Paper - The skull of a human fetus of 40 mm 1
|Embryology - 19 Oct 2020 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.
See also 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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- 1 The Skull of a Human Fetus of 40 mm I.
- 1.1 Introduction
- 1.2 The Skull as a Whole
- 1.3 Cartilage
- 1.4 Chondrocranium
- 1.5 Plates
- 1.6 Plate 8
- 1.7 Plate 9
The Skull of a Human Fetus of 40 mm I.
Charles Clifford Macklin
James H. Richardson Fellow in Anatomy, University of Toronto
Eighteen Figures (1914)
Researches upon the developing human skull have been carried on by various investigators, for a summary of whose results reference may be made to the excellent reviews by Jacoby ('94) and Levi ('00). The plate method of reconstruction, however, has been applied in such investigations only within recent years, and has been used in the study of fetuses in the following stages:
|Length (mm)||Probable age (author's estimate)||Author|
|17||50 days||V. Noorden|
|18.5||7.5 weeks||V. Noorden|
|23||8.5 weeks||V. Noorden|
|30||Jacoby (younger than the 28 mm stage of Levi)|
It will be seen that a considerable interval, occurs between the last two stages in this list, and with a view towards lessening it I undertook, at the suggestion of Professor McMurrich, the study of the skull of a fetus of 40 mm., crown-rump measurement, which would correspond, according to Mall's formula (Mall '06, p. 439), to an age of 63 days. The entire head of the fetus had been cut into serial sections in the frontal plane, the sections being 20 micra in thickness and stained with haemalum followed by erythrosin. From these sections the entire skull, together with the membrane bones and upper two visceral arches, was reconstructed by the wax plate method at an enlargement of 30 diameters, and, to facilitate the study of the nasal and otic capsules, separate models were made, at the same magnification, of the mesethmoid, ectethmoid, right otic capsule and the anlage of the right osseous labyrinth. Drawings were made of every other section by the use of Edinger's "Drawing and projection apparatus," only that tissue being included as cartilage which presented a homogeneous, blue ground-substance and could be clearly differentiated from its surroundings, i.e., the tissue which Levi ('00) has described under the terms 'alteren Vorknorpel' or older precartilage, and "Knorpel"" or cartilage. The earlier chondrogenic tissue of which there was very little at this stage, was disregarded.
The sections were not exactly in the frontal plane and to eliminate any lateral distortion in the models the amount of obliquity was determined as accurately as possible and the wax plates piled upon a plane adjusted at the proper inclination. It is also to be noted that a few sections passing through the dorsal extremity of the occiput were wanting from the series, and the small portion of the occipital region which they represented has been reconstructed by reference to other models, the parts so added being painted white, so that they may readily be recognized.
In the following description of the models I have used, as far as possible, the terms which are current in the literature, and when those referring to the human skull were exhausted recourse was had to the terminology in use in the more recent publications upon the chondrocrania of mammals. Preference is given to the BNA, and names not hitherto introduced into human chondrocraniology are usually followed by the name of the author who has employed them, the ideal being always kept in view of a system of terms uniform throughout the mammalian forms at least. New names, introduced by the writer, are indicated by making their initial appearance in italics.
An attempt has been made to select such terms of orientation as may be applied to animals having either a horizontal or a vertical long axis. Thus the terms ventral, dorsal, cranial, caudal, lateral and medial are generally used, but their respective equivalents for the human figure, such as anterior or front, posterior or back, superior or upper, inferior or lower, external or outer, internal or inner, are also employed. Oblique directions are indicated by combinations of the above terms.
Measurements appearing in the text have been taken from the models, and are thus magnified thirty times.
The Skull as a Whole
The primitive skull of homo at the 40 mm. stage presents, in general, the characters which have become familiar through the illustrations and descriptions of v. Noorden, Jacoby and Levi for younger embryos, and the model of Hertwig for a more advanced age, combined with several features that are characteristic of this period of development. The outline, when the many gaps are filled in, suggests the osseous skull.
Viewed from above (fig. 1) we note the entire absence of the roof and the extremely rudimentary character of the sides of the cranial vault. The eye meets with an irregular surface of varying depth, surrounded by a broken, ovoid contour, the smaller end being ventral. This surface we recognize as the floor of the primitive brain-case. Its dorsal half is made up of the future posterior cranial fossa, a deep, bowl-like enclosure, the steep sides of which slope rapidly down to an elongated opening in the floor - the primitive foramen magnum. In the ventral wall of this fossa is a trough-like space behind the basilar plate, flanked by two rounded eminences, the partes cochleares of the otic capsule, and terminated above by the horned, ridge-like dorsum sellae, which forms a conspicuous object in the floor of the cranium. Passing forward over this ridge a sideless pit, the hypophyseal fossa, comes into view, which marks the center of the middle cranial fossa, and here, in the region of the body of the sphenoid, the cartilaginous floor is very narrow. Lateral to the corpus sphenoidale is a large, triangular gap in the floor and sidewall of the brain-box, the apex of which meets the side of the sella, while the ventral and dorsal borders are formed by the dorsal border of the ala orbitalis and the cranio-ventral surface of the otic capsule respectively. Forming a lateral, knobbed projection beneath the ala orbitalis is the relatively small ala temporalis, and this is observed to lie ventro-caudal to the plane of the above-mentioned triangular space, just as, in the osseous skull, the greater wing lies below and in front of the plane joining the lesser wing with the petrous portion of the temporal bone. As will be seen later, this plane corresponds in a general way to the situation of a primitive floor and side-wall of this region of the skull, as found in the lizards. Two of the bones which will later wall in this space, viz., the parietal and the squamo-temporaUs, are as yet very rudimentary, while the third, or ala temporalis, has only just commenced to ossify.
We now pass forward, over the low ridge in front of the sella turcica, known as the tuberculum sellae, and come upon the plateau-like surface supporting the optic chiasma, which leads laterally into the optic foramina. Ventral to this surface is the wide, upward-slanting, anterior root of the ala orbitalis, in front of which is a broad, flattened, triangular surface. This we recognize as the floor of the anterior cranial fossa. Perforating it, in the area lateral to the median septum, which represents the developing cribriform plate, are several foramina, the largest being the paired fenestrae cribrosae. Hook-like, backwardly projecting processes mark the dorso-lateral limits of the floor, which ventro-laterally is widened by the orbital plate of the frontal bone.
The only portions of the osseous cranial vault yet in evidence are the rudimentary frontal portion of the frontal bone and the net-like parietal - these being separated by a rather wide space.
Finally, the cranial aspect discloses to view certain accessory cartilages. Above the dorsal wall of the posterior cranial fossa may be seen two cartilages, lying close together, both small, but the left considerably smaller than the right. They may be known as the cartilagines cranii posterior es. Above each parietal plate, medial to the parietal bone, is seen an isolated nodule of cartilage, that on the right being larger and more elongated than that on the left; these may be called the cartilagines craniilaterales. Below and in front of the cranial pole of each cochlea there is a small, rounded masls of cartilage, which may be termed the cartilago supracocMearis.
(anterior), almost separated by a large gap between the otic and orbitotemporal regions. The dorsal region is composed principally of cartilage, and presents a fairly smooth, convex, fenestrated surface, the most lateral part being formed by the rounded outer wall of the pars canalicularis of the otic capsule. Behind this the flat surface is seen to widen, and then quickly narrow, and to sweep backwards and inwards to unite with the corresponding plate of the opposite side in the tectum posterius. Above, the wall is heightened by the reticular parietal bone (fig. 4). These structures enclose the lower and posterior portion of the cranial cavity.
The anterior mass is composed principally of the facial parts, and contrasts with the posterior in being narrower from side to side in its lower part, and in presenting a greater proportion of osseous material. Its surface is also much more irregular and uneven. In front of the pars canalicularis we see, in a recess which lies ventro-lateral to the two parts of the otic capsule, an irregular mass of cartilaginous and osseous structures, in which we recognize the anlagen of the auditory ossicles and the upper part of Meckel's cartilage, the latter appearing above the small tympanic and goniale (fig. 2), and being covered laterally by the squamo-temporalis. Below these structures, with its hooklike proximal extremity lying in a small cavity, but unconnected with the rest of the chondrocranium, is the slender shaft of Reicherf s cartilage.
Above the otic capsule the large open space which will later form the middle cranial fossa is again apparent, and through this we see the lateral aspect of the hypophyseal fossa (fig. 3), with the high dorsum sellae, upon which are the ventrally projecting posterior clinoid processes, hmiting it posteriorly. In the anterior wall of the sella turcica is the middle clinoid process (present on the right side only) and above this is to be seen the orbital wing of the sphenoid, the gentle curvature of the latter being broken by the anterior clinoid process, and its outer and posterior extremity terminated by a sharp process, projecting dorsally. Below the ala orbitalis, and separated from it by the wide and unclosed superior orbital fissure, is the ala temporahs, supported by a short processus alaris, the rounded extremity of the latter almost touching the ventral pole of the cochlea, and being quite close to the cartilago supracochlearis, which is plainly visible from the side.
Looking at the skull from a more anterior position we see, above, the cavity of the orbit, roofed over by the cartilage of the orbital wing of the sphenoid, and by the orbital portion of the frontal bone (fig. 4) ; limited medially by the shelving posterior portion of the ectethmoid; but widely open downwards and outwards, except where cut off by the zygomatic bone and the maxilla, and but imperfectly closed behind. In the posterior portion of the orbit is seen the elongated optic foramen, and the closeness of apposition of the sphenoidal and ectethmoidal cartilages is to be observed. An open space, communicating freely above with the orbit, below with the cavity of the mouth and medially with that of the pharynx, is seen in front of the ala temporalis, the medial wall of the space being indicated by the imperfectly developed vertical plate of the palatine bone and by the rudimentary medial pterygoid plate. The position of the as yet unformed pterygomaxillary fossa is indicated in this space by the sphenopalatine ganglion (fig. 14) (not shown in the model), and lateral to this, bounded by the incomplete zygomatic arch, are the temporal and zygomatic fossae. One is struck with the lack of prominence of the zygomatic region when compared with the osseous skull, the zygomatic bone and arch being completely overhung by the lateral part of the cranial floor. That this disproportion is partly due to the shallowness of the temporal and zygomatic fossae is evident from a comparison of the model with the bony skull. It would appear that the lateral growth of the ala temporalis of the sphenoid and of the zygomatic process of the maxilla, combined with thickening of the temporalis muscle, are the principal factors which bring about the widening of the temporal and zygomatic fossae, and consequent outpushing of the lateral parts of this area. It is evident from a comparison of the skulls of the newborn and the adult that this change continues till some time after birth.
The lower part of the facial region is characterized by the gaping cavity of the moUth, bounded above by the superior maxilla and below by Meckel's cartilage with its covering bone, the mandible (fig. 4). The lack of prominence of the angle of the latter, due to the shortness and inclination of the ramus and its relative nearness to the medial sagittal plane, may be observed, and it will also be seen that there is a small space between the articular process of the mandible and the position of the future glenoid cavity of the squamo-temporalis (fig. 4). The coronoid process of the mandible is quite close to the ala temporalis of the sphenoid, medially, and to the zygomatic arch (as yet incomplete) laterally, a condition which does not obtain in the mature skull.
Dorsal to the prominent frontal process of the superior maxilla the isolated nodule of cartilage, known as the cartilago paranasalis, is to be seen, lying lateral to the cavity for the' lacrimal duct and below the small streak of membrane bone, which is the anlage of the future lacrimal. Another smaller nodule of cartilage is seen near the back of the orbit lying against the upper surface of the ectethmoid and may be known as the cartilago paraethmoidalis. Separating the orbital from the nasal cavity is to be seen the shell-like ectethmoid, bearing upon its cranioventral aspect the small nasal bone. At the front of the nasal cartilages appear the open anterior nares, separated by the ventral border of the septum.
Regarded from below (fig. 2) the model shows in the foreground the mandibular (not shown in fig. 2) and upper part of the hyoid arches. Behind these we see the semi-cylindrical ventral surface of the planum basale, separated from the elongated, flattened, ovoid partes cochleares by deep furrows. It is to be observed that the anterior extremities of the latter do not project beyond the planum. The forked structure at the root of the planum, perforated for the hypoglossal nerve, is seen to be the anterior commencement of the flattened, ring-like sides of the foramen magnum, a downwardly projecting angle marking the position of the future occipital condyle. Lateral to and above the condyle there appears a stout cartilaginous process, which supports the lower and anterior part of the otic capsule. This is known as the processus paracondyloideus (Voit), and above it is seen a wide opening, the jugular foramen (fig. 4).
The two Meckelian cartilages (fig. 3) enclose an angle, rather sharp ventrally, in which are found the structures of the floor of the mouth. The inwardly curved palatine bones, with the assistance of the inner laminae of the pterygoid processes (fig. 2), imperfectly cut off the posterior part of the nasal cavity, and between the pterygoid laminae and the planum basale is seen the space occupied by the naso-pharynx (fig. 10) . Attached to these medial laminae, and indeed developed from their caudal tips, are the cartilagines parasphenoidales (Voit, '09), the representatives of the later hamular processes. The pterygoid laminae are quite separate from the alae temporales, which appear, from this viewpoint, as rhomboidal, perforated blocks.
Lying along the caudal border of the nasal septum are seen the anterior paraseptal cartilages in front, and the long thin plates of the vomer behind (fig. 2); and within each nasal cavity is a small mass of cartilage, lying in the middle meatus (fig. 12), to which the name oariilago meatus medii has been given and whose significance will be discussed with the regio ethmoidalis.
Summing up the cartilaginous and osseous structures which we find participating in the formation of the primitive skull, we have to consider a main cartilaginous mass, or chondrocianium, a number of accessory cartilages, the upper visceral arches, and the membrane bones.
The chondrocranium is a complicated mass of cartilage of exceedingly irregular formation, in which a number of definite areas may be recognized. Upon examination it is seen to consist of a larger dorsal and a smaller ventral enlargement, united by an isthmushke part, the body of the sphenoid. A median stem, bent to enclose an angle of 115Â° open caudo-ventrally, forms the main axis, this being made up dorsally of the flattened planum basale and ventrally of the interorbital and nasal septa, or, employing the terms of Kolliker, the pars chordalis and the pars praechordalis respectively. These limbs are united by the corpus sphenoidale, or Balkenplatte of the German authors, which forms the apex of the angle. The four primary regions of the chondrocranium, which Gaupp has named, from behind forwards, the regio occipitalis, the regio otica, the regio orbitotemporalis and the regio ethmoidalis, are all represented in the median stem, in the order named, the first two being found in the pars chordalis and the second two in the pars praechordalis.
Springing out from the sides of the planum we have the walls of the posterior cranial fossa, while to the ventral end of the axis are affixed the side parts of the ethmoidal and orbitotemporal regions.
If the structures lateral to the ventral limb of this stem be removed, we have an object which roughly resembles a dipper or saucepan, the curved handle being made up of the ventral part of the stem, while the bowl, perforated below and at the sides, is formed ventrally by the planum basale, and laterally and dorsally by the walls of the posterior cranial fossa, the median lip being situated at the tectum posterius.
The chondrocranium is, at this stage, a continuous morphological unit, but there is histological evidence going to show that certain of its parts were primarily separate.
In addition to the chondrocranium proper a number of accessory cartilages, which have no direct connection with it and which have already been mentioned, also occur. They will be discussed in connection with the regions to which they refer, and are as follows:
|Cartilage cranii posterior||Regio otica|
|Cartilago cranii lateralis||Regio otica|
|Cartilago supracochlearis||Regio otica|
|Cartilago parasphenoidalis||Regio orbitotemporalis|
|Cartilago paraethmoidalis||Regio etmoidali|
|Cartilago paranasalis||Regio ethmoidalis|
|Cartilago meatus medii||Regio ethmoidalis|
Of the recorded human embryonic skulls that of Levi ('00) for the 28 mm. stage most closely resembles my specimen, but is somewhat younger. The next nearest stage, represented in the Ziegler model of Hertwig's 80 mm. embryo, is somewhat older.
The principal change which has occurred in the interval between the 28 and 40 mm. stages appears to be the development of the anterior cranial fossa. In the Levi specimen this is relatively narrower and deeper than in mine, thus indicating an adaptation of this region to the increasing size of the anterior part of the brain. Levi notes that this part of the skull has made more rapid development than any other in the interval between the 17 mm. and 28 mm. stages, and in the latter it is evidently still making rapid progress. When my model is compared with that of Hertwig, however, it is seen that in the period between the 40 and 80 mm. stages the posterior fossa has made greater strides than the anterior, thus seeking to accommodate the enlarging, backwardly-growing cerebral hemispheres. The extent of development of the posterior portion of the braincase in the interval between the 40 and 80 mm. stages will be realized when the ratio of the areas dorsal and ventral to the hypophyseal fossa, as they are found in the model of Hertwig and in my preparation, are compared. Although the dorsal area exceeds the ventral in my model the excess is by no means so great as it is in Hertwig's.
Speaking generally, since the 28 mm. stage of Levi there has been a flattening of the entire cranial floor. Between the 40 and 80 mm. stages the zone of greatest enlargement has been the upper edge of the posterior cranial fossa, the effect being as though this part had become stretched while the part around the foramen magnum had remained relatively stationary. The result is that the brain-case in this region has become more shallow and the sides more flaring, with their lateral and dorsal surfaces directed more caudally than outwardly. The region above the parietal plates has shared in this expansion, as is seen by the more widely placed parietal bones in the model of Hertwig, and this period of development has also witnessed the flattening out of the angle from the cranial aspect of the otic capsule, as may be seen when these skulls are compared.
Other expansive changes which have taken place in the interim between the 40 and 80 mm. stages are noted in the region of the middle ear, in the floor of the mouth, and in the temporal and zygomatic fossae, the details of which will be taken up in the discussion of the regions. In general one is struck with the large development of the cranial cavity, which gives to the Hertwig model a relative broadness when seen from the front, and has also resulted in a greater preponderance of the size of the cranium when compared with the facial region. A comparison of the Hertwig model with that of Jacoby for the 30 mm. stage brings out an even greater disproportion.
Of the skulls of other mammals those of Macacus cynomolgus and Semnopithecus maurus, modelled by Fischer ('03), may be mentioned. The similarity in general outline is quite striking, when these are compared with my specimen. The illustration of the caudal aspect of the skull of Macacus shows a flattened condition of the partes cochleares closely analogous to that which obtains in my model. Though the dorsal part of the cranium is relatively shorter it is very suggestive of the cranium of homo, especially from the rudimentary condition of the side-walls.
That portion of the central stem of the chondrocranium which forms its dorso-caudal limb and is traversed medially and longitudinally by the notochord, is known as the planum basale (figs. 1, 2 and 5). It is an elongated and unperf orated plate of cartilage of varying thickness, which extends from the intercondylar incisure of the foramen magnum to the dorsal border of the hypophyseal fossa, and forms the most dorsal part of the skull-base. Its cartilaginous substance is directly continuous with that of three regions; with the occipital region caudodorsally where the planum is seen to pass over into the lateral portions of the occipital anlage, with the orbitotemporal region cranio-ventrally, where it coalesces with the body of the sphenoidal cartilage at the dorsum sellae, and with the otic region laterally. The entire lateral border, with the exception of the extreme upper portion, is united to the pars cochlearis of the otic capsule, the line of union being indicated upon the medial surface of the latter by an elongated, narrow, crescentic strip (fig. 7). This line of union is formed of cartilage throughout, and in its cranial two-thirds this is of the same character as that of the adjoining parts. In the lower third, however, there is to be seen, microscopically, a distinct but thin sheet of younger cartilage, separating the adjacent parts of the planum and the cochlea, this being the last indication of the primitive separation of these parts.
It is worthy of note in passing that the 17 mm. stage of Levi shows the first indication of a union in the human skull between the otic capsule and the planum basale, the cranialmost extremity of this bridge being the first to appear.
Encircling the zone of union of the pars cochlearis with the planum is a well-marked groove. The posterior portion of this, which may be known as the dorsal basicochlear groove (fig. 5), is crescentic in outline, and is fairly well defined. It contains the inferior petrosal sinus, and appears more sharply marked than in the models of Levi and Jacoby, judging from the illustrations of these authors. The ventral portion â€” much deeper and narrower â€” is also crescentic, and may be termed the ventral basicochlear groove (fig. 2). These grooves meet, above and below, their confluences being marked by notches, designated the sphenocochlear and occipitocochlear notches (fig. 5) respectively. The sphenocochlear notch occupies the interval between that part of the lateral surface of the corpus sphenoidale which lies dorsal to the processus alaris, medially, and the medial aspect of the cranial pole of the pars cochlearis, laterally (fig. 1). It is narrow and deep. The occipitocochlear notch is the ventromedial extremity of the jugular foramen.
Viewed from behind (figs. 1 and 5) the planum is seen to be concave from side to side in its lower two-thirds, and caudocranially throughout its entire extent. The cranial third of the dorsal surface terminates above in the crista transversa, a transverse ridge from which the dorsum sellae springs upwards, and which, according to Voit, may be taken to mark roughly the boundary between the body of the sphenoid and the otic or upper portion of the planum basale. This area of the dorsal surface is convex from side to side, and covers the part of the planum which has been called the clivus of Blumenbach. The lower portion of the planum is quite steep, but the cranial portion is much more so, the inclination thus agreeing with the description given by Levi for the 28 mm. stage.
The ventral surface of the planum (fig. 2) is convex from side to side, and almost straight caudo-cranially. It is thus evident that the caudal and cranial extremities are thicker than the middle portion, the latter being in the position of a primitive gap in the cartilage as shown by Levi in the 13 mm. and 14 mm. stages and by Froriep in the 17.5 mm. stage, the latter having found the planum broken by a gap, filled with perichondrium, 1.9 mm. behind the canal of the hypophysis. This was just dorsal to the region where the occipitopharyngeal ligament, which had disappeared in older embryos (Levi), was inserted. This primitive gap (of which there is no evidence in my model, except the relative thinness of the cartilage in this region â€” â– and in this my findings agree with those of Levi and Jacoby for the 28 mm. and 30 mm. stages respectively â€” marks the site of the division which exists in early stages, between the two constituent parts of the planum, viz., that lying below it, belonging to the occipital region and representing the anlage of the future basi-occipitalis, and that lying above it, the clivus of Blumenbach, which is destined to form the basi-sphenoidalis, and has been included in the otic region.
The connection of the clivus with the body of the cartilaginous sphenoidal anlage is very primitive, according to the investigations of Levi upon young human embryos, as in the 13 mm. stage it is present while as yet there is no cartilaginous otic capsule in existence. This circumstance, combined with the relationship of the clivus to the basi-sphenoidalis of the osseous condition would seem to indicate that the upper portion of the planum should be grouped with the regio orbitotemporalis rather than with the regio otica, and this has been done by Levi, Jacoby and Van Noorden, but since Gaupp has more recently shown that the upper portion of the planum is more properly included with the otic region, and as several authors have since followed this course for the mammals (Voit, Mead) I shall adopt it in this description. The more minute details of the occipital and otic portions of the planum will be considered in the discussion of these regions.
The material composing the planum basale is mature cartilage, of uniform character, excepting one small, isolated mass of' enlarged, vacuolated, cartilage cells, the nuclei of which appear larger and more darkly-staining than those of the surrounding areas, and the ground-substance of which has stained a dark purple, thus standing in sharp contrast to the bluish matrix of the cartilage. This mass appears to be opposite the first part of the sub-basal course of the chorda dorsalis, and lies just ventrocaudal to the middle of the long axis of the planum. It belongs to the occipital portion, and is separated from the perichondrium by a thin sheet of mature cartilage. The perichondrium in the vicinity of this mass is unchanged, and contains no osseous tissue. In the 28 nmi. stage of Levi the cells were, in this area, very large, and were surrounded bj^ a very prominent capsule. We have in this area, no doubt, the beginning of the endochondral ossification center of the basi-occipitalis, which is said by Mall ('06) to appear on the 65th day.
The course of the chorda dorsalis has become familiar through numerous investigations, and I have nothing to add concerning it. The condition which I find agrees essentially with that illustrated by Fawcett for the 21 mm. stage (Fawcett '10a, fig. 2), The site of the future ligamentum suspensorium dentis, at the apex of the intercondyloid incisure, is marked by a coiled condition of the notochord, which corresponds to an intervertebral disk.
In my model the caudal portion of the planum is but slightly wider than the cranial, a condition which stands in sharp contrast to that found in the model of Hertwig, where one notices that, though the entire planum has undergone a widening, the region bordering the primitive foramen magnum has outstripped the more cranial portion in this regard. This broadening has, of course, separated the caudal extremities of the ventral and dorsal basicochlear grooves. The latter are both shallower and less strongly marked than in my model, as are also the sphenocochlear notches. Further, the side-to-side concavity in the model of Hertwig is almost obliterated, as is also that from above downward, and the inclination of the planum in the latter model is less steep than in mine.
The occipital region has, in general, the form of a ring, whose irregular circumference, stoutly built and steeply sloping ventrally, plate-like and gently shelving dorsally, encloses the lower part of the posterior cranial fossa, the floor of which is perforated by an elongated fissure, the primitive foramen magnum (fig. 1). Its appearance suggests the occipital bone, of which it is the cartilaginous precursor.
Ventrally the ring is completed by a dorsally concave plate of cartilage, the basilar portion of the occipital anlage, which has been described as the lowermost part of the planum basale. As we have seen, the basilar portion is directly continuous cranially and laterally with the regio otica; below it splits to form the condyloid or lateral portions of the regio occipitalis, which spring downwards, outwards and sUghtly backwards and enclose, with their deviating, flattened limbs, the incisura intercondyloidea, marking the ventralmost part of the primitive foramen magnum. Reaching the most caudal points of the primitive skull at the paired, downwardly projecting ventral foraminal prominences (figs. 2 and 3), upon the site of the future condyles of the occipital bone, they suddenly bend upwards and outwards, twist on their long axes so that the inner surfaces, which before looked dorsomedially now look principally cranially, and at the same time they broaden ventrally and laterally, their outermost borders coming to underlie and support the partes canaliculares of the otic capsules. The lateral wing-like plate, which is thus formed on each side, is really the ventral and narrowest part of the squamous portion, and is known as the lamina alaris (Voit) (fig. 5). Ventrally it terminates in the prominent processus paracondyloideus, (figs. 2, 3 and 4) which may be seen from the front projecting laterally from the outer surface of the condyloid portion; dorsally . it broadens into the squama, which becomes steeper, and swings medially to pass into the tectum posterius (fig. 5). The upper border, after skirting the dorsal surface of the ear capsule, passes backwards and inwards, being continuous above with the lower border of the parietal plate; the lower border forms the lateral and dorsal boundary of the primitive foramen magnum.
When regarded from the front (fig. 2) the lateral or condyloid portions appear as paired, caudo-lateral extensions of the planum basale, their outer surfaces being simply continuations of the ventral convex surface of the planum; or, stating the same thing another way, if the anterior surface of the planum be regarded as a section of a cylinder, then the outer surfaces of the lateral portions may be looked upon as localized widenings of the same. The upper boundary of each may be arbitrarily marked off by a line drawn from the tip of the occipitocochlear notch to the ventral foraminal prominence (fig. 5), cutting just ventral to the hypoglossal canal, and representing approximately the line of separation which exists between these elements as they occur in their osseous condition at birth. The caudal portion of the planum thus includes the intercondylar incisure. Piercing the outer surface of the lateral portion, which looks ventro-laterally, is seen the outlet of the hypoglossal canal (fig. 2). Upon reaching the external edge of the latter the outer surface becomes narrow, and passes directly outward upon the aforementioned processus paracondyloideus.
Seen from behind, the inner surfaces appear as caudolateral continuations of the side-to-side concavity of the dorsal surface of the planum (figs. 1 and 5). They look medially, dorsally and somewhat cranially, and present the inlet of the hypoglossal canal. The lower borders are by far the thicker, and form the lateral Umits of the incisura intercondyloidea, and each, as has been noted, passes over the ventral foraminal prominence to be thence continued dorsally as the lateral border of the foramen magnum (fig. 2). This portion of the border of the foramen, and the lower border of the condyloid portion, much resemble one another in thickness and roundness on cross-section, and when the skull is regarded from the side the ventral foraminal prominence, formed by their approximation, appears as the apex of an angle directed downwards and slightly forwards (fig. 3) .
The rounded upper border broadens dorso-laterally and bifurcates to enclose the hollow jugular recess (fig. 5), the ventral limb passing laterally to become the ventral border of the processus paracondyloideus (figs 2. and 4), while the dorsal is marked, at its termination, by a small eminence, the anlage of the future jugular tubercle (fig. 5).
The hypoglossal canal, whose inlet and outlet have been noted, pierces the condyloid portion in a direction from within outwards and forwards. It lies rather nearer the upper than the lower border, between the jugular tubercle above and the ventral foraminal prominence below. The right canal is unpartitioned, but the left presents a bar of cartilage which separates its inner third into cranio-ventral and caudo-dorsal inlets. This bar has a general direction from above downwards, forwards and inwards (fig. 5). The outer two-thirds of this canal is not divided.
When the sections are followed from behind forward it is noted that two fasciculi of the hypoglossal nerve come into close apposition, one with the other (though they remain, for a time, separated by their sheaths), just dorsal to the entrance of the hypoglossal canal. These are of about equal size, and pierce the dura as a single strand, to enter the canal (on the left side the caudo-dorsal inlet) after a short sub-dural course. A third strand, equal in size to the first two combined, may be seen to pierce the dura shortly after the first two, but remains separated from the latter (on the left side by the aforementioned septum) while traversing the canal. Upon emerging the strands unite and shortly after their exit they become intimately associated with the vagus. In the canal they are accompanied by some small veins â€” the anlage of the rete of the hypoglossal canal â€” and a small artery. The great bulk of the canal space is, however, filled with loose connective tissue.
The processus paracondyloideus,^ already referred to more than once, forms a conspicuous object as it springs from the 1 It may be here noted that Voit uses the terra "processus paracondyloideus" to apply only to the outer projecting tip (as it is found in the skull of the rabbit) of the structure which I have designated by this term. Mead, in describing the skull of the pig, uses the name "processus paroccipitalis" with the same meaning outer surface of the condyloid portion just lateral to the hypoglossal canal (figs. 2 and 4). The straight line which joins the outermost tips of the processes passes through their roots also, and meets the median sagittal plane at a right angle, thus showing that each process is perpendicular to the sagittal plane of the head. The coronal plane in which this line lies cuts the ventral foraminal prominences. Each process is prismatic in shape, and thus presents three surfaces, which meet at the most lateral point, or tip. The medial part of the cranial surface is hollowed for the recessus jugularis; the lateral part lies outside of the cranium (fig. 4), its convex area forming the outermost termination of the caudal deUmitation of the jugular foramen. Immediately above this convex surface appears the proximal, curved end of the cartilage of Reichert. The remaining surfaces look ventro-caudally and dorso-caudally respectively, and are separated by the caudal border, which projects downwards in a ridge-like manner (figs. 2 and 3), and forms a prominent object, when the skull is regarded from below, as it springs laterally from the outer part of the ventral foraminal prominence. The dorsal border is continuous with the lamina alaris. The ventral border is free (figs. 2, 4 and 5), and is thin in its medial half, where it bounds the recessus jugularis ventrally. From within outwards it follows a curved line, convex ventrally, and, in the region of the recessus jugularis, there is a small cranial concavity, over which the jugular vein and accompanying nerves pass. As has been observed before, this ventral border is the anterior extension of the bifurcated upper border of the condyloid portion. The left process presents a slight difference when compared with the right. A small foramen (figs. 2 and 5) is seen to tunnel under its ventral border, thus forming a passageway from the recessus jugularis within to the ventro-caudal surface of the process on the outer aspect of the skull. The outlet lies just lateral to that of the hj^oglossal canal. Though the right side * as Voit gives to the term "processus paracondyloideus." I have selected the latter term, and used it in a more extended sense, as applying to the entire structure corresponding to the transverse (and perhaps costal) process of the occipital vertebra, since this represents a morphological unit.
does not present this foramen the cartilage in this locality is very thin. The foramen contains nothing but loose connective tissue and its direction is from within downwards and forwards. It may be known as the paracondyloid foramen, and appears in the model of Hertwig on the right side only.
The squamous portions form the dorsal, and most of the lateral, part of the occipital ring. The architectural, and, as we shall later see, possibly the developmental foundation of each half, is the crescentic bar of cartilage which forms the lateral boundary of the primitive foramen magnum, and extends between its two prominences (figs. 1, 2 and 5). Rounded in cross-section it is seen to diminish in size gradually and uniformly from before backwards. Ventrally it is directly continuous with the condyloid portion (and may even be looked upon as a backward extension of this), the area of union being marked by the anlage of the future condyle, which has been termed the ventral foraminal prominence. Its principal direction is dorsal and slightly cranial, in contrast to that of the condylar portion, which, as has been noted, is caudal, lateral and slightly dorsal. Its concavity looks medially and slightly caudally, the latter curvature being evident when the skull is regarded from the side (fig. 3). Dorsally it terminates in the paired dorsal foraminal prominences, which mark the entrances into the incisura occipitalis superior. As will be seen later this bar corresponds to the neural arch of the occipital vertebra, and will he hereafter referred to as such. Medial to it is the anlage of the future medulla oblongata. Just above its ventral portion appears the jugular tubercle, and, upon examining the cartilage in this location, the cells are seen to present, from the dorsal part of this tubercle to a point about midway between the foraminal prominences, a condition similar to that which obtains in the central part of the basilar portion. This would seem to point to the beginning endochondral ossification of the ex-occipital portion of the occipital bone, the center for which, according to Mall ('06) appears on the 56th day. The ventral part of this center is confined to the jugular tubercle, but, as the sections are followed backward, it is found that it gradually comes to involve the entire core of the neural arch. The involve merit in ossification of the jugular tubercle is to be noted, as this, as we shall later see, is probably to be regarded as the superior articular process of the occipital vertebra.
Springing laterally and dorsally from this neural arch we find the upward-shelving squama, which narrows ventrally into the lamina alaris, and dorso-medially participates in the formation of the tectum posterius. Its lateral portion is widened. The upper border of the squama may be divided into ventral and dorsal portions, the former being connected with the otic capsule, and the latter with the parietal plate (figs. 3 and 5). The ventral part is fitted closely to the caudal and dorsal surfaces of the pars canalicularis of the otic capsule, the line of union being crescentic in shape, with concavity looking upward, forward and outward. This border extends cranio-dorsally from the outer angle of the jugular foramen to the fissura capsulooccipitalis (Voit) , and its position is marked on both the inner and outer aspects of the skull by crescentic grooves, formed by the approximation of the flattened occipital and rising otic surfaces. These furrows, which may be known as the medial and lateral capsulooccipital grooves (figs. 3 and 5), are not equally well marked, that on the inside of the skull being much the deeper. It contains part of the transverse sinus.
Evidence of an earlier separation between the pars canalicularis and the squama is afforded by the microscopic appearance. Between these structures there is seen, ventrally, a thin sheet of perichondrium, its plane being parallel with the transverse planes of the head, and when the sections are followed dorsally this is found to give place to a cartilage of younger type than that surrounding it, this being traceable almost as far back as the capsulooccipital fissure. The younger condition of the intervening tissue in the region of the jugular foramen as compared with that farther back would seem to indicate that fusion of the parts has taken place in the more dorsal part first, and has gradually progressed forward, and this assumption is born out by examining the illustrations of Levi. In his 14 mm. model (which is the earliest stage in which the otic capsule appears) the pars canalicularis and squama are almost entirely separated by an elongated fissure extending from the jugular foramen to a small bridge of cartilage which cuts off the capsulo occipital fissure, and it is believed that this long cleft represents the metotic fissure of the lower forms. Upon examining the illustration of Levi's 17 mm. stage it is found that the aforementioned small bridge of cartilage has considerably widened ventro-medially, and in the subsequent 28 mm. stage the squama and the pars canalicularis are united as far as the jugular foramen. In both the 17 and the 28 mm. stages of Levi the union was marked by a separating sheet of perichondrium.
At the caudo-ventral extremity of this union there is seen a small notch, passing laterally from the outer part of the jugular foramen to lose itself upon the external surface of the skull, just above and behind the tip of the paracondyloid process.
It may be noted that the otic capsule, between its dorso-lateral connection with the squama and its ventro-medial connection with the planum basale, forms a bridge, uniting these structures, roofing the recessus supraalaris, and affording an upper delimitation for the foramen jugulare.
Dorsal to the capsulooccipital fissure the upper border of the squama proceeds backwards and inwards, and describes a curve with concavity upwards, to reach a small eminence, seen in the Hertwig and other models, which may be known as the dorsal occipital prominence (figs. 1 and 2). Beyond this it falls away to join with the upper border of its partner of the opposite side, this junction resulting in the formation of a dorsal concavity, directed upwards, which marks the upper edge of the tectum. Between the capsulooccipital fissure and the dorsal occipital prominence the squama is continuous cranially with the parietal plate. The line of union of the two lies at the bottom of a groove, seen from the inner aspect of the skull. It may be known as the occipitoparietal groove (fig. 1) and presents, on the right side two perforations, on the left one, through which small veins pass. The paired foramen (known as the occipitoparietal fissure, (figs. 3 and 5), is the larger, is elongated, and is situated about midway between the extremities of the groove. It perforates the groove at the most caudal part of its course. The smaller foramen, which is limited to the right side, appears just in front of the larger. On the right side the terminal fourth of the upper border of the squama is separated from the slender dorsal tip of the parietal plate by a narrow slit. The parietal plate appears to end freely, but dorsal to this there are what appear to be degenerating cartilaginous cells, connecting the end of the parietal plate with the squama. This may indicate that these structures were united at an earlier time.
I regret that the dorsal extremity of the head of my embryo is missing, and that I am, on that account, unable to ascertain the condition in this region. On the right side the sections terminate in the dorsal occipital prominence, and show that the parietal plate has come to an end before this, as described. Owing to the fact, however, that the sections were cut obliquely, being deeper on the right side than on the left, I am unable to say whether or not the termination of the left parietal plate, and the relations which it bears to the squama, are the same as those found on the right side. I have, however, assumed that they are, and have so constructed my model; this having been done there was only the gap between the dorsal occipital prominences to be filled in, and this I did by reference to the Levi illustrations and Hertwig model. There are indications, on the left side, that the separation of the posterior extremity of the parietal plate will take place, as it has on the right, the cartilage connecting it to the squama, in the last few sections, being very thin.
In the membrane just lateral to the tip of the right parietal plate and dorsal occipital prominence there appears the weakly staining spicule of the interparietal bone. I have not represented it in the model, since only a small fragment is available, the remainder being included in the missing sections.
The sections go back sufficiently far to show that the occipital squamae unite dorsal to the tip of the superior occipital incisiu-e,to form the tectunf posterius, thus differing from the findings of Voit ('09) in lepus, who states that the squamae never reach the midline, the edges of the superior occipital incisure being the dorsal borders. I am, of course, unable to ascertain the exact width of the tectum.
The ventral part of the lower border has been described as the neural arch. Dorsal to this the character of the border changes completely, for, after the dorsal foraminal prominence is passed it loses its thickness and roundness on section, and becomes thin and serrated. This portion borders the superior occipital incisure, and really represents the lower portion of the original dorsal border, the upper portion being united to form the tectum posterius. The latter, according to Levi's investigations, is formed somewhere between the 14 and 17 mm. stages by dorsal fusion of the squamae.
The surface outlined by the above-described boundaries has been seen to become narrower ventrally in the lamina alaris (fig. 5). This is a wing-like plate of cartilage, bounded ventrally by the processus paracondyloideus. The outer edge of its upper surface bears the caudal extremity of the pars canalicularis of the otic capsule, and medial to this the upper surface, which looks principally cranially, particijiates ventrally in the structure of the hollow recessus jugularis, and forms the floor of* the recessus supraalaris. The latter is a cleft between the lamina and the overhanging pars canalicularis of the otic capsule (fig. 5) through the lateral part of which the transverse sinus passes. The lower surface of the lamina alaris is also hollowed in the more central parts of the paraforaminal area (fig. 2, p. 372), and the plate is, therefore, quite thin â€” in fact on the right side it is perforated by a minute foramen, just lateral to the neural arch (fig. 1), and posterior to the jugular tubercle, through which a small vein passes. This foramen is doubtless the representative of the condylar foramen of the adult condition. Dorsally the lamina alaris becomes wider, thicker, and more vertical in slope, as it passes into the larger posterior portion of the squama. The inner surface is smooth, and presents, dorso-lateTal to the tip of the superior occipital incisure a small oval foramen on the right side, but not on the left, through whichÂ»a small vein passes (fig. 1). A very shallow groove runs parallel to the neural arch, just lateral to its upper aspect.
Upon the outer surface the most prominent object is a rounded eminence, â€” the lateral occipital eminence â€” (fig. 3) which appears immediately dorsal to the otic capsule, and is separated therefrom by the cranial portion of the lateral capsulooccipital groove. Its posterior part projects slightly into the cranial cavity (fig. 5), and its edges are ill-defined, fading gradually into the surrounding cartilage. This marks the thickest part of the squama. It may be followed caudally as a low ridge on the outer surface, and is seen to meet, almost at a right angle, a second ridge, which extends from the tip of the paracondyloid process to the dorsal foraminal prominence, and to which the name crescentic ridge (figs. 2-3) may be given, the point of union being about the center of the latter.
The crescentic ridge, seen only on the outer surface of the skull, is a low elevation which sweeps dorsally and medially, between the extremities above mentioned, and in so doing describes a curve with convexity backwards and outwards. At its ventral end it is sharply defined, and is separated from the ear capsule by the small notch which stretches outward from the jugular foramen. Dorsally it is wider, and its margins are not so clearly outlined. It is of considerable thickness throughout, and is the lateral boundary of a semi-crescentic area â€” well seen in E. Fischer's ('03) illustration of the skull of macacus cynomolgus, and also in Voit's picture of the skull of lepus â€” which may be known as the paraforaminal area (fig. 2) . It is bounded medially by the rounded edge of the neural arch, and ventrally by the caudal border of the paracondyloid process. Its hollowed central portion, which represents the lower surface of the lamina alaris, has been before referred to, and on the right side has been seen to be perforated by the minute condylar foramen. We find, therefore, that the thickest parts of the squama are situated immediately behind and below the ear capsule, and are represented by the lateral occipital eminence and the crescentic ridge, the former, just behind the capsulooccipital fissura, being slightly the thicker. The area dorsal to a line passing from the dorsal foraminal prominence to the occipitoparietal fissure is very thin, and presents a short distance behind this line evidence of commencing ossification. The cartilage appears to be undergoing greatest change in the region immediately dorso-lateral to the tip of the superior occipital incisure, and dorsally the modified cartilage may be traced as far as my sections go. This area no doubt represents the center of ossification of the supraoccipital, which according to Mall ('06) ossifies from four centers, the first pair appearing in the region immediately dorsal to the foramen magnum on the 55th to 56th day, and being followed by other paired centers in the region ventral to this.
In all there have been noted in the occipital region five commencing centers of ossification, one, median, for the basilar portion; two, lateral for the condyloid portions, and two, lateral, for the squamous portions. In the vicinity of these centers the cartilage grades off into that surrounding them. Of the rest of the cartilage it may be noted that the material of the ventral border of the paracondyloid process, of the ventral part of the jugular tubercle, and of the body of the condyloid portion lateral to the hypoglossal canal shows a more advanced character than the remainder.
The atlantooccipital capsules are present; each is a sac composed of a dense sheet of perichondrium, covering the applied facets of the atlas and condyloid portion of the occipital cartilage. They are richly cellular, and above them the cartilage is of a younger type than that found in the remainder of the mass, but gradually grades off into the more mature type.
The primitive foramen magnum (figs. 1-2) is, at this stage, worthy of examination. It consists of three parts; a large central area, with which are contjinuous ventral and dorsal incisures. The central portion is bordered by the crescentic, rounded neural arch, and its plane looks upward and slightly forward. Ventrally is to be seen the intercondyloid incisure, its plane looking dorso-cranially, and this plane forms an angle with the main portion, which is open cranio-dorsally. This incisure remains a part of the foramen, later undergoing widening, and rounding out at the tip, as may be seen by examining the Hertwig model and the osseous skull. Behind the central portion, and appearing as a dorsal prolongation of it, is the superior occipital incisure, filled by the membrana atlantooccipitalis dorsalis. The plane of this incisure is somewhat steeper than that of the part in front, and thus it forms an angle, looking cranially and slightly ventrally, with that of the main area. This is shown upon the lateral contour of the skull as the point of an angle, directed downward and backward, marking the tip of the dorsal foraminal prominence (fig. 3).
The superior occipital incisure, though representing the dorsal part of the primitive foramen magnum, is by no means a part of the adult foramen magnum, as its edges unite later to complete the caudo-dorsal closure of the occipital region, the dorsal limit of the foramen magnum becoming set by the approximation of the dorsal foraminal prominences, as shown by the researches of Bolk ('04).
I find no evidences of the condition which Bolk describes at the posterior extremity of the foramen magnum, viz., a central cartilaginous mass (formed by the fusion of paired pre-existing masses) which lies between the upturned dorsal extremities of the occipital side-walls.
It seems clear from the work of various investigators, beginning with Froriep ('86), that although the occipital region of the mammals has been developed from the skeletogenous elements of four metameres, only the most caudal ever attains the status of a mature sclerotome, the three cranialmost being undifferentiated and playing but a minor part in the construction of the adult bone. This being true it follows that the story of the evolution of the occipital anlage is largely the story of the development of the caudal segment, or, as it will be hereafter called, the occipital scleromere or primitive occipital vertebra.
As in the spinal region, so in the occipital, the sclerogenous tissue passes through successive and overlapping membranous or blastemal, chondrogenous and osteogenous phases (Bardeen '05, '08), and in each phase the condition in the occiput recalls that of the corresponding phase in the vertebrae. Thus in the blastemal stage the occipital scleromere shows paired chordal processes joining across the midline in the region of the notochord, and paired neural processes embracing the neural canal as in the spinal scleromere; the costal processes are, apparently, poorly developed in the occipital scleromere. But along with this marked similarity to the vertebrae the occipital scleromere shows certain individual peculiarities. Instead of uniting with the cranial portion of the segment immediately caudal to it, after the fashion of the other scleromeres, it retains its connection with the cranial portion of its own segment, and this, in turn, becomes united with the tissue of the three cranial, undifferentiated sclerotomes, the membranous anlage thus formed being known as the occipital plate (Bardeen '08 and '10). According to Froriep the occipital scleromere is marked off from the undifferentiated sclerotomes by the caudal root of the hypoglossal nerve.
The middlepiece of the occipital plate is made up in its caudal portion of the chordal processes of the occipital scleromere, and in its cranial part it also contains the elements of the body masses of the undifferentiated segments. So also while the lateral portions are mainly formed of the neural processes of the occipital scleromere they also contain, in the region cranial to the hypoglossal canal, remnants of the condensed lateral masses of the undifferentiated segments (Froriep, Levi).
In the chondrogenous stage of the occipital anlage of man, for the knowledge of which we are principally indebted to Levi, there are also striking resemblances to the vertebral conditions. The 13 mm. stage, studied by this author, shows the beginning of the transition from the membranous to the cartilaginous condition and in this paired masses of condensed chondrogenic mesenchyme, separated by the perichordal septum, were situated dorso-caudally in the middlepiece or basilar portion, and in the dorsal part of each mass, medial to the hypoglossal nerve, a small cartilaginous nodule occurred, recalling the paired chondrous centers of the body of a spinal vertebra. The mesenchymatous masses, representing the chordal processes of the occipital scleromere, were joined ventrally ; they are the first portions of the occipital anlage, and indeed of the entire chondrocranium, to undergo chondrification, and eventually form the diverging, caudolateral portions of the pars basilaris bordering the foramen magnum. In the matter of priority in time of chondrification of the body over the arch processes the occipital scleromere resembles those of the atlas and axis.
In the 14 mm. stage of Levi chondrous paired centers for the neural processes of the occipital vertebra arise, lateral to the hypoglossal nerve roots, and speedily unite by continuity of cartilage with the nodules just mentioned, which also join with one another in the same way, at first ventrally. The fused basilar centers are a little later joined by the chondrifying middle part of the undifferentiated sclerotomes to form the median part of the pars basilaris, and the chondrifying lateral masses of the undifferentiated sclerotomes join the neural processes. Thus the basilar portion, which may be taken to extend to the ventral margin of the hypoglossal canal is built up from the middle parts of all of the primitive occipital segments, the portion included in the anterior margin of the foramen magnum arising from the body mass of the occipital scleromere, and the cranioventral part coming from the body masses of the undifferentiated sclerotomes.
A lateral outgrowth from the neural arch is seen in a membranous condition in the 13 mm. stage of Levi, and is somewhat later in chondrifying than the neural arch of either the occipital vertebra or of the atlas. It represents, doubtless, the transverse process of the occipital vertebra, and is spoken of by Levi as the Querleiste. The costal process of the occipital vertebra has not been shown to have a separate center of chondrification.
It is to be noted that in the 13 mm. stage of Levi the bodies of the occipital vertebra, atlas and axis are represented by paired masses of chondrogenic mesenchyme, separated by the perichordal septum, and each mass contains a small nodule of cartilage (except in the case of the axis, where the two nodules have fused), their neural arch processes being entirely membranous. In the 14 mm. stage of Levi not only are the bodies chondrified but also the neural arches; for in the occipital vertebra a chondrous center appears just lateral to the hypoglossal foramen, and the arches of the atlas and axis also present each a small nodule of cartilage. Thus chondrification takes place simultaneously in corresponding parts of the occipital and first and second spinal vertebrae.
The paired cartilages of the body of the occipital scleromere of Levi's 14 mm. stage appear to be slow in joining dorsally, being found separated in this region by the perichordal septum, while their ventral parts are united. This is possibly to be explained as a result of expansion in this region, from intracranial pressure.
The above identification of the cartilaginous neural arch of the occipital vertebra and its appendage, the transverse process, as found in the early Levi models does not agree with this author's own interpretations of his findings. In his earliest human skull, from a 13 mm. embryo, Levi shows, lateral to the roots of the hypoglossal nerve, what he calls the lateral portions, from whose lateral surfaces the Querleisten project directly outward, and he figures the latter in all four of his stages. In the 14 mm. stage he finds in each lateral portion, just external to the hypoglossal nerve roots (Levi, text fig, 2) a cartilaginous center, which speedily joins with that for the body mass, which latter, as has already been noted, represents the chondrification of the body of the occipital scleromere. It seems evident that the center of chondrification in Levi's lateral portion is the center for the neural process of the occipital scleromere, with possibly the addition of the center for the undifferentiated portion, and hence it follows that the club-like membranous mass in which this nodule is found is the neural process of the occipital scleromere, which, with its partner, builds the lateral part of the neural canal of the occiput. Furthermore, these lateral portions are in direct alignment with the spreading arch-processes of the underlying cervical vertebrae, as is shown by the illustrations of Levi, and, though this author does not label them as the arches of the occipital vertebra, Bardeen, in his copy of Levi's illustration of his 13 mm. stage in Keibel and Mall's "Human Embryology" (vol. 1, p. 401) gives them what I regard as the c6rrect designation, "Arcus vert, occip." Though the lateral portions of the occipital vertebra are here considerably larger than the arches of a cervical vertebra this extra size is probably a local adaptation. Again, at the same time (14 nfim. stage) according to Levi the arch mass of the undifferentiated sclerotomes has chondrified. It would, indeed, be surprising if the neural arch of the occipital vertebra were still membranous at a time when the arch processes of the undifferentiated sclerotomes were chondrified (those of the atlas and axis being also chondrified), as would be the case if Levi's interpretation were correct. My identification obviates this difficulty.
Accepting this interpretation it follows that the small processes or Querleisten which project laterally from the lateral processes cannot be the tips of the neural arches, as Levi describes them; they are really the anlagen of the transverse processes of the occipital vertebra, Levi, who does not account for the dorsal tips of the lateral portions of the occipital vertebra at all, is led to conclude that the Querleisten represent the tips of the neural arches, apparently, by the histological resemblance of their tissue, in the early stages, to that composing the tips of the neural arches of the underlying cervical vertebra. He remarks, however, that the Querleiste is very tardy in chondrifying when compared with the neural tip of the atlas, a detail which is, if anything, opposed to his identification of it as the tip of the neural arch of the occipital vertebra, but is what might be expected if it be homologized with a transverse process. The Querleisten are shown in the Levi models to be in direct alignment with the transverse processes of the cervical vertebrae, and they never come together dorsally and unite, after the fashion of the dorsal extremities of the neural arches. They are identical with what I have called in my model the paracondyloid process, following Voit, who described similar structures in the rabbit, and identified them as the representatives of the transverse processes, also remarking that the rectus capitis lateralis muscle, which each has attached to its lower surface, is to be regarded as the morphological equivalent of an intertransversarius muscle. Mead ('09), too, finds a similar and very strongly marked process in the skull of the pig, and calls it the paroccipital process. Both Levi and Voit state, correctly I befieve, that the process ultimately becomes the jugular process of the occipital bone. .
My identification of the primitive elements forming the pars basilaris also is not exactly in agreement with Levi's, but the difference depends largely on where the line between the basilar and lateral portions of the occipital anlage is drawn. 1 have assumed the separation between these portions to be approximately as it exists at birth, while Levi includes in his lateral portions the nodules which I believe represent the paired body of the occipital vertebra. If my interpretation of these structures is correct they should be regarded as constituents of the basilar portion, which would thus represent the body masses of the undifferentiated sclerotomes plus the body mass of the occipital vertebra, while according to Levi's view it would represent only the body masses of the undifferentiated vertebrae.
In the later development of the chondrogenous stage the forerunners of the individual features of the occipital bone begin to show themselves, and we find cartilaginous representatives of the body, pedicles, inferior and superior articular processes, transverse (and possibly costal) processes, laminae and spinous processes, these almost altogether differentiating from the occipital scleromere. The development of the body (pars basilaris) has already been discussed. The pedicles are, of course, represented by the cartilaginous tissue in the region of the hypoglossal foramen, but here we have material added from the lateral mass of the cranial sclerotomes, (Froriep, Levi), which results sometimes in the partitioning of the foramen, as may be seen on the left side of my model, and as has frequently been found by other observers in young embryos of homo and other mammals. The inferior articular process is, doubtless, represented by what has been designated the ventral foraminal prominence, the forerunner of the condyle, though the condyle of the mature bone is partly formed by the pars basilaris. The rudiment of the superior articular process is, perhaps, to be seen in the jugular tubercle. The transverse process, as we have seen, is to be found in the paracondyloid process, and there is some evidence to indicate that in this latter there may be included the costal process as well. It will be remembered that the paracondyloid process was perforated on the left side by the paracondyloid foramen, and that on the right side the corresponding area of cartilage was thin, and, further, that the Hertwig model also presented this foramen on one side. It pierces the process from above downwards and forwards and is in series with the costo-transverse foramina of the cervical vertebrae below. If it be regarded as the costo-transverse foramen of the occipital vertebra, then the bar of cartilage which closes it in front must be looked upon as the costal process of this vertebra. I have not been able to find that the costal process of the occipital anlage has a separate center of chondrification as it has in the spinal vertebrae. Though there is no vessel or nerve passing through the paracondyloid foramen yet it is possible that this represents an old channel of the vertebral artery, which has become obsolete on account of the change of course of this vessel. In this regard the foramen may be analogous to the costotransverse foramen of the 7th cervical vertebra, which no longer transmits the vertebral artery.
I regard as the representatives of the laminae the crescentic, tapering, hornlike masses of cartilage which form the lateral borders of the foramen magnum, and which I have spoken of as the neural arches. Since the squamous portions are continuous with the outer borders of these they may be considered as extensions of the laminae, and the tips of the latter (dorsal foraminal prominences) as the representatives of the spinous processes.
Not only on developmental, but also on histological grounds, does it appear evident that the margin of the foramen magnum is formed from the primitive neural arch of the occipital vertebra. Upon an examination of my slides it is seen that the cartilaginous tissue of this portion bears a strong resemblance to that composing the arches of the upper cervical vertebrae, and even more striking is the situation of the ossification center. It appears, as I have described, in the arch of the occipital vertebra, just dorsal to the root of the transverse process â€” exactly the same relative position as the ossification center in a cervical vertebral arch occupies. This point is beautifully brought out in the model of Hertwig, where, upon either side of the neural canal a series of ossification centers presents itself, the uppermost member of the series being found in the exoccipital, in the position in which I have described it in my model, and being followed caudally by the ossification centers for the 1st, 2nd, 3rd and 4th cervical vertebrae; each center being uniformly situated with reference to its respective arch. In the basi-occipital, too, the single median ossification center recalls the center of a typical vertebral body, and indeed the osseous elements of the occipital bone at birth are strikingly similar to those of a vertebra, especially the fifth lumbar vertebra.
Keeping in mind the intimate and peculiar relationship which a typical vertebral arch bears to the cord which it encloses it seems reasonable to suppose that this relationship would be retained even after the vertebra had been taken up into the skull. It seems much more reasonable, even on purely theoretical grounds, to assume that the arch of the occipital vertebra goes to form the margin of the foramen magnum of the mature skull than to postulate that it forms the jugular process, as Levi maintains, and when it is realized that the latter is formed from the transverse process of the occipital vertebra, and that the margin of the foramen magnum is merely the modified arch of the occipital vertebra the proper relationship of the parts becomes intelligible.
Not only do the neural arches of the occipital vertebra exhibit a striking resemblance to those of the cervical vertebrae in each stage which has been studied, but their behavior in growth recalls very strongly that of the arches below. This parallelism in manner and time of development between the arch of the occipital vertebra and those of the cervical vertebrae is. clearly shown by an examination of the 13 mm., 14 mm., 17 mm., and 28 mm. stages of Levi, my own 40 mm. stage, and the 80 mm. stage of Hertwig. The neural arches of the occipital vertebra, small at first, are seen to grow backwards and outwards, and then to come together medially and dorsally, thus hedging in the dorsal part of the foramen magimm, this process being duplicated coincidently by each of the upper cervical vertebral arches.
In each successive stage the tips of the neural arches, both occipital and cervical (which retain their original alignment), are seen to be farther advanced than in the last in their enclosure of the spinal cord, a condition strikingly brought out by a comparison of the foramen magnum and the underlying vertebral arches of my model with those of the oldest Levi model, on the one hand, and the Hertwig model on the other. In the 28 mm. Levi model the tips of the neural arches of the occipital and upper cervical vertebrae are separated by a considerable interval, in my model they are almost united, and in the Hertwig model, as has been noted, they are all completely joined; in the adult bone the tips of the occipital vertebra are represented by the internal and external occipital crests (representative of a spinous process). This closure of the foramen magnum takes place, accordingly, somewhere between the 40 and 80 mm. stages in man, and it bears a striking resemblance to that of the segments of the spinal canal. Growth seems to progress uniformly throughout the series, and dorsal closure is apparently completed at about the same time in each segment. Thus, with the fusion of the dorsal foraminal prominences there, is completed what amounts to the closure of the cranial extremity of the spinal canal.
From what has been said regarding the formation of the foramen magnum it will be evident that what is found in the 40 mm. stage is something more than the foramen of the adult condition; it is this plus the superior occipital incism*e. Further, the structure described as the tectum posterius is not the dorsal delimitation of the real foramen magnum at all, but merely that of the superior occipital incisure. The edges of the latter unite in the form of a median seam upon the union of the dorsal extremities of the neural arches of the occipital vertebra, and thus is effected the closure of the portion of the floor of the occipital region dorsal to the foramen magnum. This conception of the development of the foramen magnum explains why the primitive foramen is relatively so much larger than the adult condition.
In the condition of the occipital anlage at birth we find a basilar portion, formed in its cranial part from the body mass of the undifferentiated sclerotomes and in its foraminal portion from the body mass of the occipital vertebra. The exoccipitals have been developed, as we have seen, principally from the neural arches of the occipital scleromere, but in the region of the foramen hypoglossi there has been added material from the lateral masses of the undifferentiated scleromere. The supraoccipitals, which ossify separately, are to be regarded as primarily connected with the neural arch of the occipital vertebra, and their separateness of ossification is analogous to the condition which we find in the 5th lumbar vertebra.
The otic region, like the occipital, is transversely somewhat ringlike in form, and its irregular sides, for the most part flattened from within outwards, are united by their caudal edges with the upper border of the occipital anlage, except where sundry foramina occasion interruptions. The otic ring, accordingly, heightens the dorsal part of the cartilaginous brain-case. In it we recognize four distinct elements,, two unpaired, the otic portion of the lamina basalis and the tectum synoticum, and two paired, the otic capsule and the lamina parietalis. In addition to the parts entering into the composition of the ring there are also to be considered in the otic region the small, paired, isolated nodules known as the cartilagines supracochleares, cranii laterales and cranii . posteriores.
When the skull is viewed from within (figs. 1 and 5) the upper or otic portion of the lamina basalis is seen to unite the ventromedian portions of the otic capsules. Passing laterally the eye meets the large, irregular mass of cartilage known as the otic capsule, which forms the ventro-lateral delimitation of the posterior cranial fossa, as well as part of the floor of the middle cranial fossa. Caudo-laterally the capsule is continuous with the lamina alaris, and dorsal to this with the wider portion of the squama of the occipital anlage, while cranio-dorsally the commissura capsuloparietalis (figs. 3 and 5) is seen uniting the larger dorsal portion of the otic capsule with the lamina parietalis the latter having also a union with the capsule below the capsuloparietal fissure. Dorsal to the capsuloparietal commissure the flattened parietal plate appears, and we note that it is wide ventrally, but becomes narrow dorso-medially. With the parietal bone it assists in the formation of the wall of the cranium in this region. Below, the parietal plate is continuous with the upper border of the squama; dorsally it is represented in the model as terminating freely just before reaching the dorsal occipital prominence, but there is microscopical evidence, as far as my sections go (as I have already stated) which seems to indicate that there was here a previous union of parietal plate and squama. Of the unpaired, dorso-median tectum synoticum, described by several authors, I can, unfortunately, make no statement, as my sections for this dorsalmost region are lacking. The otic ring, as I have represented it in the model, is therefore incomplete dorsally. This may possibly be its actual condition, and in this connection it may be noted that Mead states that the otic ring in Sus is incomplete dorsally.
The pars otica of the lamina basalis, which is the most cranial part of the chordal portion of the base of the skull, has already been described. The fissura basicochlearis is incomplete above, being represented by the lowermost part of what I have designated the spheno-cochlear notch - filled with connective tissue and a few small veins. The abducens nerve passes above the notch, lateral to and below the outwardly-projecting posterior clinoid processes, and in this the condition is similar to that described in such mammals as the rabbit (Voit) and pig (IMead), except that the basi-cochlear fissure in the latter types is closed above by a cartilaginous bridge joining the upper surface of the pars cochlearis with the lamina basalis, the abducens nerve passing over this bridge. In the model of Sus by Mead this nerve passes throught a foramen formed by cartilaginous connection of the posterior clinoid process with the cochlea. There is no evidence of this in my preparation.
The connections of the otic capsule with the planum basale, squama occipitalis and parietal plate have been noted. The model also shows it continuous ventro-laterally with the incus, but histologically a sheet of perichondrium intervenes. There is no connection with the processus alaris of the temporal region through the commissura alicochlearis, such as Jacoby shows in the 30 mm. stage of homo and Voit figures in his model of the skull of lepus, but there is what I regard as a rudiment of this, viz., the cartilago supracochlearis, (figs. 1 and 3) which will be later described. The otic capsule roofs over the recessus supraalaris and recessus jugularis, and bridges the foramen jugulare (fig. 5).
In the otic capsule (figs. 6 and 7) we may distinguish a larger dorsolateral portion, which contains the semicircular canals, and which may therefore be known as the pars canalicularis (Voit), and a smaller, ventro-median portion, which contains the cochlear part of the membranous labyrinth, and which may therefore be termed the pars cochlearis. Voit has restricted the use of the term 'pars vestibularis' to the dorsal part of the pars cochlearis, which presents the fenestrae vestibuli and perilymphatica and the fenestrae for the vestibular division of the eighth cranial nerve. It contains the first, or unwound, portion of the ductus cochlearis. I shall adopt this usage of the term in this description.
The cartilage of the two portions is directly continuous, the zone of union being marked cranially by a notch, open above, which may be known as the superior otic notch (figs. 6 and 7), and ventro-laterally by a recess formed by the union of the lateral surface of the pars cochlearis with the ventral surface of the pars canalicularis; this may be known as the ventro-lateral otic recess (fig. 6). It contains the anlagen of the auditory ossicles.
The pars canalicularis is an irregular, somewhat flattened, ovoid mass of cartilage, hollowed for the passageways of the semicircular canals and utriculus. It presents for examination three surfaces, ventral, lateral and medial. Of these the lateral and medial are convex, and are approximated above and behind, their -ventral edges being widely separated. The lateral surface (fig. 6) is smooth, and somewhat triangular in shape, being wider above than below. The cranial border is rounded, and is formed by the out-bulging of the anterior semicircular canal; it is known as the prominentia semicircularis_ anterior. Ventrally this prominence terminates in the prominentia utriculo-ampullaris superior, a conspicuous rounded eminence at the cranio-ventral extremity of the pars canalicularis, marking the upper approximation point of its three surfaces. It is formed principally by the wall of the ampulla of the anterior semicircular canal (fig. 8). Springing upward from the dorsal part of the anterior semicircular prominence the capsuloparietal commissure may be seen, its connection with the otic capsule being shown in figures 6 and 7. Dorsal to this the border is marked by the capsuloparietal fissure (fig. 3), and caudal to this again by the lower union of the parietal plate with the otic capsule; under the latter union is to be noted the capsulooccipital fissure (fig. 7). The lowermost part of this border is formed by union with the squama. These borders separate the lateral from the medial surface of the capsule.
Both the capsuloparietal and capsulooccipital fissures appear in other models of the human skull (Levi, Hertwig), and they have also been shown to be present in the primitive skulls of other mammals, as the ape (Fischer) and rabbit (Voit). The capsuloparietal fissure is sometimes known as the foramen jugulare spurium, and the capsulooccipital fissure as the foramen petrosooccipitale.
The ventral border, which separates the lateral from the ventral surface, is, below the superior utriculoampullary prominence, marked off mainly by the conspicuous crista parotica, below this by the mastoid process, and below this again by the prominentia semicircularis posterior (fig. 6), which passes over the root of the mastoid process at this point.
The most prominent object upon the lateral surface is the lateral otic eminence (fig. 6), which lies in its dorso-cranial area, separated from the dorsal part of the anterior semicircular prominence by a very shallow groove. It slopes backward into the parietal plate between the two post-otic fissures, and is formed by the backward and outward projection of the massa angularis, a large mass of cartilage lying in the enclosure formed by the anterior and lateral semicircular canals, the crus commune, and the upper part of the posterior canal (fig. 8). The two lateral otic eminences mark the extremities of the greatest transverse diameter of the primitive skull.
The dorsal extremity of the posterior semicircular prominence forms a gentle rise in the caudal area of the lateral surface, and then passes over the root of the mastoid process, as we have seen, to become prominent in its ventralmost portion, the prominentia utriculoampullaris inferior (figs. 6 and 7), which forms the conspicuous border between the ventral and medial surfaces and acts as the upper border of the lateral part of the jugular foramen.
The prominentia semicircularis lateralis is an indistinct swelling passing downward and backward from the crista parotica in the region of the incus to the dorsal extremity of the posterior semicircular prominence.
The medial surface (fig. 7) is more extensive than the lateral. Its cranial and dorsal borders are the same as those of the lateral surface; its ventral border is marked above by a rounded ridge passing downwards from the superior utriculoampullary prominence to the superior otic notch, and below by the posterior semicircular prominence, which, as has been seen, terminates ventro-medially in the inferior utriculoampullary prominence, the latter bearing a ventrally-projecting process, the processus interperilymphatica (Voit). The middle portion of the ventral boundary is formed by the transition of the medial surface of the pars canalicularis into that of the vestibular portion of the pars cochlearis. As has been mentioned the medial surface is convex, and presents in its central area as its most prominent object the prominentia cruris communis (Voit), formed by the crus conmiune within (fig. 9). Upon the dorsal part of this prominence is seen the long, almost horizontal, slit-like foramen endolymphaticum, for the outlet of the ductus endolymphaticus. Both lips of this foramen are formed of a young type of cartilage and it may be noted that the upper lip projects medially in its dorsal part to ovei^hq-ng the duct, and is continued dorsally past the foramen to form a groove, in which the duct lies (fig. 7). The dorsal extremity of this upper lip appears as a short, free process, overlying the duct.
The condition of the terminal portion of the ductus endolymphaticus is of interest. This does not end in a sac, but becomes a long, narrow fusiform dilation shortly after emerging from the foramen, and gradually decreases in size, to be prolonged, at its dorsal extremity, into a fine, lumenless filament or cord of cells. After leaving the foramen endolymphaticum it passe's medial to the transverse sinus in the sub-dural space, outward and backward, and ends in the loose sub-dural connective tissue just medial to the occipitoparietal groove, about 1.8 cm. dorsal to the capsuloparietal fissure. It is not intimately associated with the cartilage of the ear capsule after its exit therefrom, and hence cannot retard the development of this locally to bring about a thinness of the wall, which is found in my preparation just dorsal to the endolymphatic foramen, in the area corresponding to that in which the small foramen which Voit describes in the developing otic capsule of the rabbit appears. This thin region of the wall (which is unperforated) is caused by encroachment upon it from within of the cavities surrounding the dorsal extremities of the anterior and posterior semicircular canals, and not by pressure of the saccus endolymphaticus from without, as Voit assumes in the skull of lepus.
The upper part of the medial surface is marked by the crescentic inner aspect of the anterior semicircular prominence (fig. T), which is more distinct here than on the lateral surface, and sweeps backwards, from the superior utriculoampullary prominence to the dorsal end of the prominence of the crus commune. It corresponds to the arcuate eminence of the adult bone. Below this prominence is to be seen a distinct fossa, the fossa subarcuata anterior (Voit), delimited caudally by the prominence of the crus commune. This fossa invades the substance of the massa angularis, and upon examining the slides microscopically it is found that it is filled with a mass of loose connective tissue, covered by the dura.
The medial surface below the prominence of the crus commune looks downward, backward and inward in its upper portion, and almost directly downward in the lower. The latter is thin, composed of more darkly staining cartilage with thickset cells and little ground substance, and forms the roof of the supraalar recess. In its ventral area may be seen the inferior utriculoampullary prominence, continuous dorso-laterally with the posterior semicircular prominence and ventrally with the short interperilymphatic process. A spur of the inferior utriculoampullary prominence caused by a localized thickening of the wall, projects backwards and upwards as a low ridge to disappear somewhat below the endolymphatic foramen. It overhangs the transverse sinus in this region. The fossa subarcuata posterior, which Voit mentions in his description of the skull of the rabbit, is not represented here.
The boundaries of the ventral surface (fig. 6) have already been described in connection with the discussion of the ventral boundaries of the lateral and medial surfaces. Its medial part is concerned with the junction of the pars canalicularis with the vestibular portion of the pars cochlearis. The lateral part of the ventral surface forms the dorsal wall of the ventro-lateral otic recess, which has been already referred to, and which contains the structures entering into the formation of the middle ear.
In the cranial area of the ventral surface there appears, projecting forward from the ventral surface of the superior utriouloampullary prominence, a distinct, almost vertical, ridge (figs. 5 and 6), which lies immediately medial to the body of the malleus, but is separated therefrom by a sheet of connective tissue. This represents the medial part of the cartilaginous tegmen tympani, or processus perioticus superior. The lateral portion of the tegmen, such as is shown in Voit's model of the skull of lepus, is not present, but its position is indicated by a low ridge, which arches downward and outward from the upper end of the medial portion of the tegmen and marks off the ventral from the lateral surface, terminating below in the crista parotica (fig. 6). The cartilage is not developing rapidly in this location, as in Voit's specimen. Upon examining the model of Hertwig it is found that the tegmen tympani has grown forward and outward to overlie partially the bodies of the malleus and incus, but the lateral portion has evidently made no further development, and so it may be concluded that the ossicles in man do not occupy a deep cartilaginous recess formed by the tegmen tympani as they do in the rabbit. In the oldest rabbit skull examined by Voit the lateral portion of the tegmen had become quite a prominent plate, covering the ossicles, and reaching out toward the lateral wall of the middle cranial fossa. It appears evident that the tegmen tympani is rudimentary in man.
The crista parotica (figs. 2, 3 and 6) forms a conspicuous object upon the border between the ventral and lateral surfaces. It is narrower, as well as more prominervt, below than above, and its edge shows younger cartilage than the adjacent regions. The cartilage of the incus, though in the model it appears to be attached, is really quite separate from that of the ear capsule, there being an intervening sheet of perichondrium.
The lowermost part of the ventral surface lies in a somewhat more posterior plane than the upper, and forms the dorsal wall of a small recess, open in front and below, bounded laterally by the crista parotica and medially by the interfenestral septum of the vestibular portion of the parscochlearis(or promontorium). In the upper and medial part of this recess appears the lower portion of the fenestra vestibuli, while in the lateral portion, sheltered by the lower part of the crista, the facial nerve is to be found, this region becoming later the lower part of the facial canal or aqueduct of Fallopius. The proximal end of the cartilage of Reichert may be seen just medial to the lower extremity of the crista (fig. 2).
Just below the crista, and separated from it by a small notch, there appears, on the right side, a short, free, anteriorly projecting conical spur of cartilage, slightly younger in character than that of the adjacent otic capsule, and representing the mastoid process of the adult condition (fig. 6). Its substance is directly continuous with that of the ear capsule dorsally, but medially it is separated therefrom by perichondrium. On the left side the same formation is to be seen, except that a portion of the intervening sheet of perichondrium is, near the point of the process, replaced by cartilage. Immediately medial to each process is to be seen the origin of the stapedius muscle.
A brief word as to the course of the facial nerve may here be in place. After entering the internal acoustic meatus it traverses the facial foramen in a direction outward and slightly forward and enters the ventro-lateral otic recess. Here it becomes associated with the geniculate ganglion from which the great superficial petrosal nerve may be followed forward. Leaving the geniculate ganglion the facial nerve now passes downward and slightly outward over the large cartilaginous bar which unites the pars cochlearis with the pars canalicularis and which is found between the facial foramen above and the vestibular fenestra below; thence it proceeds backward over the incudostapedial articulation. It now is to be found just medial to the crista parotica, and runs steeply downward, the relatively small stapedial muscle lying medial to it here. Passing lateral to the upturned end of the cartilage of Reichert, just between the latter and the lower tip of the crista* parotica, it gives off the chorda tympani, and turns suddenly forward, following the line of the shaft of Reichert' s cartilage, being situated shghtly above and lateral to it, and almost immediately lateral to the auditory or Eustachian tube. The relations of the facial nerve at the proximal end of Reichert's cartilage are those shown in Low's ('09) plate, figure 3. The chorda follows its wellknown course through the middle ear anlage.
It is to be noted that the facial nerve does not go through a secondary facial foramen formed by the connection of the tegmen tjrmpani ventrally with the cochlea, as is the case in the rabbit (Voit), and hence there is no true fovea genicularis in the skull of man in this stage, or, indeed, in any stage, judging from the evidence at hand.
The slightly younger condition of the cartilage along the ventral margin of the crista parotica would seem to indicate that the facial canal was closing here, but in the Hertwig model it is still ooen at this region.
The walls of the pars canalicularis are for the most part thin, and composed of mature cartilage. The largest mass of cartilage is formed by the massa angularis, mentioned above, the ventro-median side of which lies immediately lateral to the fossa subarcuata anterior, while the dorso-lateral side projects outward as the lateral otic eminence. Within the mass, just below the floor of the anterior subarcuate fossa, there is a small isolated cavity, and as this is within the arch of the anterior semicircular canal I regard it as a remnant of a portion of the otocyst, which is, as yet, unclosed here. The ground substance of the cartilage of this mass is abundant and pale staining, the nuclei being relatively scattered, and surrounded by capsules not increased in size.
The remainder of the cartilage of the pars canalicularis is made up of masses filling the interstices between the canals and ampullae.
The pars cochlearis is the anterior and smaller part of the otic capsule, and lies immediately lateral to the upper end of the basal plate (figs. 1 and 2). Like the pars canalicularis it is of flattened, ovoid form, and contains the sacculus and ductus cochlearis. Upon it we may recognize two principal surfaces, medial and lateral, to which may be added a third or caudal surface, made up of the structures in the ve^ibular portion surrounding the foramen perilymphaticum.
The medial and lateral surfaces are separated by a rounded border, which runs from the perilymphatic foramen below, around the ventral part of the pars cochlearis, over the cranial pole, and thence backward to terminate by passing over the suprafacial commissure to become continuous with the pars canalicularis at the superior otic notch. The lowermost part of this border is deflected outward to form the promontory; within it is the first and uncoiled part of the cochlear duct, and it is known as the prominentia cochlearis inferior (Voit) (figs. 2, 6 and 7). This prominence passes at first inward, forward and upward, then almost directly upward to reach the cranial pole, and finally passes backward into the prominentia cochlearis superior (Voit) (figs. 1, 6 and 7) as the cranialmost border of the pars cochlearis, which roofs the coiled part of the cochlea (fig. 8), is called. Above the cranial pole the cartilago supracochlearis appears (fig. 3).
The lateral surface of the pars cochlearis (fig. 6) is smooth and gently convex in its ventral portion, and here presents a shallow groove, lying between the promontory and the cranial pole.
though it falls a little short of reaching either of these extremities (fig. 6). It is known as the sulcus caroticus (Voit) and contains a portion of the internal carotid artery (fig. 13). The sulcus caroticus does not correspond to the line of attachment of the lamina spiralis within, but crosses its cranio-ventral convex portion. Its lower part forms a low rounded projection into the lumen of the uncoiled portion of the cochlear duct, which appears in the figure of the cast of the cavity (fig. 8) as a shallow fossa.
The dorsal part of the lateral surface is made up principally by the outer wall of the vestibular portion, and forms the medial wall of the ventro-lateral otic recess. Above, the outlet of the facial foramen is to be seen, bridged by the suprafacial commissure. Below this opening is a small groove, the sulcus facialis (Voit), for the facial nerve, and below this, again, appears the elongated, crescentic, fenestra vestibuli, lying in a general direction from above downward and backward, and presenting a concavity downward and forward. It contains the anlage of the footplate of the stapes, which, however, fills only a small portion of the space of the fenestra, the remainder being occupied by the connective tissue representative of the annular ligament of the base of the stapes. Below the fenestra vestibuli is the cartilaginous septum which separates it from the fenestra perilymphatica below. This septum, which acts as a commissure to join the promontory of the pars cochlearis with the ventral surface of the pars canalicularis, has been referred to by Voit as the promontorium (fig. 6).
Passing below the lower, downwardly concave, border of the promontorium we come upon the small caudal surface of the vestibular portion (fig. 2), marked centrally by the large fenestra perilymphatica, which will later be separated by the processus interperilymphaticus into the larger lateral fenestra cochlearis or rotunda (over which is stretched the membranous anlage of the membrana tympani secondaria), and the smaller, medial foramen for the aquaeductus cochleae, within which may by seen the saccus perilymphaticus (Voit). The interperilymphatic process, more prominent on the left side than on the right, has been referred to, and appears as a short, conical projection directed forward from the inferior utriculoampullary prominence. The cochlear fenestra is apparently closed off on the left side of the Hertwig model, and almost so on the right.
The perilymphatic fenestra (fig. 2) is sharply concave, from before backward, the direction of the concavity being downward. Its inner wall is formed by the lower edge of the massa pyramidalis of the median wall of the vestibular portion. When regarded from below the circumference of the perilymphatic fenestra appears to have been formed by the bifurcation of the inferior cochlear prominence at the promontory, the lateral limb forming the promontorium; the medial the lower border of the massa pyramidalis; the two limbs uniting dorsally in the inter-perilymphatic process.
The boundaries of the medial surface have already been noted (fig. 7). It is quite smooth, and is more flattened than the lateral. Ventrally the elongated, narrow, crescentic line of union with the basal plate may be seen; immediately ventral to and parallel with this, the everted, narrow, extra-cranial surface, formed by the medial aspect of the inferior cochlear prominence, makes up the outer wall of the ventral basicochlear groove (fig. 2) as a strip 1.5 mm. wide.
Dorsal to the basal lamina the medial surface is intracranial, the strip immediately bordering the lamina being concerned in the formation of the outer wall of the dorsal basicochlear groove (fig. 5), which is sharply marked throughout, but more so above than below. In the dorso-cranial area of the medial surface the large, deep, meatus acusticus internus appears (fig. 7); below, the surface passes into the caudal surface of the pars cochlearis and behind into the medial surface of the pars canalicularis.
If we now consider, briefly, the passageway of the ductus cochlearis (figs. 8 and 9) we find the first, or uncoiled part, outwardly deflected at and for a short distance beyond its entrance from the perilymphatic fenestra. The lateral wall is here quite thin, but the opposite medial wall presents a pronounced conical thickening, to which reference has been made as the massa pyramidalis (fig. 2). The apex of the pyramid projects laterally into the first portion of the cochlear duct, and, indeed, it is owing to this circumstance that this part of the duct is thrust outward to form the promontory on the outer surface. From a point just ventro-cranial to the tip of this pyramid a small commissure, known as the commissura laminopyramidalis (fig. 8), springs to join the lamina spiralis which is immediately ventral, and this commissure passes over the uncoiled part of the cochlear duct; at the same time it divides the crescentic fissure in the floor of the internal acoustic meatus into ventral and cranial parts. The caudoventral surface of the pyramidal mass forms the medial wall of the first part of the cochlear duct; the cranio-ventral surface constitutes the medial and steepest part of the floor of the internal acoustic meatus, while the border between these delimits laterally the slit-like foramen, piercing the ventralmost part of the meatus, which transmiits the cochlear division of the acoustic nerve. Dorsally the base of the pjo'amid is seen to be pierced from above downward and backward by the foramen singulare (fig. 9), which leads into the cavity of the ampulla of the posterior semicircular canal, the region of exit appearing as an indentation of the inner wall just medial to the inner edge of the fenestra perilymphatica. It appears in the cast of the cavity of the capsule as a projection (fig. 9). A small portion of the dorsal side of the pyramid is concerned in the formation of the ventral wall of the vestibular space; the remainder, together with its border joining the cranio-ventral surface, is directly continuous with the cartilage of the medial wall of the vestibular part of the pars cochlearis.
The ductus cochlearis, shortly after passing the level of the lamino-pyramidal commissure (fig. 8), emerges from the vestibular part of the pars cochlearis, and enters the ventral, completely enclosed pars cochlearis (sensu stricto) which contains its coiled part. The only entrances into the closed portion of the cochlea are the passageways for the cochlear duct and the cochlear root of the acoustic nerve. The medial wall is here quite thin, while the lateral wall presents the coiled lamina spiralis (fig. 5).
If the suprafacial commissure were removed (fig. 7) it would be seen that the superior cochlear prominence is continued downward and backward as the first and widest part of the spiral lamina, which here separates the upper coiled portion of the cochlear duct, in front, from the internal acoustic meatus behind. In front of the upper part of the ventro-medial edge of the lamina spiralis, at a point marked by the widened cranio-ventral extremity of the foramen for the cochlear root, the medial downward continuation of the superior cochlear prominence passes over upon the medial surface of the pars cochlearis. When the medial wall of the pars cochlearis is removed it is seen that the lamina spiralis is attached to the lateral wall of the pars cochlearis, the line of attachment being in the form of a helix, which makes but little more than one turn. If the lower edge of the internal acoustic meatus (figs. 5 and 7) be followed forward and inward it passes over the upper edge of the foramen for the cochlear root to reach the medial edge of the lamina spiralis; thenpe it may be followed along the edge of the narrowing lamina, whose curvature becomes progressively sharper, ending on the lateral wall in a downward turn. In this way there is formed a commodious recess for the upper coiled part of the cochlear duct and its surrounding space. In all the cochlear duct makes about two turns (fig. 8).
The internal acoustic meatus (figs. 5 and 7) presents a rounded border, although its edges are somewhat straightened below and behind. The dorsal portion of the upper border is s'harp, and represents the medial edge of the foramen faciale. Passing caudally the edge becomes less sharply marked on the dorsal side, the cartilaginous surfaces which form it here meeting at a right angle. Ventrally we come upon the caudal edge, which is very sharp indeed, and represents the upper edge of the base of the massa pyramidalis. The dorsal and caudal edges form a rounded angle, and about 1 cm. below this point the entrance of the foramen singulare appears. Passing upward from the ventral end of the lower edge we come upon a crescentic and illdefined border which delimits the meatus cranio-ventrally, and passes dorsally into the medial edge of the suprafacial commissure.
The floor of the meatus is composed of three distinct portions; the ventral, formed by the first part of the lamina spiralis, the dorsal, which is the ventral edge of the wall of the vestibular portion in this region, and the medial, formed by the cranioventral surface of the pyramid, as we have seen. These surfaces increase in steepness in the order mentioned, so that the crescentic fissure (fig. 7) formed by their approximated deep edges, is deeper caudo-ventrally than cranio-dorsally. Looked at from within the ventral and dorsal are the only surfaces visible, the ventral presenting much the greater area. The borders of the latter where they join with the margins of the meatus present the enlarged extremities of the crescentic fissure, the upper of which serves for the passage of the facial nerve, the lower and anterior for the upper part of the cochlear division of the acoustic nerve.
Five foramina appear in the internal acoustic meatus, and of these the foramen singulare has been considered. It transmits the posterior ampullary nerve to the inferior cribriform macula. The other four are parts of the crescentic fissure. This latter is divided into almost equal limbs by the lamino-pyramidal commissure, which has been noted overlying the first part of the cochlea, and joining the dorsal surface of the first part of the spiral lamina with the pyramid. The ventral limb is long and slit-like, widest in its ventro-cranial end, and transmits the fibres of the cochlear root of the acoustic nerve. It will later become the spiral foraminous tract. The cranial limb is separated by two cartilaginous septa into three foramina, the upper, which we have seen, being large, and transmitting the 7th cranial nerve, and being known as the facial foramen, the lower two being of about the same size, and transmitting the superior and inferior branches of the vestibular root of the acoustic nerve (figs. 5 and 7).
I have also reconstructed a model of the cavity of the otic capsule, and from the illustrations of this a conception of the general plan of the cavity may be gained (figs. 8 and 9). This cast includes not only the membranous labyrinth but the space surrounding it, together with the entrances of the various foramina. In the illustration the laminopyramidal commissure appears as a foramen behind the coiled portion of the cochlear tract. In general form the cast resembles the later osseous labyrinth.
The cartilago supracochlearis (figs. 1, 3, 13) may now be considered. This is a small, rounded mass of cartilage, situated upon the cranial pole of the pars cochlearis, and, in the model, is about 8 mm. wide, and almost as 'long dorso-ventrally. It is quite free from cartilaginous union with the underlying cochlea, but the two are more closely approximated posteriorly than anteriorly, where the intervening connective tissue is thicker. The cartilage is immediately beneath the anterior part of the semilunar ganglion (fig. 13), and the material of which it is formed is mature cartilage of apparently the same age as that in the adjoining pars cochlearis.
It is difficult to say what may be the significance of this cartilage. Certainly it cannot be any one of the Restknorpeln which Voit describes in his Stage II (43 mm.) of the rabbit, since only one of these, Restknorpel b, corresponds at all in position with this cartilage, but it is distinctly above the semilunar ganglion while the siipracochlear cartilage is below it. I am inclined to regard it as a rudiment of the commissura (or trabecula) alicochlearis, which Jacoby describes in his 30 nrni. human embryo as a cartilaginous bridge extending between the anterior part of the pars cochlearis of the otic capsule and the ala temporalis. There is no evidence in my model of such a commissure, though the surfaces of the processus alaris of the temporal wing and the ventral surface of the pars cochlearis are very close together, and in the later stage modelled by Hertwig (80 mm.) there is no evidence of either commissure or rudimentary cartilage in this location, indicating that the cartilage in my embryo is probably undergoing retrogression. Voit describes and figures such a commissure in the skull of the rabbit, which encloses the carotid foramen laterally. He states it is a direct forward continuation of the planum supracochleare of the pars cochlearis.
The parietal plate (fig. 3) is a thin, semi-crescentic plate of cartilage, situa.ted above and. behind the pars canalicularis of the otic capsule, and bearing upon its median surface a concave impression for the brain (fig. 5) . The ventral extremity is wide, and is surmounted by an irregularly formed and rudimentary portion, the upper border of which is overlaid laterally by the caudal edge of the parietal bone. Above this part, and lying in the membrane within the parietal bone is to be seen, on the right side, an elongated nodule of cartilage, which may be known as the cartilago cranii lateralis (fig. 3) â€” probably a remnant of the side wall in this region. On the left side there is a somewhat smaller nodule. That this portion is undergoing retrogression is evident from a comprison with the models of Levi, on the one hand, and with the model of Hertwig on the other, when it is seen that the 14 nam. stage of Levi marks, perhaps, the stage of greatest development of the parietal plate, there being, after this, a progressive reduction, moderate in the 28 nam. stage of Levi and in my model, and pronounced in that of Hertwig.
With the otic capsule the parietal plate is connected at two points â€” in front through the capsuloparietal commissure, and below this through the bridge of cartilage between the capsuloparietal and capsulooccipital fissures. The ventral edge is indented and presents no evidence of the ^heno-parietal commissure, such as exists in certain of the lower mammals, as the rabbit and pig, and which represents, according to Gaupp ('00) the taenia marginalis of reptiles. The upper border is concave upward, and in its ventral portion there may be seen a small incisure, open behind, formed by an overhanging, backwardly projecting spicule from the uppermost part of the plate. This incisure appears to be the representative of what Mead calls the fissura laminae parietalis in the skull of Sus, where it is quite conspicuous. The upper border is continuous above with the membrane covering the brain.
The lower border is continuous ventrally with the otic capsule at the upper edge of the capsulooccipital fissure. Behind this it follows the elongated occipitoparietal groove (figs. 1 and 5) which runs backward to the dorsal occipital prominence, and marks the zone of union with the squama. Several small blood vessels are found in the ventral portion of the occipitoparietal groove, but in its dorsal part there is but a single small vessel. Just above the groove, and running parallel with it, is a low rounded ridge. The dorsal, scimitar-like extremity of the parietal plate is shown projecting freely dorso-medially. The outer surface is convex and is biit indistinctly marked off from the underlying squama.
In the membrane forming the posterior and superior part of the cranium, considerably above the tectum posterius, are to be seen two small cartilages lying side by side, the cartilagines cranii posteriores. The cartilage on the right side, though small, is relatively very large when compared with its partner of the left, which is insignificant.
These cartilages appear to represent the unpaired mass described by Bolk as lying above the tectum synoticum, which subsequently disappears. Possibly in my embryo they are undergoing reduction. Mead describes a somewhat similar small free nodule in Sus, but this is single and, although in the midline, it lies immediately above the tectum. He calls it the processus ascendens of the tectum posterius, and thinks it may possibly be the homologue of the processus ascendens of the tectum posterius of reptiles.
Fig.1. Wax plate reconstruction of the chondrocranium of a 40mm human fetus (seen from above).
Fig.2. Wax plate reconstruction of the chondrocranium of a 40mm human fetus (seen from above).
Fig. 3. Wax plate reconstruction of the chondrocranium of a 40mm human fetus (Seen from left side).
Fig. 4. Skull of a 40mm human fetus showing the right anterior aspect.
Fig 5. The Left Otic and Occipital Region - viewed from within
Fig. 10. Anterolateral Aspect of Detached Sphenoid Region
Fig. 11. Shows the nasal ami intraoibital septa from the right side. Unions wit ii ventral loot of ahi orliitalis, tectum nasi of ectethmoitl, ventro-lateral process, and anterior paraseptal cartilage are seen. The superior paraseptal cartilage appears in the more dorsal area. The dorsal cut surface of the inter orbital septum could not be represented from this position, but it begins above a little below the level of the lower edge of the ventral root of the ala orbitalis. Dorsal to this root the posterior edge of the illustration represents the dorso-cranial border of the septum interorbitalis.
Fig. 12. Medial aspect of left ectethmoid. Unions with the nasal septum are shown.
Fig 13. Coronal section through head at level of hypophyseal fossa, showing the cartilago siipracochlearis overlaid by the semilunar ganglion.
Fig. 14. Coronal section through head showing dorsal part of ethmoidal and ventral part of orbitotemporal region. Asymmetry is due to obliquity of section. The posterior cupular process on the left side (right side of illustration) is shown joining with the side wall; on the other side, in which the ])hin(> of section is posterior, the process is completely joined to the ectethmoid.
Fig. 15 Coronal section through the dorsal part of ethmoidal region, showing posterior cupidar process, the nasal septum, the interorbital sejitum with attached alae, and the palatine and vomer.
Fig. 16. Coronal section through the ethinoidal region, showing superior paraseptal cartilages, paraethnioidal cartilages, the vomer, maxilla, and palate bones.
Fig 17. Coronal section through ethmoidal region, showing nasal septum, ectetmoid, with attached conchae, cartihigo paranasalis, and vomer, maxilla and lacrimal bones.
|AM - Anlage of the maxilla
CP1 - Cranio-ventral process of medial lamina of anterior Jacohsonian cartilage
CP2 - Lateral lamina of anterior Jacobsonian cartilage
CP3 - Body of medial lamina of anterior Jacobsonian cartilage
CP4 - Caudal process of medial lamina of anterior Jacobsonian cartilage
|CP5 - Dorsal extremity of lamina of medial anterior Jacobsonian cartilage
Ep - Epithelium of nasal cavity
J - Organ of Jacobson (right)
Jd - Duct of left organ of Jacobson
Jm - Meatus of duct of right organ of Jacobson
|M - Mesethmoid
MS - Suspensory membrane of Fawcctt, the anlage of the vomer
OM - Maxilla
OV - Vomer
PV - Ventro-lateral process
Fig. 18. Is a Series of eight coronal cross-sections, through caudal portion of mesethmoid, in the region of the anterior paraseptal cartilages. Sections numbers 35. 45, 59, (35, 71, 78, 83, and 90. Magnif.
Section 35 shows the ventral end of the ventro-lateral process, PV, and its connection with the cauchil border of the mesethmoid. In section 45 the dorsal ends of these processes are shown in cross section, with the ventral extremity of the maxilla OM; the membranous anlage of this bone appears in sections 35 and 45 - AM. Part of the epithelium of the nasal cavity is represented in all the sections Ep. Section 59 is through the ventral end of Jacobson's cartilage and shows the cranio-ventral process of the medial lamina CP1 on the right side attached to the mesethmoid, while the outer lamina, CP2 lies free in the mesenchyme. Owing to the section having been cut somewhat more deeply on the right than on the left side, the left side of the illustration shows a somewhat more dorsal plane than does the right. The meatus of the right Jacobsonian duct opens at Jm in section 59. Section 65 shows, on the right side the dorsal tip of the left outer lamina of Jacobson's cartilage, and medial to this the main part of the inner lamina below, and the cranio-ventral process above. The organ of Jacobson ajipears in this and following sections, and it will be observed that the mesethmoid has become quite thin in its vicinity, ihv. organ lying in a concavity of the cartilage. In section 71 the main lamina of Jacobson's cartilage is seen CP3, and it will be noted that at its caudal extremity the relationship to the maxilla is very intimate. In the next section, No. 78, the caudal process is seen separated from the main mass of the cartilage, and in the next section. No. 83, this process has disappeared. In No. 99 the dorsal tips of the Jacobsonian cartilages are seen CP5, and below the mesethmoid, lying in a mass of condensed mcsenchj'me. the anterior tips of the vomer are seen as two small spicules of bone.
Note the bibliography is included at the end of part 2 of this paper.
Continue to Part 2. - The Skull of a Human Fetus of 40 mm
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Cite this page: Hill, M.A. (2020, October 19) Embryology Paper - The skull of a human fetus of 40 mm 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_skull_of_a_human_fetus_of_40_mm_1
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