dotted line. The basal plate is not quite horizontal, the cranial end being a little the closer to the eye of the observer. Model 1. X6.25.
Fig, 2. Chrondrocranium from below with cartilaginous branchial arch skeleton extirpated. Frontal and parietal bones are shown on right side. The basal plate is not quite horizontal, the caudal end being a httle the closer to the eye of the observer. The view is directly into the anterior nares. Model 1. X6.25.
Fig. 3. Skull from front, showing membrane bones on right side. Face is seen in frank view. The cervical vertebrae and cartilaginous branchial arch skeleton are also seen. Model 1. X6.25.
Fig. 4. Skull from back, giving a frank view of the foramen oocipitale magnum. The cervical vertebrtae are seen, their arches beiag as yet unclosed dorsally. Note the alignment of the hemiarch tips with the dorsal foraminal prominences, representing the extremities of the hemiarches of the occipital vertebra. The right half of the interparietal bone is seen. Model 1. X6.25.
Fig. 5. Skull from right side, showing membrane bones. The cervical vertebrae and cartilaginous branchial arch skeleton are included. Only the right half of the skull is shown. Model 1. X6.25.
Fig. 6. Left half of chondrocranium, cervical vertebrae, and cartilaginous branchial arch skeleton as seen from left side. Model 1. X6.25.
Fig. 7. Profiles of external form of head, brain and upper end of spinal cord, and skull, in their normal relation to one another, as seen from the right side. Drawn from a profile reconstruction. Xl-9.
Fig. 8. Condensed mesenchyme enveloping the vomer, seen from front, side, and above. The anterior extremities of the vomer are indicated. The gutter in the center is for the lower edge of the nasal septum. There is a slight amount of lateral curvature. The cut edges of the mesenchyme are indicated. Model 22. X12.5.
Fig. 9. Two halves of the vomer from the same point of view as that of figure 8. They are very slender spicules of bone lying along the lower border of the nasal septum. Model 21. X12.5.
Fig. 10. Median stem of skull as seen from right side. It consists of the basal plate behind and the interorbital and nasal septa in front, forming ai- "btuse angle at the body of the sphenoid. The adjoining exoccipital cartilage is shown in part. Junctions with cartilage lying laterally are shown. Model 4. X12.5.
Fig. 11. Right half of basal plate and parts of upper two cervical vertebrae, sectioned in the mid-sagittal plane. The cut surface is seen in frank view. Shows the preossification center for the basioccipital, the notochord, the pharyngeal bursa with a little of the epithelium of the roof of the pharynx, the temporal wing, dorsum seUae and a portion of the exoccipital. Model 8. X12.5.
Fig. 12. Left cartilage of Jacobson from left side with neighboring septum. Models 2 and 25. Xl2,5.
Fig. 13. CartUages of Jacobson from below in relation to nasal septum. Models 2 and 25. X12.5.
Fig. 14. Right half of occipital cartilage and parietal plate from in front and within, with the ascending process and right half of inter-parietal bone in its mesenchyme. Connections with adjoining cartilages are shown. Model 24. X6.25.
Fig. 15. Interior of right otic capsule from within and above. The inner wall of the capsule has been cut away to show the lateral wall of the cavity. Figure 21 shows this cavity modeled as a solid. The superior and posterior semicircular canals are shown, with the entrances to the lateral canal. A good view is afforded of the spiral septum in the wall of the cochlear space. The membrane fiUing in the vestibular window is indicated. Other features are the medial end of the tuba auditiva with its entrance into the pharynx, the alar process of the temporal wing of the sphenoid, the aUcochlear commissure, the internal carotid artery, and views of the fifth and seventh cranial nerves. Model 14. XlO.
Fig. 16. Right otic capsule and associated structures seen from right side, front, and a little below. The suprafacial commissure has been removed. The facial nerve is shown in its relation to the otic capsule and styloid process, with its off-shoots, the chorda tympani and the great superficial petrosal nerves, the latter arising from the geniculate gangUon. The tip of the long crus of the incus appears in relation to the chorda tympani. A prominent object is the tympanic cavity fundament, of which the lateral surface is shown, presenting an impression at the site of the future tympanic membrane. The auditory tube is shown in its full extent. The immense semilunar ganglion with its root and branches, the internal carotidartery, and the processus alaris of the temporal wing of the sphenoid are seen. Model 14. XlO.
Fig. 17. Right otic capsule, medial surface, frank view, showing connections with adjoining cartilages and openings toward the cranial cavity. Model 5. XlO.
Fig. I8. The space within the right otic capsule seen from within, modeled as a solid; the surface presented fits into the cavity shown in figure 23. Openings toward the cranial cavity are shown. Note the large volume of this cavity in comparison with that of the membranous labyrinth wliich fills it (fig. 19). Compare also figures 21 and 22 in this respect. Model 6. XlO.
Fig. 19. Membranous labyrinth of right otic capsule contained within the space shown in figure 18. Figures 17, 18, and 19 were all drawn from approximately the same point of view, so that an accurate idea may be gained of the space contained within the otic capsule and the membranous labyrinth within that. Model 7. XlO.
Fig. 20. Right otic capsule, lateral surface, frank view, with openings looking outward. The attachments of the cartilage of Reichert and of the short process of the incus are seen. Model 5. XlO.
Fig. 21. The space within the right otic capsule, seen from without, modeled as a sohd; the openings are indicated. Model 6. XlO.
Fig. 22. Membranous labyrinth of right otic capsule contained within the space shown in figure 21. Figures 20, 21, and 22 were all drawn from approximately the same point of view. Model 7. XlO.
Fig. 23. Medial wall of right otic capsule, seen from without, the lateral wall having been cut away. The cutting of the capsular wall was not done in quite the same way as in the model shown in figure 15, so that the cut edges do not fit together exactly. The inner wall of the space is seen with the endolymphatic and internal acoustic foramina. Model 26. XlO.
Fig. 24. View from front of mass of cartilage (massa angularis) partially inclosed by the semi- circular canals of the right otic capsule. Above is seen the superior canal leading into the space for its ampulla and, farther downward and to the right, into that for the utriculus. To the left is the space for the ampulla of the lateral canal and, farther back, the lateral semicircular canal appears. Model 15. XlO.
Fig 25. View of same mass of cartilage shown in figure 24, but seen from below. The lateral semicircular canal is conspicuous to the left, passing above into an enlargement for the ampulla of this canal and for the utriculus, with the beginning of the superior canal above. Below, the lateral canal passes medially into an erdargement for the inferior extremity of the posterior canal and for the crus commune. Model 15. XlO. Views of the angular mass from other aspects are seen in other figures, as from without in figures 20, 5, 6, and from within infigure 15.
Fig. 26. Right maxilla, palate, medial pterygoid plate with hamular process, temporal wing, sphenopalatine ganglion, and associated nerves, seen directly from within. Cut surface showing junction of temporal wing with alar process is seen. Note the incisive suture partially separating the maxilla and premaxilla. Model 11. X12.5.
Fig. 27. The same structures as those seen in figure 26 with the exception of the medial pterygoid plate, seen from above. The relations of the maxillary division of the trigeminal nerve and its branches to the different structures are shown. Model 11. X12.5.
Fig. 28. Lateral aspect of the right palate bone and medial pterygoid plate, with their investment of condensed mesenchyme. Model 18. X12.5.
Fig. 29. The same structures seen in figures 26 and 27, but viewed from below. The tooth gutter of the maxilla and premaxilla is conspicuous and the lower end of the incisive suture appears. The zygomatic bone also is shown. Compare with figure 30. Model 11. X12.5.
Fig. 30. Condensed mesenchyme enveloping the right maxilla, palate, and medial pterygoid plate. The developing teeth are seen in their gutter in the maxilla. (Compare with figure 29 drawn from approximately the same point of view.) The cartilaginous hamular process is seen projecting from the medial pterygoid lamina. Compare also with figure 36, forbdeveloping teeth of the right lower jaw. Model 25. X12.5.
Fig. 31. Frank view of the right mandible, Meckel's cartilage, and associated structures, seen from within. The cartilaginous precursors of the auditory ossicles are seen above, in relation to the facial and chorda tympani nerves. A ghmpse of the squama temporalis is given and also of the goniale. Note the relations of the mandibular division of the trigeminal nerve. Model 9. X12.5.
Fig. 32. Lateral aspect of lower end of the right Meckel's cartilage, showing especially the area applied closely to the mandible, where the cartilage is showing the changes preliminary to ossification. (Compare with figures 33 and 35.) Model 16. X12.5.
Fig. 33. Right mandible from same viewpoint as in figure 31, showing the tooth gutter and area of close apposition to the lower end of Meckel's cartilage. Model 10. X12.5.
Fig. 34. The same structures as those shown in figure 31, but the model was rotated somewhat medially around its long axis. While presenting all the structures from a new angle, it shows especially the relation of the nerves to the mandible. Model 9. X12.5.
Fig. 35. View of right mandible with the model in the same position as that shown in figure 34; it is rotated so as to afford a good view of the tooth gutter. Model 10. X12.5.
Fig. 36. Condensed mesenchyme around right mandible viewed from approximately the same point as that of figure 35. It shows the developing teeth of the right lower jaw. The mesenchyme is connected across the midhne with its partner of the opposite side, the cut edge being shown. Model 23. X12.5.
Fig. 37. Mucous membrane of inner wall of right nasal cavity, overlying the septum; it is cut away to show the right organ of Jacobson. (Compare with figure 10.) Model 20. X12.5.
Fig 38. Mucous membrane of lateral wall of right nasal cavity, overljing the right ectethmoid, showing folds for the developing conchce. The mucous membrane fits over the structures seen in figure 41. The elongated nasopharyngeal canal, flanked by the developing palate and medial pterygoid plate, is well seen. Model IS. X12.5.
Fig. 39. Lateral aspect of right ectethmoid from the front, side, and a little below, showing especially the nasolacrimal duct, with the nasolacrimal sac and the lacrimal ducts above, and, below,the expanded end, applied to but not perforating the external aspect of the mucous membrane of the inferior meatus. The tip of the paranasal cartilage hes just lateral to the duct and the tiny shred of osseous tissue representing the lacrimal bone is seen lying along the posterior maxillary process. The cupular process of precartilage is conspicuous in the lower part of the figure. The broad plate of epitheUum, which represents the future inferior meatus, but which has not yet undergone cleavage except posteriorly, is plainly shown. Model 18. X12.5.
Fig. 40. Anterior end of right ectethmoid, with the epithelial plug in the anterior naris, embraced medially by the cupular process of precartilage. The terminal portion of the nasolacrimal duct is shown, entering the space for the inferior meatus, with a small portion of the mesenchyme of the maxilla. Model 19. X12.5.
Fig. 41. Medial aspect of right ectethmoid, showing the developing concha. Precartilage is especially evident in the superior concha, the small process of the middle meatus, and the cupular process. The other conchce are edged with it. Themesenchyme envelopes of the maxilla, the palate, and the medial pterygoid plate are seen. The cartilaginous hamular process is conspicuous, as are also the developing teeth of the right side of the upper jaw. (Compare figure 30.)
Note also the tip of the nasolacrimal duct in the space for the inferior meatus. Model 25. X12.5.
Fig. 42. View of right hyoid arch from without, below, and behind. The connection with the otic capsule is seen above and below appears the lesser cornu of the hyoid cartilage. The relations of the facial nerve, chorda tympani, and tympanic cavity are well seen, and the handle of the malleus is plainly shown in a concavity representing the stratum mucosum of the future tympanic membrane. Model 17. X12.5.
Fig. 43 Medial aspect of left ectethmoid (compare with figure 41), showing developing conchse. The anterior portion of the tectum nasi has been trimmed a little farther laterally than on the right side, and hence the cut surface is not quite the same in the two figures. The cupular process is omitted. Model 3. X12.5.
Fig. 44. View of right hyoid arch from within and slightly above, with its membranous connection with the lesser cornu of the hyoid below (as in fig. 42) and the cut edge of its connection with otic capsule above. Fitting into the curvature of its upper portion is the epithelium of the developing tympanic cavity, which from this point of view is almost parallel with the plane of the paper. The ring-like stapes is seen and to it is attached the tendon of the stapedius muscle, with the muscle itself passing medial to the facial nerve and to the upper end of the styloid process. The handle of the malleus is also seen, with the chorda tympani lying just medial to it. Model 17. X12.5.
Fig. 45. Cartilages of the hyoid, thyroid, cricoid, and upper end of the trachea, seen from the front. On the left side of the model the precartilaginous edging of the upper three cartilages is shown. Model 12. X12.5.
Fig. 46. Right half of the same structures seen in figures 45 and 47, viewed from within. The precartilage of the arj'tenoid is seen and should be compared with the young cartilage of the same structure shown in figure 47. Model 13. X12.5.
Fig. 47. Same structures seen in figure 45, but viewed from behind. The asymmetry evident in the arytenoids and in other places is due to the fact that on the left side of the model the precartilage was shown, whereas on the right side only the cartilage and young cartilage appear. Model 12. xl2.5.
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