Paper - The skull of a human fetus of 40 mm 2
|Embryology - 19 Apr 2021 Expand to Translate|
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|Week 10 (GA week 12). There are 2 historic papers describing this fetal skull and later 1921 papers describing a slightly older fetal skull.
Macklin CC. Preliminary note on the skull of a human fetus of 43 mm greatest length. (1921) Anat. Rec. 22(4): 251-265.
Macklin CC. the skull of a human fetus of 43 millimeters greatest length. (1921) Contrib. Embryol., Carnegie Inst. Wash. Publ., 48, 10:59-102.
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The Skull of a Human Fetus of 40 mm II.
Charles Clifford Macklin
James H. Richardson Fellow in Anatomy, University of Toronto
- Part I of this paper was published in the July number of the Journal, vol. 16, no. 3.
The orbitotemporal region, in the present stage, is composed of the cartilage of the sphenoidal anlage and unites the otic region, behind, with the ethmoidal region in front. In it we recognize an unpaired, median portion, made up of that part of the central stem of the chondrocranium which contains the bend; this represents principally the cartilaginous body of the sphenoid, and is directly continuous dorsally with the otic portion of the planum basale and ventrally with the nasal septum of the ethmoidal region. In addition there are two paired, lateral parts, the forerunners of the greater and lesser wings. The internal pterygoid plate, which is laid down in membrane bone, will be considered in the section devoted to the discussion of the purely osseous elements.
If we examine, successively, the parts of the median portion, beginning dorsally, we note first a prominent transverse ridge, the crista transversa (fig. 5), which marks the boundary between the orbitotemporal and otic portions of the median stem. Directly continuous with this ridge, and springing upward from it, is the prominent dorsum sellae (figs. 1 and 3), here showing no median perforation, as it does in the rabbit (Voit), and it is owing to this circumstance that the upper edge of the crista is, in homo, entirely obliterated.
The dorsum sellae forms the conspicuous posterior wall of the hypophyseal fossa. Its upper and lateral corners are thickened and project upward, outward and forward in a horn-like manner, so that their ventral extremities overlie, to some extent, the hypophyseal fossa. These are the posterior clinoid processes. Their upper extremities rise higher than the cranial edge of the dorsum sellae between them, and hence the latter is concave cranially, as well as ventrally. The dorsal surface of the dorsum sellae is gently concave from side to side, and from above downward, passing uniformly over upon the surface of the otic portion of the planum basale below.
Histologically there is here, at this stage, no evidence of the primary separation of the chondrous anlagen of the dorsum sellae, such as Fawcett describes in earlier stages (19 and 21 mm.), except a slightly younger condition of the cartilage at the ventral side of the junction of the dorsum sellae and crista transversa.
In front of the dorsum sellae we come upon the wide, flattened floor of the hypophyseal fossa ([[:fig. 1), or sella turcica, which opens laterally into the side-parts of the middle cranial fossa, and is thus more correctly a short wide groove than a fossa. Ventrally the wall rises abruptly, - almost vertically, - to reach a transverse ridge, - the anlage of the tuberculum sellae. The lateral edge of the ventral wall presents, on the right side, but not on the left, a small, conical backwardly projecting middle clinoid process. The tuberculum sellae of my model is much more prominent than it is in that of Hertwig. It is interesting to note in passing that Levi is of the opinion that in the development of the human skull the sella turcica is the only part to retain its primitive position, the other parts moving cranially, and he finds in the 28 mm. stage, among other evidences of this, the appearance of the tuberculum sellae as the anterior wall of the sella turcica. There is no evidence of the tuberculum sellae in the 17 mm. stage, while in the 28 mm. stage the sella has a vertical ventral wall, as in my model.
The part of the central stem of the chondrocranium composing the floor and ventral wall of the sella turcica has been termed the Balkenplatte or lamina trabeculi. It is wide, and, when compared with the parts in front and behind it, quite thin. It shows no evidence of perforation. From the ventral half of each lateral edge of the floor there is seen to project, in a direction downward and outward, the short, rod-like processus alaris.
When the model is regarded from the side (fig. 3), the floor of the sella tm'cica, which is slightly concave cranially, appears as the upper expanded end of the planum basale, the ventral wall of the sella appearing as a continuation upward of the ventral surface of the planum; indeed if the ventral border of the latter (which we have noted is almost straight) is projected upward it will pass just ventral to, and parallel with, the ventral wall aforementioned. From the same position, too, the ventral wall of the sella appears as the dorsal expanded end of the cranio-ventral or horizontal limb of the central stem of the chondrocranium, and the appearance is as if the caudo-dorsal edge of this had been applied to the cranio-ventral edge of the vertical limb, making an angle of 115°, open caudo-ventrally, the flattened extremities forming the ventral wall and floor of the sella turcica respectively. Ventrally (fig. 2) this angle is seen to be quite sharply marked, and to lie on the line between the ventral edges of the roots of the processus alares, or, just below the level of the ventral wall of the sella. Ventral to this angle the cartilage of the central stem gradually becomes narrower from side to side, and expanded caudo-cranially, passing ventrally into the nasal septum (fig. 10).
If we now turn our attention again to the upper surface of the median stem, we pass forward from the sella turcica, over the rounded, transverse, tuberculum sellae, and come upon the flattened lamina hypochiasmatica (fig. 1) (sulcus chiasmaticus of Levi), triangular in shape, with the apex placed ventrally in the midline, and the base formed by the upper edge of the ventral wall of the sella turcica. This surface underlies the optic chiasma, and is horizontal, thus making almost a right angle with the ventral wall of the sella. Ventrally the apex rises upon the dorsal edge of the interorbital septum, forming therewith an angle of 120°. Immediately lateral to this junction is a small, slit-like foramen, which may be known as the foramen praechiasmaticum (figs. 1, 10 and 14); it has been shown in the models of the skulls of several mammals, as the ape (Fischer) and the rabbit (Voit), as well as man (Hertwig). It contains nothing but loose connective tissue. It does not appear in Jacoby's figure, but the optic foramen extends all the way to the interorbital septum, and one may assume from this that the small isthmus of cartilage, which cuts off this small aperture from the optic foramen, and which may be known as the commissura praechiasmatica (figs. 1, 10, 14), has been developed between the 30 and 40 mm.. stages. In my sections this commissure shows a rather younger condition of cartilage than that found in the surrounding chondrocranium. The foramen praechiasmaticum evidently disappears later, as it is not to be found in the osseous condition. At the dorsal extremity of the optic foramen the lamina hypochiasmatica is seen to pass over upon the dorsal root of the ala orbitalis.
A feature which I have not noticed in the description of human primitive skulls, but which is described by Voit in the skull of lepus, as the ala hypochiasmatica, is a small but strong crescentic ridge which projects antero-laterally from the surface of the lamina hypochiasmatica just ventral to the origin of the dorsal root of the ala orbitalis, and which is continuous dorsally with this root. It appears in figure 10 and may be seen from above as a projection into the optic foramen (fig. 1). It presents a convex ventral edge, and is separated from the surface of the interorbital septum, lying within, by a distinct furrow containing only connective tissue. In the Voit model of the skull of rabbit this shows beginning ossification, but such is not the case in my model, in which the ala presents a somewhat younger type of cartilage, especially in the ventral edge, when compared with that of the adjacent cartilage.
Ventrally, as we have seen, the lamina hypochiasmatica is continuous, medially, with the dorsal border of the interorbital septum. From the sides of this septum the ventral roots of the alae orbitales are seen to spring (fig. 1), and its cranialmost edge, delimiting the orbitonasal fissure medially, passes over directly upon the nasal septum.
The interorbital septum (figs. 10, 11, 14 and 15) is narrow cranially, being delimited dorsally by the roots of the alae orbitales and ventrally by the dorsal extremities of the side-walls of the etlimoidal region (fig. 11). It is thicker below than above, its lower surface being keel-like, and passing horizontally forward upon the lower edge of the nasal septum; the upper part becoming increasingly thinner as we follow it ventrally, to coalesce with the upper portion of the same structure. It represents a transition from the flattened Balkenplatte to the thin nasal septum, in which the median stem appears to undergo a torsion of 90Â°, and forms the median delimitation of the narrow inner recess of the orbit.
The interorbital septum of man is homologous with the structure bearing the same name found in the Saurians, but, as is the case in the other mammals, it is rudimentary when compared with these lower forms. A theory which seeks to account for the shortness of this septum when compared with the condition in the lower forms is that, in the mammals, the nasal side-wall has gradually grown backwards, to encroach more and more upon the territory of the interorbital septum, to the advantage of the nasal septmn (Mead).
If we start at the floor of the sella turcica (fig. 1), and proceed upward and forward we come upon three successive steps, at about equal intervals apart, formed, as we have seen, by the lamina hypochiasmatica, the uppermost edge of the interorbital septum, and finally, in the most cranial, which reaches the highest point of the chondrocranium, by the crista galli of the mesethmoid (fig. 11).
The orbital wings (figs. 1 and 10) are the cranio-ventral and the larger of the paired lateral extensions of the sphenoidal anlage. Each wing has the form of an imperfectly defined, triangular plate, with the irregular base parallel with the median plane, and the apex lateral and turned dorsally. The plate is gently concave downward, forming the roof of the orbit; above it takes part in the formation of the floor of the anterior cranial fossa. Of the unions, two are with the central stem of the chondrocranium, the dorsal and ventral roots, and one which is elongated and broken by several foramina is with the sidewall of the ethmoidal region. The two wings lie almost horizontally, and thus differ from the earlier conditions, where they are considerably higher laterally than medially (Jacoby).
The dorsal root is the smaller, and is directed outward and slightly upward. Thick and rounded in section, its cartilage is directly continuous with that of the lamina hypochiasmatica, from the side of which it has been seen to project. A short distance from its origin it widens out, and at the same time rises cranially to form a plate, flattened from above downward, outward and slightly forward, the dorsal edge of which presents, near the median line, a backward projection, the anterior clinoid process (fig. 3). Between this process and the edge of the Balkenplatte, rendered more sharply defined on the right side by the middle clinoid process, there is to be seen a distinct notch, which lies dorsal to the root, opposite the most posterior extremity of the optic foramen, and conveys the internal carotid artery. The dorsal border of the wing, after leaving the anterior clinoid process, passes upward, forward and outward, making a dorsolateral convexity, and finally turns abruptly backward upon the dorso-lateral process. The ventral border of the root forms the dorsal and part of the lateral border of the foramen opticum, while its lower surface, together with that of the wing lateral to it, forms the cranial delimitation of the superior orbital fissure.
The ventral root is wide and flat, and is directed outward and slightly backward. It is considerably the longer, as well as the broader, and is directly attached to the dorsal border of the interorbital septum along the line indicated in figure 11. In addition it is connected, through the praechiasmatic commissure, with the ventro-lateral edge of the hypochiasmatic lamina (fig. 1). The ventral root occupies a somewhat higher level than the dorsal. The dorsal border curves outward and backward, to assist in the delimitation of the foramen opticum; the ventral border passes almost directly outward, and forms the dorsal border of the orbitonasal fissure. Through the portion of the wing lying lateral to the optic foramen it is continuous with the dorsal root; and in this way is formed the shelving side of a recess, the floor of which is made up of the hypochiasmatic lamina, in which is found, among other structures, the optic chiasma.
The optic foramen is pear-shaped in outhne, with the narrow end ventro-medial. The lateral border is somewhat higher than the medial, and is downwardly and inwardly concave. The optic nerve and ophthalmic artery may be seen to pass through it, the former overlying the latter.
The ventralmost portion of the orbital wing has been known by the name of cartilago sphenoethmoidalis. This is a triangular plate of cartilage, somewhat thinner than that composing the remainder of the wing, and showing ventrally a connection with the superior prominence of the lateral nasal cartilage through the sphenoethmoidal commissure (fig. 1), and dorsal to this connections at several points with the lateral nasal cartilage as far back as the orbitonasal fissure, the bonds of attachment being broken by intervening foramina. Thus there is formed here a secondary, lateral cribriform plate, similar to that shown in the model of Hertwig, which leads from the anterior cranial fossa, not iato the nasal capsule, but into the orbit. The dorsal margin is irregular and forms the outer half of the ventral margin of the orbitonasal fissure, the medial half being formed by the dorsal surface of the ectethmoid.
The ventral border of the orbitonasal fissure is somewhat lower than the dorsal. The fissure is elongated, its long dimension being directed laterally, and in this it differs from the condition shown in the skull of lepus (Voit) , where it is directed ventro-laterally. In the latter animal, too, there are no connections with the ectetlimoid dorsal to the sphenoethmoidal commissure. As in the skull of lepus (Voit) so in man, the fissure is principally filled with connective tissue, but in its ventral region it transmits the anterior ethmoidal nerve and vessels from the orbit to the anterior cranial fossa. It is thus, in this region, representative of the anterior ethmoidal foramen of the adult, skull.
The lateral border of the ala orbitalis is serrated, and passes directly backward and outward to terminate in the hornlike, dorso-lateral process (fig. 1). It overlies, except at its tip, the median edge of the orbital plate of the frontal bone (fig. 2). The dorso-lateral process marks the lateral extremity of the ala, and also represents the ventral rudiment of the primitive taenia marginalis of this region, which in the lower forms stretches over the foramen sphenoparietale to make a connection with the parietal plate, as has been already noted.
The ala temporalis (fig. 10) is the smaller of the paired lateral appendages of the median portion of the sphenoidal anlage. It lies lateral to the Balkenplatte, and, for the most part, in front of and below the level of the floor of the sella turcica (fig. 3). As H. Fuchs remarks one must distinguish in the temporal wing of mammals two portions; a medial, sloping steeply downward and outward, and a lateral, ascending part. The medial portion, or processus alaris, is a short, straight, rodlike mass of cartilage, directly continuous with the outer edge of the Balkenplatte, from which, as we have seen, it projects downward, outward and slightly backward. Its dorsal surface comes into close contact with the cranio-ventral pole of the pars cochlearis, but is not connected therewith by a cartilaginous bridge (commissura alicochlearis) , as is the case in the Jacoby model, and in the models of several of the lower mammals. What I regard as a remnant of this bridge is, however, to be found in the supracochlear cartilage, which has already been described. Levi was unable to find any trace of this commissure in his specimens.
It may be noted in passing that Jacoby states that this bridge extends from the lateral edge of the sella turcica to the anterior edge of the ear capsule, but he has evidently included in the median portion of this the structure which other authors refer to as the processus alaris. Jacoby states that the ala temporalis springs outward from this bridge; perhaps a better way of stating it would be to say that the ala temporalis, through the processus alaris, springs from the edge of the floor of the hypophyseal fossa, and that the bridge, or commissura alicochleaiis, connects the ala with the anterior surface of the pars cochlearis of the otic capsule. By the disappearance of the commissura alicochlearis the carotid foramen is left open laterally. It is quite small, being delimited ventrally by a small notch between the root of the alar process and the Balkenplatte, and dorsally by the ventral basicochlear groove, which passes backward into the sphenocochlear notch.
The ventral surface of the alar process is almost completely taken up with the attachment of the lateral portion of the wing. The rounded outer extremity projects freely into the surrounding mesenchyme (fig. 1).
The larger lateral portion of the temporal wing lies immediately below the dorsal portion of the orbital wing, from which it is separated by the superior orbital fissure, now open laterally (fig. 4). It is rhomboidal in shape, the long axis being directed upward, outward and slightly forward, towards the lateral extremity of the ala orbitalis. The central portion of its mass is perforated, from before backward, by the large foramen rotundum, which transmits the second branch of the trigeminal nerve. The nerve, however, by no means fills the foramen, the greater portion of the space within it being occupied by connective tissue. The foramen rotundum is found in Levi's 28 mm. stage, but not in Jacoby's 30 mm. stage, the second branch of the fifth nerve here occupying a groove upon the upper surface of the wing. It is well shown in the illustrations of Fischer's ('03) ape skulls.
Immediately below the foramen rotundum the dorsal surface is concerned in the union with the ventral surface of the alar process, the long axes of the medial and lateral portions crossing at a right angle, open above. When regarded from above the effect is as if the caudo-medial corner of the lateral portion had been applied to the ventral surface of the medial portion in such a way as to leave the lower extremities of both free. The foramen ovale is not as yet formed.
Histologically there is to be seen at the junction of the alar process and lateral portion a sheet of younger cartilage and procartilage cells, which almost completely separates the two portions. This is evidently the last trace of the primitive separation of these parts, of which Levi, Fawcett and other authors speak. In the 14 mm. stage Levi finds the temporal wing represented by two procartilaginous anlagen, separated by a sheet of connective tissue.
The portion of the wing above the foramen rotundum is known as the lamina ascendens, and ends laterally in a somewhat sharp angle, which projects freely outward and forward. The uppermost angle, rather more blunt, is to be seen almost immediately under the lateral edge of the optic foramen. The dorsal surface of the lamina ascendens is convex, and terminates dorsally in a ridge, bordering the foramen rotundum laterally.
The lowest portion of the wing is marked by a blunt angle, lying below and a little medial to the foramen rotundum, and representing the processus pterygoideus. The innermost extremity of the wing presents a more sharply marked angle, which projects freely inward, where it comes into close contact with the parasphenoid bone, or internal pterygoid plate, the upper extremity of which lies immediately ventro-medial to it. Medially and caudally this angle is separated from the alar process by a well-marked groove, - the deepest part of the circular groove which surrounds the union of the alar process and the lateral portion.
The outer margin of the lateral portion projects farther forward than the medial (fig. 14) so that its ventral face looks inward as well as forward. There is no trace of a lamina pterygoidea, perforated by the internal maxillary artery, such asVoit describes in the skull of the rabbit.
Histologically, modification of the cartilage cells in the upper lateral portion of the wing indicates beginning endochondral ossification; Mall found the first trace of the alisphenoid bone in an embryo of 58 days. Medial to the foramen rotundum the cartilage is thinner and of younger character than that elsewhere in the lateral portion, indicating that this was the part which was, perhaps, latest to form.
Gaupp has pointed out that the brain-case of the mammals has, in the orbitotemporal region, been enlarged by the inclusion of a space which, in the lizards, lies below the primitive side-wall of this region, and to which he has given the name 'cavum epiptericum.' This space is the ventral continuation of the cavity which has been described by Voit in lepus as the cavum supracochleare. Voit has found in an early rabbit embryo evidences of the medial attachment of the primitive side-wall of the orbitotemporal region in the form of three rudimentar}^ cartilages, and membrane connecting them, and stretching out from them. From behind forward he finds, first, Restknorpel a, surmounting the commissura suprafacialis (continuous with this on one side but not on the othfer), next Restknorpel b, overlying the semilunar ganglion, and connected by a sheet of connective tissue, which overlies the abducens nerve, with the pillar of the dorsum sellae. Restknorpel b is quite free. A third rudimentary cartilage, Restknorpel c, appears only on the left side of Voit's youngest rabbit skull, and this fact appears to point to its transient nature. It presents itself at about the site apparently of the middle clinoid process, and is attached directly to the side of the Balkenplatte. Between Restknorpeln b and c the boundary betweeji the primitive cranial cavity and the cavum epiptericum is not clearly marked. From these three Restknorpeln, and from the cartilage intervening, anchorage is afforded for a stout sheet of connective tissue which stretches outward and upward to find its cranial attachment in the lower edge of the taenia marginalis, or, as Voit calls it in this region, the commissura orbitoparietalis. Underlying this membrane, which Voit thinks is, to some extent, the precursor of the dura mater of this region, are several important structures which are prunarily outside of the primitive brain case, as in the lizards, but are later taken into the brain case of the mammals; among these have already been mentioned the semilunar ganglion and part of its nerve trunks, part of the facial nerve, the geniculate ganglion and part of the great superficial petrosal nerve. In addition might be mentioned the nerves to the eye muscles, and the internal carotid artery - indeed all the structures in the cavernous sinus. The carotid artery is shown in the youngest stage of Voit winding around the caudo-ventral surface of Restknorpel c to enter the primitive cranium, this point marking its original inlet.
From the researches of Voit it would appear that the new floor and side-wall of the cavum epiptericum and the cavum supracochleare are formed by the upper part of the ala temporals the tegmen tympani and its forward extension, and the commissura ahcochlearis, together with the osseous elements formed by the parietal, and squamous portion of the temporal, bones. These completely shut off this space, except for the foramina.
In my embryo there do not appear to be any of the nodules corresponding to the three Uestknorpeln of Voit. The space corresponding to the above cavities is filled with loose connective tissue containing vessels and nerves. The dura has not condensed, is loose, and spreads outward and upward from the level of the upper part of the pituitary body. Thus the primitive cranial floor and side-wall of this region is not represented in my model. There is, however, what may be a rudiment of the cartilaginous secondary cranial floor - the floor of the cavum epiptericum, viz., the small nodule which T have called the cartilago supracochleare. Voit considers the ahcochlearis a part of this floor, which dorsally is directly continuous with the planum supracochleare; this being so it follows that, if the cartilago supracochleare be regarded as a rudiment of the commissura ahcochlearis it is then a rudiment of the cartilaginous floor of the cavum epiptericum, or, more accurately, the cavum supracochleare.
The ethmoidal region is the most ventral of the primary divisions of the chondrocranium. With the orbitotemporal region it is directly continuous at three points, as we have seen; medially the septum nasi passes directly backward into the septum interorbitale, and laterally the upper portion of the lateral wall of the ectethmoid is united, on each side, with the sphenoethmoidal cartilage through the sphenoethmoidal commissure and the line of bridges of cartilage dorsal to the latter.
Architecturally considered the principal elements entering into the construction of the ethmoidal region are those going to form the nasal capsule, but this region also includes certain accessory cartilages, of which there are, in relation to the septum the anterior and superior paraseptals; in relation to the lateral wall (intracapsular) the cartilago meatus medii, and (extracapsular) the paraethmoidal and paranasal cartilages.
The nasal capsule resembles, roughly, a tent, partitioned into equal, paired, lateral rooms or cavities by the septum, or mesethmoid, which lies in the median sagittal plane of the skull. The highest point, or peak, is marked by the crista galli. The roof and sides are formed by the tectum nasi and paries nasi respectively, the posterior or subcerebral portion of the former being broken by the long, paired, irregularly-contoured fenestrae cribrosae (fig. 1), destined to become the cribriform plate. This portion of the superior surface of the capsule takes part in the formation of the median part of the anterior cranial fossa, and represents the anlage of the upper surface of the ethmoid bone.
The incomplete floor (fig. 2), or solum nasi, is formed laterally by the inwardly-turned lower edges of the side-walls, and medially by the lower border of the septum and the anterior paraseptal cartilages. Between the side-wall and the septum is the gaping and elongated basal fissure, which extends ventrally into the incisura narina and dorsally into the cupuloseptal fissure, the latter being a very narrow space between the dorsalmost extremities of the nasal septum and the nasal wall, almost completely filled by perichondrium. The floor is almost entirely covered in by membrane bones - the maxilla, the palatine and the vomer - these closing off the inferior nasal meatus below, and marking the upper delimitation of the oral region. The three elements, tectum, paries and solum nasi, combine to form the shell-like structure known as the ectethmoid. Laterally the dorsal portion of this is in relation to the orbit.
The nasal septum or mesethmoid (fig. 11) is a vertical and roughly pentagonal plate of cartilage, constituting the ventral end of the central stem of the chondrocranium. The dorsal border is marked above by the interrupted line of attachment of the dorsal surface of the tectum nasi, which separates the surface of the nasal from that of the interorbital septum; below this these surfaces have no definite delimitations. The cranio-dorsal border, after passing over from that of the interorbital septum, runs horizontally forward for a short distance, and then mounts rapidly and evenly to reach the highest point of the chondrocranium in the conspicuous crista galli. The latter marks the widest part of the septum. The outer edges of this border, except in the upper portion of the crista galli, show short lateral eminences, which project into the fenestra cribrosa. These projections will later coalesce with others from the upper edge of the side-wall, and thus form the dorsal portion of the cribriform plate. In front the crista galli passes over upon the cranioventral border, which is straight, and sharply inclined downward, while laterally it is directly continuous throughout with the ventral portion of the tectum nasi. By reason of the fact that the tectum rises somewhat, as it springs from the septum, the upper border of the latter lies here at the bottom of a shallow furrow, the sulcus supraseptalis (fig. 1), which reaches from the crista to the ventral tip of the capsule.
The ventral border of the septum is straight and free, and marks the anterior extremity of the mesethmoid, forming the medial limit of the incisura narina, the representative of the fenestra narina of some of the lower forms. At its lower end it meets the caudal border at an angle of 113°. The caudal border is almost straight, horizontal, and thickened throughout, but much more so dorsally than ventrally, so that it resembles a cone, this similarity being rendered more striking by the fact that the transition to the thin part of the septum above is quite abrupt, thus resulting in the formation, on each side, of a shallow furrow (figs. 14 to 18). Near the ventral extremity the caudal border shows on the right side, but not on the left, a lateral connection with the cartilages of Jacobson (fig. 11), and, in front of this, bilateral unions with the ventro-lateral processes (Fawcett '11), the latter appearing immediately behind the front end. Projecting backward from the posterior of these attachments, lying parallel with and below the caudal border, in a position corresponding to about the middle of its ventral half, may be seen the cartilages of Jacobson, or the anterior paraseptal cartilages (figs. 2 and 18). Behind these the dorsal portion of the border, in its caudo-lateral aspect, is covered by the thin plates of the vomer (fig. 2).
The surfaces of the septum are for the most part smooth, but in the region below the crista galli there is a deflection to the left; on the right side this appears as a well-marked furrow, running from above downward and forward (fig. 11). In the area just above the anterior paraseptal cartilage the furrow lying above the lower, thickened border is somewhat deepened, and in this hollow is to be found the anlage of the organ of Jacobson (figs. 11 and 18).
An interesting featm'e of the mesethmoid is found about the middle of its dorsal half (the part corresponding to the future lamina perpendicularis of the ethmoid bone), in the form of two paired, elongated cartilages, very small in size, which lie parallel with, and close to, the surface. These may be termed the superior paraseptal cartilages (figs. 11 and 16), and though mature, their cartilage is younger in type than that composing the adjacent septum. The cartilage on the right is somewhat the longer, measuring 13 mm., the left being 11 mm., and in direction the right is parallel with the caudal border of the septum, while the left runs slightly upward, as w^ll as backward. The nasal septum, opposite the anterior part of each cartilage, and for a short distance in front of it, shows a slight swelling, and the anterior part of the paraseptal cartilage lies just lateral to and somewhat below this. So closely does the paraseptal cartilage lie to the nasal septum that it is difficult to make out a separation, but by the aid of the high power and close examination it is seen that the cartilage is separate from the septum, except at two points on the right side, and one on the left. On the right side the connection points are at the anterior extremity, and about the middle, while on the left the sole union is at the anterior end. Thus each cartilage presents a free, posteriorly projecting extremity. From the caudal edges of each cartilage there stretches downward a sheet of young connective tissue. Figure 16 shows the relationships of the superior paraseptal cartilage to this sheet, and it will be seen that, where the cartilage is free from union with the septum, as it is on each side in this figure, the relationship of the cartilage to the membrane is somewhat similar to that of a sesamoid bone to its tendon, for the real upper connection of the membrane appears to be somewhat above the cartilages, where it becomes continuous with the perichondrium of the septum, as shown in this figure. The two sheets unite below the septum, and in this region, which marks their thickest part, they contain the slender spicules of membrane bone which represent the vomer. Though they are found throughout the extent of the vomer they disappear shortly beyond its extremities.
Fawcett ('11) in his description of the paraseptal cartilages finds a similar sheet of condensed mesenchyme, and gives to it the name "suspensory membrane", stating that it envelops the anterior and posterior paraseptal cartilages, and extends between them; further that in the interval between these cartilages the vomer is developed (following Zuckerkandl '08). I am unable to discover in my model any trace of this "suspensory membrane" in front of the ventral extremity of the vomer, so that it has, obviously, nothing to do in this stage with the support of the anterior paraseptal cartilages; moreover it is not found behind the dorsal extremity of the vomer, and hence cannot function in the suspension of the processus cupularis posterior (posterior paraseptal cartilage of Fawcett) which evidently represents the last rudiment of the posterior transverse lamina of such forms as the rabbit (Voit). Furthermore, since the cellular composition of this membrane is apparently the same as that which forms the membranous anlage of any of the membrane bones, and its situation is that which will be occupied by the future upwardgrowing vomer, and since the vomer is to be found within its thickened caudal portion, it would appear that it is simply the membranous anlage of the vomer. The term 'suspensory' would seem to be misapplied, since the bony elements enclosed by it cannot be said to be suspended, any more than the early osseous spicules of any other membrane bone may be said to be suspended in their membranous anlagen, and the cartilaginous elements are not enclosed by it; indeed the so-called posterior paraseptal cartilages, as Fawcett himself states, are continuous with the lateral walls of the nasal capsule (fig. 14). Fawcett evidently believes that this membrane once sustained the cartilago paraseptalis communis, of such forms as the rabbit, and that the vomer is a covering bone which surrounds and takes the place of this cartilage when it disappears, although the vomer is present along with the cartilago paraseptalis communis in the skull of the rabbit (Voit). It appears evident from a study of my slides that the vomer attains its adult condition by advancing upward in this sheet of mesenchyme, and thus comes to enclose the septum. What part, if any, the superior paraseptal cartilages play in the development of the vomer, or indeed what their real significance is, I am unable to say.
At the ventral end of the caudal border of the mesethmoid, and lying almost parallel with it, there are to be seen, upon either side, the small, straight, rod-like ventro-lateral processes (figs. 2, 3 and 18), 9 mm. in length in the model, connected by their ventral extremities with the septum (fig. 11), but having their dorsal ends free, the greater part of their length being separated from the septum by perichondrium. The condensed mesenchyme of the ventral tip of the maxilla appears immediately beneath them (figs. 2 and 18), and their material is cartilage of the same character as that of the adjacent septum. Though their dorsal extremities come into close contact with the cranio-ventral process of the median Jacobsonian cartilage, they are not connected therewith.
The Jacobsonian or anterior paraseptal cartilages (figs. 2, 3 and 18) consist of two paired masses, medial and lateral, found immediately dorsal to the ventro-lateral processes. The medial mass is a quadrangular, inwardly concave plate, 28 mm. long, whose long axis is parallel with the lower border of the mesethmoid, with which its upper edge is in close apposition, being, for the most part, only separated by perichondrium. It is much the larger of the two, and lies at a lower level than the lateral mass. The ventral extremity of the plate is drawn out to a rather sharp free point, known as the ventral process (fig. 2), its tip lying just below the septum. The lower border is marked, rather nearer the ventral than the dorsal extremity, by a projection, directed downward, which, however, is terminated by a sharp, backwardly turned point of cartilage, this structure being known as the caudal process (figs. 2 and 18). The dorsal termination is very blunt; it may be described as the dorsal border. The cranial border, almost parallel with the caudal border of the mesethmoid, presents near the ventral process a long, slender limb 1 cm. in length, which curves upward and forward, to come into close contact with the dorsal tip of the ventrolateral process, and thus to reach the most ventral extremity of this cartilaginous mass. On the left side this off-shoot, which may be known as the cranio-ventral process (figs. 2 and 18), is disconnected from the septum, though only separated therefrom by a thin sheet of perichondrium, but on the right side a connection to the septum appears near the ventral end of the process. This connection, however, is very meager, the surrounding perichondrium almost cutting it off, and it would seem to be secondary. The cartilage of which the plate is composed is similar to that of the septum within. The two plates enclose a space, open below, which is filled with dense connective tissue, and at the dorsal border of the cartilages the ventral tips of the vomer are seen in it. The ventral process is connected laterally with the lateral Jacobsonian cartilage by cells of procartilage.
The lateral member of the anterior paraseptal cartilages (figs. 2-3) is a short, curved rod, 1 cm. in length, whose concavity is directed downward, forward and slightly inward, lying almost parallel with the cranio-ventral process of the medial mass which is to be found immediately internal to it. The entire lateral mass is of a younger type of cartilage than that composing the larger Jacobsonian cartilage, and upon examining the sections it is seen that it is connected with the latter at a point near the ventral process by procartilage. Otherwise the lateral mass is quite free. The adjacent portion of the lateral wall is marked by the prominent paraseptal process (fig. 2), whose tip is composed of young cartilage, and between this and the lateral Jacobsonian cartilage there is a zone of loose tissue which suggests an earlier connection between these points such as exists in the rabbit (Voit) in the form of the anterior transverse lamina. The ectethmoid would then be united with the medial Jacobsonian cartilage, the lateral member being probably a rudiment of this lamina.
In the ectethmoid (fig. 12), as we have seen, there may be recognized a roof, or tectum nasi, a lateral wall, or paries nasi, and a floor, or solum nasi. Its only connections are made medially with the septum (figs. 11 and 12) and laterally with the cartilage sphenoethmoidalis (fig. 1), both unions being through the roof. The posterior, or subcerebral portion of the tectum nasi is as yet imperfectly developed, and is concerned principally in the formation of the lateral deHmitation of the fenestra cribrosa, the representative of the future cribriform plate. Dorsally this subcerebral portion is wide and flattened, and forms part of the ventral boundary of the fissura orbitonasalis. jNIedially it is connected with the nasal septum by a short line of attachment, interrupted by small foramina (fig. 12) while laterally it is bounded by the broken line of union with the cartilago sphenoethmoidalis. This portion narrows as it is followed ventrally, and forms the rather uneven lateral boundary of the fenestra cribrosa.
The ventral border of this fenestra is formed by the dorsal edge of the prominentia superior (fig. 1), which is a medial continuation of the sphenoethmoidal commissure connecting this to the nasal septum in front of the crista galli. Projecting backward into the fenestra cribrosa from the point of union of the sphenoethmoidal commissure with the superior prominence is a short spicule of cartilage, known as the processus cribroethmoidalis (fig. 1), also present in the model of Hertwig. It forms the median boundary of a small incisure, known as the incisura cribroethmoidalis (fig. 1) and appears to be the representative of the foramen cribroethmoidale of the rabbit (Voit), through which the anterior ethmoidal branch of the ophthalmic division of the 5th nerve passes into the nasal from the cranial cavity, to emerge, as we shall see, through the foramen epiphaniale as the external nasal ramus. In the model of Hertwig the incisure is still unclosed dorsally. It represents the ethmoidal fissure of the mature bone.
The ventral, or precerebral, portion of the roof is attached, throughout its entire extent, to the nasal septum (fig. 12). It is almost straight, in the sagittal plane, there being a slight depression between the superior and the supraconchal (Sakterwulst - Voit) prominences. In the coronal plane the tectum is convex upwards, and, when regarded from above, it is seen to be widened by the afore-mentioned prominences.
The paries nasi may be divided into a smooth dorsal portion, the planum antorbitale (fig. 3), which forms the ventro-medial wall of the orbit, and a ventral portion, which presents a much more uneven surface, the two grading into one another in the region of the lacrimal duct. Mead states that in the reptiles a line joining the corresponding place (commissura sphenoethmoidalis) with the processus maxillaris posterior would separate the paries nasi from the planum antorbitale. The same is true also of the mammals, although here the planum antorbitale is usually oblique instead of transverse." Upon comparison with such forms as the pig the paries nasi (sensu stricto) of man, comprising the part ventral to this line, is very rudimentary. Dorsally the planum antorbitale terminates in a rather sharp point, and upon examining the inner surface of the ectethmoid it is seen that this tip is turned inward and forward, the cartilage being directly continuous, to form the processus cupularis posterior (figs. 12 and 14). The border above this extends upward to the bridge of cartilage joining the dorsal portion of the tectum with the septum; it presents near its cranial extremity a small notch (fig. 12), and below this it is fitted closely to the contour of the septum within, the narrow space between being known as the cupulo-septal fissure (figs. 2 and 14) completely filled with connective tissue. This fissure marks off, upon the septum, the delimitation between its interorbital and nasal portions. The upper portion of the planum antorbitale is in close relationship to the ventral root of the ala orbitalis on account of the very rudimentary condition of the interorbital septum in the mammals. A theory accounting for the shortening of the latter is given by Mead who states: "In the evolution into the nasal capsule of the mammals the posterior part of the capsule of the reptiles has been expanded by the backward rotation of the posterior wall (reptilian planum antorbitale), the pivot being the more solid lateral side." Below the dorsal extremity of the planum antorbitale is a wide notch, the dorsal palatine notch (fig. 12), partly filed by the developing upper portion of the palate bone; below this is a rounded angle, from which the sharply-marked lower bqi'der runs forward and outward, with a slight concavity downward, to end, by an upward and inward bend, upon the posterior prominence (fig. 3). When the capsule is looked at from below it is seen that the ventral two-thirds of this lowermost border is concerned with the attachment of the solum nasi (fig. 2). Above, the line of attachment of the sphenoethmoidal cartilage marks off the upper surface of the paries nasi from the tectum, and just beneath this line, and close to the surface of the planum antorbitale, but separated therefrom by connective tissue, there is to be seen a small nodule of cartilage, the cartilago paraethmoidalis (figs. 3 and 16) apparently of the same age as that of the adjacent wall. It is oval in shape and bears no apparent important relationship to the neighboring structures. Within the capsule the superior nasal meatus corresponds to the dorsal area of the planum, while the middle meatus is medial to its anterior portion.
The ventral portion of the paries nasi (fig. 3) (paries nasi sensu strido), is thrown into a number of eminences, between which lie corresponding hollows. In its dorsal area is to be seen the prominentia posterior, a small swelling upon which the posterior maxillary process appears. The prominence does not extend so far laterally as the lower border of the antorbital plane behind and below it; the latter is continued upward and inward, curving over the sunmiit of the prominence to enclose, in the downward concavity thus formed, the posterior maxillary process. The latter is wide and flat, projects vent rally and is cut off in front and above by a sharply marked furrow, being unbounded below and behind (fig. 17). The slender la'crimal bone lies along its upper border, and a short distance lateral and below is an elongated nodule of cartilage, the anlage of the processus paranasalis, which may be known as the cartilago paranasalis. This presents a free ventral extremity and a dorsal extremity closely applied to, but not continuous with, the lower aspect of the posterior prominence. It is separated from the underlying cartilage by a thin sheet of perichondrium, and between it and the processus maxillaris posterior the anlage of the lacrimal duct may be seen. The lower surface of the prominence is rounded, and thus contrasts sharply with the lower border of the planum antorbitale immediately dorsal to it, which is sharply marked, and projects outward and. downward beyond the prominence.
The cranial area of the ventral surface presents a smaller eminence, the prominentia superior (figs. 1, 3 and 12), to whose dorso-lateral edge the sphenoethmoidal conmiissure is attached and which medially goes over into the septum. It is to be found just ventral to the fenestra cribrosa, and the small foramen epiphaniale (fig. 3) is to be seen piercing its ventro-lateral edge. It represents a small cavity, found in the inner capsular wall. This prominence is separate from a larger one, situated upon the lateral wall, some distance in front and below, which I regard as the structure called by Voit the Sakterwulst in the skull of the rabbit. Between these prominences a shallow furrow may be observed, and this opens below into a prominent pit, delimited by the Sakterwulst, and the superior and posterior prominences. This pit corresponds, I believe, to the sulcus lateralis anterior which Voit describes in the skull of lepus, and it follows that the region dorsal to it must represent the wall of the recessus lateralis. The latter is, however, very rudimentary in man, and the corresponding area of the lateral wall in my model appears to be collapsed, when compared with the condition in the rabbit as shown by Voit's illustrations. The anterior prominence of Voit is not to be seen.
The Sakterwulst (supraconchal prominence) is the most conspicuous of all the prominences in the human ectethmoid at this stage,' and presents a sharply-marked summit. Behind the latter may be seen an undulating ridge, which passes successively backward, across the sulcus lateralis anterior, over the ridge of the posterior prominence, to reach the lower border of the planum antorbitale (fig. 3). The Sakterwulst corresponds to the ventral extremity of the middle meatus of the nose; below it is a groove leading downward and backward to an incisure in the lower border of the ectethmoid, the incisura post-transversalis (figs. 3 and 12), behind the processus paraseptalis, the latter being the representative of the anterior transverse lamina of the rabbit (Voit). The ventral surface of the Sakterwulst slopes downward, forward and inward to the upper part of the incisura narina. Upon the edge of this there js, at this point, a small projection, the representative of the cartilage cupularis of the lower forms (fig. 12).
The ventro-caudal portion of the paries nasi is raised into a slight eminence, and upon the ventral edge of this, which bounds the incisura narina, the processus alaris superior appears (figs. 2 and 12) this being, however, but rudimentary in man when compared with such fonns as the rabbit. The most prominent feature of the lower edge is the elongated and slender paraseptal process, which points inward, backward and downward.
The solum nasi is formed mainly by the inferior nasal concha, which appears as the inturned lower edge of the paries nasi, and in part by the rudiments of the laminae transversales anterior and posterior, represented respectively by the processus paraseptalis and the processus cupularis posterior. The dorsal portion of the inferior concha is corrugated, and presents a free posterior edge, which meets its median border at a right angle. This dorsal edge is inwardly continuous with the lower extremity of the middle concha (fig. 12), where it joins the inner aspect of the planum antorbitale; with the latter it forms a notch, lodging the ventral portion of the upper border of the palate bone, and known as the ventral palatine notch (figs. 2 and 12). A rounded ridge, directed backward and inward, appears upon the lower surface, and separates two grooves, the medial being the more well-marked. This ridge is represented in the floor of the middle meatus by a wide groove which runs backward and inward to terminate by passing over the dorsal edge of the inferior concha, which thus shows a marked concavity upward.
The ventral portion of the solum is narrow and more sloping than the dorsal, and is also smoother. It terminates ventrally at the post-transverse incisure. The medial border of the inferior concha is thickened except at its extremities, and forms the lateral boundary of the basal fissure (fig. 12).
The medial surface of the ectethmoid is comphcated, but is, in a general way, concave. Dorsally may be seen (figs. 12 and 14) the processus cupularis posterior projecting forward freely into the mesenchyme. This is the last remnant of the lamina transversahs posterior which is found in the rabbit (Voit), in which animal it connects ventrally with the paraseptal cartilage, but in my model there is no such connection. This is the only part of the dorsal border which turns inward, and is thus the sole representative of the median part of the cupola posterior, described for many of the lower forms. A portion of the dorsal border, a little below the lower margin of the bridge of cartilage which joins the tectum nasi to the septum is partially cut off by a thin sheet of perichondrium, and it also presents a slight thickening. Its cartilage at the ventro-cranial edge is slightly younger than that found in the adjacent wall. This may be the homologue of one of the ethmoidal conchae of the lower forms (fig. 12). Below the dorsal portion of the tectum is to be seen a small fossa, probably the representative of the sphenoethmoidal recess of the adult condition, and this is bounded below by the anlage of the superior nasal concha (fig. 16). The latter is a small ridge of cartilage which stretches across from the medial surface of the planum antorbitale behind to the attachment of the middle concha with the wall, in front.
Below and behind the superior concha is to be seen the superior meatus of the nose (fig. 12), and it may be noted that this narrows below by reason of the fact that the line of attachment of the middle concha, which forms its lower and anterior boundary, runs from above downward, as well as backward.
The middle nasal concha (figs. 12 and 16) is somewhat larger than the superior, and presents an irregular lower edge which may be followed from a swelling of the lateral wall (which corresponds to the antero-lateral sulcus of the outer surface) backward and downward to tenninate at the apex of the ventral palatine notch by running over upon the dorsal edge of the inferior concha. The edge of the middle concha is composed of very young cartilage. The middle concha is perforated, near its upper end, by a small foramen, filled by connective tissue. Its medial surface, in its ventral and upper portion, looks inward, but as it is followed dorsally it is seen to become directed backward, and also to become much narrower, so that the dorso-caudal extremity even looks slightly outward, being separated from the adjacent medial wall of the planum antorbitale by a perceptible interval, the lower limit of the superior meatus, while its dorsal edge shows a slight prominence projecting backward a short distance before the lower end is reached.
The middle meatus (figs. 12 and 16) is quite capacious - almost cavernous - in appearance. Above, it is roofed in by the overhanging middle concha, and below it is delimited by the shelflike inferior concha, these two conchae meeting dorsally to bound its dorsal extremity, this latter being a deep recess. Ventrally the deepest portion of the meatus is marked by a furrow, which corresponds to the Sakterwulst on the lateral surface. At approximately the center of the middle meatus, close to the lateral wall, but separated from it by a layer of connective tissue, is a small nodule of very young cartilage, surrounded, by a shell of procartilage, the latter connecting the nodule dorsally with the wall of the ectethmoid, which shows a prominence at this point. This nodule, which is known as the cartilago meatus medii, is situated at a point of the lateral wall just opposite to the posterior maxillary process of the lateral surface (fig. 17), and is surrounded by loose connective tissue. It occurs on both sides, but what its significance may be is uncertain.
At the root of the paraseptal process there may be seen a small hollow, corresponding to the eminence upon the outer surface. The under surface of the ventral portion of the tectmn nasi is smooth, concave, and presents, laterally and above, the foramen epiphaniale, aforementioned (fig. 12).
Only the upper two visceral arches, representing Meckel's cartilage with the auditory ossicles and Reichert's cartilage, are shown in the model.
Meckel's cartilages (figs. 3) comprise two irregularly curved rods, each of which passes from the ventro-lateral recess of the otic region to the site of the future mandibular symphysis, and form by the approximation of their ventral, upturned extremities the apex of an angle, the sides of which enclose the triangular area occupied by the structures composing the floor of the mouth. In cross-section each rod is seen to be of the mature type of cartilage and shows an almost uniform diameter, although the middle of the shaft is characterized by a slight but elongated spindle-shaped thickening, while the ventral upturned extremity is thickened and flattened ventro-dorsally. The dorsal extremity of the shaft, too, just before it becomes continuous with the malleus, shows a short fusiform expansion.
After leaving the ventro-lateral recess of the otic capsule, where it is directly continuous with the malleus, the cartilage proceeds forward, inward and downward, lying quite close to the pars cochlearis of the otic capsule (4 mm. in the model). It then changes its direction, passing almost directly forward, and only slightly downward and inward, thus forming, in the portion below the ala temporalis of the sphenoidal anlage, a wide curve with its concavity directed outward, forward and upward. This direction is maintained until the cartilage reaches a point a short distance in front of its termination, when it turns rather abruptly upward, and its tips become flattened, their medial edges being approximated in the midline, but being separated by a thin sheet of connective tissue, as is usual in homo. Thus a second curve, with its concavity upward and slightly outward, is formed. Upturning, enlargement and flattening of the anterior end is noted by Low first in the 18 mm. stage, and is persistent, the tip becoming later constricted off, and appearing in the 95 mm. stage of Low as a small nodule above the symphysis.
Excepting the small portion made up of the posterior fusiform expansion the shaft is f anked laterally by the covering membrane bone of the mandible, which also overlaps the ventral extremity in the manner shown in the Hertwig model, and which was noted by Low as early as the 18 mm. stage. The terminal upturned portion of the cartilage shows a general enlargement of the cells, and to the lateral surface of this the covering bone is applied very closely - indeed it would appear that this area is encased by a thin plate of perichondral bone. Within this the cells are still larger and vacuolated, and where it fuses with the mandibular bone there is what appears to be a beginning center of endochondral ossification. Here the ground substance is much more deeply staining, and at one point a bud of osteoblastic tissue appears to be invading the cartilaginous mass. Elsewhere, however, the cartilage and covering bone are separated by a narrow interval, containing connective tissue, and nowhere else in the cartilage are there any indications of ossification.
Just beneath the posterior expansion of the shaft is seen the small, flattened tympanic bone (fig. 2), while medial and above the latter, lying with its long axis parallel to that of the Meckelian cartilage, and immediately applied to it, is the slender goniale. Above and lateral to the posterior extremity of the shaft is to be seen the squamo-temporalis.
Ventrally the mylohyoid muscle is attached to the perichondrium of Meckel's cartilage, but in the middle third its attachment is to the inner table of the mandible and its dorsal prolongation, this connection being established just above the cartilage. Hence it would appear that the mylohyoid ridge of the mature bone indicates the original position of Meckel's cartilage.
By reference to the Hertwig model it is seen that in the interval between the 40 mm and 80 mm stages there has been a modification of the curve of the cartilages, the dorsal curvature having been eliminated in the older stage, so that the proximal portion of the cartilage is forced away from the cochlea and the angle at the symphysis considerably widened. It would appear that in the process of development the cartilage grows more rapidly in length than in thickness, and hence with advancing age it becomes progressively more slender. This is brought out clearly when my model is compared with the earlier Low ('09) models on the one hand and with the Hertwig model on the other, the cross-section of the rod in the latter being less when compared with its length than is the case in my model, and much less than that of the earlier Low models.
Evidence of beginning ossification and resorption of the cartilage opposite the interval between the lateral incisor and canine tooth germs is first noted by Low ('09) in the 31 mm. stage, in the form of enlargement of the cells, while in the same region, in the 36 mm. stage, he mentions ossification of the perichondrium with vacuolization and enlargement of the cells. Again, in the 55 mm. stage, he figures this region of Meckel's cartilage almost surrounded by bone, and undergoing resorption. Hertwig's model shows a complete investment of covering bone in this region. My model is intermediate between the 36 and 55 mm. stages of Low, the cartilage being only half surrounded by bone, and ossification just commencing within.
Of the auditory ossicles all are represented in the model (figs. 2 and 3). The malleus, as has been stated, is directly continuous ventrally with the shaft of Meckel's cartilage. Dorsally it is cut off from the incus by perichondrium, though in the model the two are represented as continuous, a lateral furrow marking the intervening boundary. The head of the malleus is large and rounded, and lies just lateral to the tegmen tympani, being separated by connective tissue; its manubrium is long, thickened proximally, and its tip is closely applied to the promontory of the cochlea, a condition which is strikingly different from that found in the model of Hertwig, where there is a considerable interval between these parts, caused apparently by the expansion of the middle ear region, which has thrust the upper end of Meckel's cartilage, with its affixed structures, outward. In the angle between the manubrium and Meckel's cartilage the tympanic and goniale are seen. The goniale, which represents the future anterior process, is at present unattached to the malleus, and remains so till the end of the fifth month (Broman '99) .
The incus is completely separated from the malleus, in front, and from the otic capsule, behind, by perichondrium, and presents a body to which is attached a long and a short limb, morphologically resembling the adult condition. The incudostapedial articulation is formed of condensed mesenchyme, and marks the apex of a right angle, open upward, outward and backward, formed by the long limb of the incus and the two limbs of the stapes, and in this angle the facial nerve is to be found.
The stapes at this stage is circular in form, the base being imperfectly developed. The limbs are represented by well defined, round cartilaginous rods.
Reichert's cartilages (figs. 2 and 4) are the paired cartilaginous rods, lying below the posterior half of Meckel's cartilage, which forai the posterior part of the hyoid arch. Each is roughly the shape of a walking cane with a curved handle and a somewhat bent shaft. The handle-like portion is found between the lower end of the crista parotica and the lateral extremity of the jugular foramen, the fenestra perilymphatica being above it, and the paracondyloid process below. It lies quite free in the mesenchyme, separated by that tissue from the otic capsule, a condition agreeing with that described by Jacoby for an earlier stage, in which the cartilages were in contact with the otic capsule, but not fused with it, as they are shown to be in the Hertwig model of a later stage. Each cartilage is of almost uniform diameter throughout, and its direction is from behind forward, inward and slightly downward, the shaft presenting a slight outward and downward bowing, where it sweeps around the promontory of the cochlea, and a gradual inturning toward the distal extremity. As it passes forward the shaft gradually separates from the promontory, to which it is closely approximated, without being actually in contact, and at the same time draws closer to Meckel's cartilage. The terminal anterior tip lies just medial to the angle of the mandible (fig. 2), and above the greater cornu of the hyoid cartilage, which it overlaps for a short distance, without actually becoming contiguous, and it shows a rather younger type of cartilage than the rest of the shaft. The latter differs from that of the chondrocranium in being more reddish in stain.
All of the purely membrane bones of the skull are represented in my embryo, though they are as yet but imperfectly developed. The interparietal appears, as has been noted, just lateral to the dorsal occipital prominence as a faintly staining spicule of osseous matter, but owing to the lack of the dorsal sections the extent of the bone could not be ascertained, and hence it was not shown in the model. Though this bone does not appear in the model of Hertwig it must have been present, as this embryo was considerably older than mine. According to Mall ('06) the centers for the interparietal (one on each side) appear on the 57th day and unite on the 58th day.
Situated some distance above the otic capsule, and forming a part of the side wall of the cranial cavity, is the network of bone which is the anlage of the future parietal (fig. 4). It is roughly of diamond shape, with the long axis directed cranio-caudally, and medial to its lower border is the disappearing parietal plate, the two layers, cartilage and bone, being separated by a slight interval filled with connective tissue. There is a considerable area separating this bone from the frontal, and it is also widely separated from the interparietal, there being a very large area at the back of the cranium, behind the parietals and above the tectum posterius, uncovered by bone or cartilage.
The parietal bone is not so dense as the frontal, showing that it is probably later in appearing. Its structure presents no definite center, it being a fine reticulum throughout, though, for technical reasons, it has been represented in the illustrations as a solid plate.
The frontal bone (fig. 4) is similar in structure to the parietal, and encloses the lateral and ventral parts of the anterior cranial fossa. In it two main parts may be distinguished, a vertical portion, convex antero-laterally, and a horizontal portion, which is slightly arched upwards for the accommodation of the structures of the orbit ; these being the anlagen of the corresponding parts of the adult bone. Joining these portions is a wellmarked, rounded ridge, the representative of the future supraorbital ridge, and here the bone is most densely deposited, especially in the central part of its extent, indicating, probably, that in this position the first masses of osseous tissue were laid down, and not in the region where the frontal eminences will later appear, as is stated in the textbooks. The entire margin of the bone, as well as the greater area of the posterior half, is but a tessellated plate composed of intercrossing osseous spicules. The anterior extremity is rounded, and shows no definite resemblance to the mature condition; it is separated by a considerable interval from the nasal bone, and almost touches its neighbor of the opposite side, this being the part of the bone which most nearly reaches the median plane. Posteriorly it gradually recedes from this plane, the entire medial edge of the horizontal portion underlying the outward-slanting ventro-lateral edge of the sphenoethmoidal cartilage. Its dorsal extremity is separated by a considerable interval from the parietal bone. It also, for technical reasons, is figured as a plate.
The squamosal (figs. 2, 4 and 13) is a somewhat fan-shaped bone, in which two distinct portions may be recognized; a posterior flattened plate, covering laterally the auditory ossicles and the upper part of Meckel's cartilage, and a ventral, elongated spicule of bone, the zygomatic process, whose tip is found just above the dorsal tip of the zygomatic bone. The upper edge of the flattened portion is convex upward, and is somewhat serrated, the lower border being slightly concave, and passing directly into the zygomatic process. The latter, owing to its lying parallel with the sagittal plane, meets the flattened portion at an angle, as the latter looks outward and forward (fig. 2).
The zygomatic bone (figs. 2 and 4) is somewhat quadrilateral in shape, and thus bears a resemblance to its adult condition. It already shows a body, with four angles, three of which terminate in marked projections. The body is flattened and thin, and the ventral part of its medial surface is in close apposition, though not in union, with the zygomatic process of the maxiUa. From the dorso-caudal angle there projects backward a spicule of bone, whose dorsal extremity, underlies the ventral extremity of the zygomatic process of the squamosal, thus identifying it with the zygomatic process of the zygomatic bone; the zygomatic arch is, accordingly, incomplete. From the cranial angle there projects upward and slightly backward the rather blunt, but strongly marked, frontal process, lying somewhat ventro-lateral to the upper extremity of the ala temporalis, and a slender elongated and inwardly curved infra-orbital process overlies the outer border of the zygomatic process of the maxilla. The caudo-ventral angle of the body is well-marked, and represents the anlage of the future malar* tubercle.
The lacrimal (figs. 4 and 17) is a slender spicule of membrane bone lying along the upper and lateral part of the cartilaginous posterior maxillary process, its long axis being directed from behind forward, inward and slightly upward. Its middle part presents a slight thickening, and the naso-lacrimal duct lies immediately lateral to it.
The nasal bone (figs. 1 and 4) is represented in the model as a thin plate, rounded in outline, flattened, and about 1 cm. in diameter, lying upon the part of the upper surface of the tectum nasi which represents the medial surface of the supraconchal prominence. Behind this plate, and lying close to it, are several minute nodules of membrane bone, which also contribute to the formation of the adult bone. The nasal anlagen lies in a mass of dense mesenchyme, which overlies the entire nasal capsule, similarly to the formation described by Fawcett ('10 a and '10 b) in a 30 mm. human embryo.
The tympanic (figs. 2 and 4) is a short, flat plate of membrane bone, lying beneath the posterior extremity of Meckel's cartilage, and in front of the malleus. Its anterior part is slightly wider than the posterior, and its long axis lies parallel with that of the cartilage, its flattened surface being applied to the latter. The bone is found in an isolated anlage of condensed osteogenic mesench3mie, and is placed lateral to and below the goniale. When compared with the Hertwig model it is seen that the only part which is as yet laid down is the anterior widened extremity, there being no evidence of the ring-like foran of the later bone.
By the subsequent outward swinging of the ossicles and attached Meckelian cartilages consequent upon the expansion of the region of the middle ear the plane bordering upon Meckel's and Reichert's cartilages is changed in direction, so that its outer surface looks almost directly downward, as shown in the model of Hertwig, instead of outward, as is the case in my model, and at the same time the interval between these two cartilages is relatively narrowed. This interval the tympanic bone comes to occupy, its dorsal half, as yet undeveloped in my embryo, being applied to the cartilage of Reichert in the Hertwig model, the long process of the malleus occupying a position within the ring.
The goniale (figs. 1-2) is a short, somewhat flattened, rod of membrane bone, lying immediately below and medial to the posterior fusifomi enlargement of Meckel's cartilage, and so close to the latter that it appears to be developed from the perichondrimn covering its surface. Its long axis is parallel with that of Meckel's cartilage, and its anlage is quite separate from that of the tympanic, which lies on a lateral and caudal plane. Its posterior end approaches close to the neck of the malleus, but does not actually become continuous therewith until the end of the fifth month (Broman '99), when it becomes the anterior or Folian process of that ossicle, a fact which was demonstrated by Dreyfuss ('93). Gaupp ('05 and '11) has given strong reasons for the identification of the processus Folianus with the goniale of the reptiles, and it is on his authority that I have used that name for it here. The bone is shown in the Hertwig model, and also appears in Low's illustration from a 95 mm. human embryo under the name of 'processus folianus.' The vomer (figs. 2, 16, 17 and 18) is represented by two slender strips of bone lying one on either side, immediately caudolateral to the lower border of the mesethmoid, the two almost enclosing this border excepting for a narrow strip between their lower edges. The dorsal extremity of each strip is blunt, but the ventral is drawn out to a fine point, and near the latter there is a bridge between the two bones. The ventral point projects only a short distance in front of the dorsal border of the larger Jacobsonian cartilage.
The maxilla (figs. 2, 4, 16 and 17) is an irregular mass of cancellous bone which lies in a notch on the ventro-lateral aspect of the ectethmoid, and it already shows the frontal, alveolar, palatal and zygomatic processes. The central mass of the bone is irregularly triangular in form upon coronal section (fig. 17), the longest side of the triangle being applied to the ethmoidal cartilage, while the medial angle represents the palatal process, the upper angle the frontal process, and the remaining angle, which is a right angle, the alveolar process. The pointed, upwardly-directed frontal process reaches a level somewhat above that of the lacrimal; its posterior surface forms the anterior wall of the space containing the naso-lacrimal duct (fig. 4). The palatal process does not quite reach to the median plane and anteriorly it is separated by a notch from the extremity of the alveolar process: its medial border is below and lateral to the vomer, and its posterior border underlies the palatine bone for a considerable distance.
The alveolar process is a very irregular ridge of bone, occupying the inferior and lateral edge of the maxilla, and its anterior end extends medially almost to the mid-line, coming to lie in advance of the palatine process. In its lower aspect may be seen the depressions for the future dental alveoli. The zygomatic process is a more lateral coarse spicule of bone, which projects backward and outward until its extremity almost touches the inner surface of the zygomatic bone. It is perforated from above downwards by foramina, and just before it joins the body of the bone it sends up a small process, which marks off a groove occupied by the second branch of the trigeminal nerve (fig. 17). There is as yet, therefore, no foramen for this nerve trunk. The posterior edge of this process is continuous inwardly with the posterior edge of the palatal process. Anteriorly a spicule from the upper part of the anterior extremity of the alveolar process projects forwards, and lies lateral to and below the anterior part of the anterior paraseptal cartilage, to which it is closely applied. This is the anlage of the anterior nasal spine.
The palate bone (figs. 2, 3, 14, 15 and 16) is a thin, delicate lamella of osseous tissue, strongly concave inward, which walls in the dorsal portion of the nasal cavity. Its borders are serrated; the upper being fitted into the dorsal palatine notch of the ectethmoid, and ventrally to this into the ventral palatine notch. The incurled lower edge (fig. 2) which becomes the palatal plate, approaches but does not meet its fellow of the opposite side, being separated from it in the model by a space of 15 mm. Dorsally the anlage of the pyramidal process is represented by a thickened and irregular projection which lies ventral to the medial pterygoid lamina.
The medial pterygoid plate (figs. 2, 3 and 10) is represented by an elongated and somewhat spiral rod, lying behind the palate bone and medial to the lateral portion of the ala temporalis. Two distinct portions may be recognized in it, the upper part being osseous and the lower cartilaginous. The former is bent dorsally and from its most posterior extremity, which is about 3 mm. from the medial angle of the lateral portion of the ala temporalis, a short but stout process projects cranio-medially, marking the highest part of the rod. Below it terminates in the cartilaginous hamular process, which points almost straight downward.
The membrane bone of the upper portion is typical in structure, and when followed downward shows a gradual transition into the cartilage of the hamular process. The cells of the membrane bone enlarge, their capsules swell, and at the same time the ground substance becomes lighter in color. Nearer the cartilage which forms a cap to the extremity of the osseous rod the capsules become smaller, the ground substance increased in amount, and the line of transition is not sharply marked. Though the tip of the cartilage partakes of many of the properties of normal cartilage - staining lightly, having a homogeneous matrix, the nuclei surrounded by capsules, and the whole being enveloped by a closely-fitting, sharply defined, thin sheet of perichondrium - yet there are several points of difference which mark out this mass of cartilage as different from that found elsewhere in the primitive skull. The matrix stains slightly more darkly, the nuclei are larger and lighter in color, and the capsules surrounding them are relatively smaller, when compared with the size of the nuclei. Altogether the cells resemble those of membrane bone more than they do those of typical cartilage.
The mandible (figs. 3-4) is a plate of membrane bone lying immediately lateral to Meckel's cartilage, and separated from this throughout its extent by connective tissue, except in a small area in the region of the lateral incisor and canine tooth germs, where, as has been noted, the bone is directly applied to the cartilage, appearing like ossified perichondrium. The posterior part of the bone is wider than the anterior, and is marked cranially by a distinct indentation, the representative of the future sigmoid notch, which separates the anlagen of the condylar and coronoid processes. Just external to the latter is the zygomatic bone, and internal to it is the cartilaginous pars temporalis of the sphenoid. The lateral surface of the plate shows, about the junction of the anterior and middle thirds, a distinct opening, the mental foramen, and at this point there is a bend in the bone, the direction of the long axis changing from downward and forward to upward, inward and forward. A short distance above the lower border of the bone, and parallel with it, a groove may be seen upon the lateral surface, more distinct posteriorly than anteriorly. This is also shown in the illustrations of Low's older specimens, and appears to represent the line of insertion of the masseter.
The anterior half of the bone shows an inner table, regarded by some authors as the splenial element, which separates the vessels and nerves from Meckel's cartilage, becoming continuous anteriorly with the ossified perichondrium immediately surrounding the cartilage. About the center of the long axis of the bone the so-called splenial element dwindles in height, and terminates as a thin, backwardly projecting spindle of bone, to which the mylohyoid muscle is attached. Anteriorly the membrane bone overlaps the termination of the cartilage of Meckel, and thus shows the condition observed by Low as early as the 18 mm. stage.
On the whole, the shape of the mandible suggests the adult condition, though it is considerably more slender, and the angle is more obtuse. Immediately caudo-medial to the latter is the tip of Reichert's cartilage. There are no accessory cartilaginous nuclei observable, as Low describes in an older model.
Low ('05) has held that each half of the mandible is laid down as a single skeletal element, the dentary, the so-called splenial element being simply an extension of this. The condition in my embryo supports this finding, the main portion of the bone being fonned of the dentary, the splenial element being but a thin lamella of osseous tissue, directly continuous below with the anterior portion of the dentary, and posteriorly terminating in a slender process, as described. Within this medial plate the tooth gutter is well-marked. The condition is very similar to that shown by Low ('09) in his text figure 4.
From the foregoing brief account of the morphology of the human fetal skull of 40 mm. it will be seen that it presents, at this stage, several unique features. The structure of the parts surrounding the foramen magnum is significant in casting, perhaps, some further light upon the genesis of this interesting region. The presence of a fairly clearly defined neural arch of the occipital vertebra has been dwelt upon, and its interpretation discussed. The finding of a small nodule of cartilage above and in front of the cochlea is, so far as I am aware, a contribution to our knowledge of the parts in this area. Other new features are the cartilagines paraethmoidalis, meatus medii and paraseptalis superior, all of which have been briefly described ; in addition may be mentioned the lateral and dorsal cranial cartilages, and the paranasal cartilage, lying lateral to the lacrimal duct.
Finally, I wish to express to Professor McMurrich my sincere appreciation of his kindness in placing at my disposal the material for this research, and the freedom of the Anatomical Laboratory of the University of Toronto, as well as of his abundant advice and encouragement.
Anatomical Laboratory, University of Toronto September, 1913
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