Behind the posterior boundary of the squamosal, constituted by the two diverging lines above described (Fig. 55), lies all that portion of the temporal bone which is known as the pars mastoidea. But, as I shall have occasion to demonstrate, when explaining the mode of development of the temporal bone, this pars mastoidea is, in reality, made up of extensions of two of the primitive constituents of the pars petrosa, and of a third element, the epiotic. The posterior margin of the squamosal, as above described, may be said roughly to form two sides of a parallelogram. The third side is the thick part of the upper edge of the pars mastoidea, corresponding with the termination of the upper and anterior surface of the pars petrosa on the inner side of the bone. If a fourth side is made by an imaginary line connecting the ends of the others, the bony surface which lies above and in front of the line will, as nearly as possible, belong to the pro-otic element, while that which lies below and behind it, including the mastoid process, appertains to the epiotic. On the other hand, a certain amount of the pars mastoidea internal to the digastric groove belongs to the opisthotic.

136

LECTURE VIII.

ON THE STRUCTURE OF THE SKULL.

THE DEVELOPMENT OF THE HUMAN SKULL.

As might be expected from the nature of the case, it has not yet been possible to obtain a series of human embryos, in every stage of development, sufficiently large to enable embryologists to work out all the details of the formation of the human skull. But all higher vertebrate embryos so nearly follow one and the same type of early developmental modification, that we may reason, with perfect confidence, from the analogy of the lower Vertebrates to man, and fill up the blanks of our observations of human embryos by investigations of the chick, the dog, the rabbit, or the pig.

In the chick, the first indication of the body of the embryo is an elongated, elevated area of the blastoderm, the axis of which is traversed by a linear groove. The one end of the elongated area is wider and more distinctly raised up from the rest of the blastoderm, than the other: it is the cephalic end (Fig. 31, A, a), and the linear groove stops short of the rounded extremity of this part of the elevated area. A peculiar cellular cylinder, tapering off at each end, the notochord, is soon discerned occupying the bottom of this groove, beneath the outer, serous, or neuro-epidermic layer of the germ.

A laminar outgrowth of the convex summits of the ridges which bound the primitive groove now takes place, in that part of the embryo, which will eventually become the middle region

*See Lecture IV., pp. 64-66.

of the head; and the dorsal laminæ, thus produced, extending forwards and backwards, like parapets, upon each side of the primitive groove, lay the foundations of the lateral walls, not only of the skull, but of the spinal column.

Very early, however, the boundary line between skull and spinal column is laid down, by the appearance in the substance of the bases of the dorsal lamina and the adjacent middle layer of the blastoderm, of the first pair of those quadrate masses of condensed tissue, the proto-vertebræ ("Urwirbel" of the German writers), which are the foundations, not only of the bodies of the vertebræ, but of the spinal muscles and ganglia. The proto-vertebræ increase in number from before backwards; and, at length, extend through the whole range of the spinal column, while none ever make their appearance in the region which will be converted into the skull.

The edges of the dorsal lamina now unite, the coalescence taking place first in the middle cephalic region, and extending thence backwards and forwards; at the same time, the cephalic canal becomes separated into three distinct dilatations, or cerebral vesicles, of which the anterior is by far the most marked (Fig. 57, A, I, II, III).

The rudimentary cranial cavity next becomes bent upon itself in such a manner, that the longitudinal axis of the first cerebral vesicle takes a direction at right angles to the axis of the third, and of the spinal canal generally. In consequence of this change, the middle cerebral vesicle occupies the summit of the angulation, and becomes the most anterior point of the whole body (Fig. 57, C, D).

The bend thus produced is the cranial flexure. It results in the division of the floor of the cranial cavity into two parts, an anterior and a posterior, which are at right angles to one another (Fig. 57, C, D, E). Hitherto, no trace of the notochord has been observed in the anterior division, that structure ending in a point behind the flexure (Fig. 57, D, E, h).

As development proceeds, the anterior cerebral vesicle becomes divided into two portions,-an anterior, the vesicle of the cerebral hemispheres (I); and a posterior, the vesicle of the third ventricle (I'). In the upper wall of the vesicle of the third

Fig. 57. Successive stages of the development of the head of a chick.-I, II, III, first, second, and third cerebral vesicles; Ia, vesicle of the cerebral hemispheres; 1,

ventricle the rudimentary pineal gland (e) makes its appearance in the middle line. From the middle of the lower wall grows out a process, the infundibulum, terminating in a glandular appendage, the pituitary body, which last is lodged in the deep fossa situated in the floor of the anterior division of the skull, immediately in front of, and beneath, the termination of the notochord (Fig. 57, B, D, d).

The three pairs of sensory organs appertaining to the higher senses, the nasal sacs, the eyes, and the ears,―arise as simple cæcal involutions of the external integument of the head of the embryo. That such is the case, so far as the olfactory sacs are concerned, is obvious; and it is not difficult to observe that the lens and the anterior chamber of the eye are produced in a perfectly similar manner. It is not so easy to see that the labyrinth of the ear arises in this way, as the sac resulting from the involution of the integument is small, and remains open but a very short time (Fig. 57, C, b). But I have so frequently verified Huschke's and Remak's statement that it does so arise, that I entertain no doubt whatever of the fact.* The outer ends of the olfactory sacs remain open, but those of the ocular and auditory sacs rapidly close up, and shut off their contents from all direct communication with the exterior. The olfactory nerve is developed from the anterior division of the anterior cerebral vesicle. The optic nerve is primarily developed from the posterior division of that vesicle, its connection with the middle vesicle (which eventually gives rise to the corpora quadrigemina)

vesicle of the third ventricle; a, rudiments of the eyes and optic nerves; b, of the
ears; g, of the olfactory organs; d, the infundibulum; e, the pineal gland; c, proto-
vertebræ; h, notochord; 1, 2, 3, 4, 5, visceral arches; V, VII, VIII, the
trigeminal, portio dura, and eighth pair of cranial nerves; k, the fronto-nasal process;
, the maxillary process; x, the first visceral cleft.

A, B, upper and under views of the head of a chick at the end of the second day.
C, side view at the third day.

D, side view at seventy-five hours.

E, side view of the head of a chick at the fifth day, which has been subjected to slight pressure.

F, head of a chick at the sixth day, viewed from below.

F1, the cartilaginous crauium of the same.

P, pituitary space; tr, trabecula; Qu, quadrate cartilage; Sc, semicircular canals ;
Co, cochlea.

G, head of a chick at the seventh day, from below.

* See also Kölliker's "Entwickelungs Geschichte," p. 300, et seq.