Skull and Hyoid 263 



In lower vertebrates the basal part of the brain case is divisible into a posterior 

 portion, the parachordal bars, and an anterior part composed of the trabeciila: cranii 

 (Fig. 21 1). In the human skull there are no signs of such cartilaginous bars, although 

 there is some indefinite suggestion of the conditions in the precartilaginous state, but 

 the. portion of the base between the foramen magnum and the pituitary fossa can never- 

 theless be termed the parachordal part, because the notochord runs in relation with 

 it as far as the dorsum sellae. In front of this, however, there are no indications 

 of trabecular structure whatever, and this front part of the base of the skull, 

 moreover, is possibly of a quite different morphological value, so that it should 

 not be called the " trabecular part," but might be termed with more propriety 

 the prechordal part. 



The notochord does not lie altogether in the parachordal part of the base (see 

 Fig. 216), but below it, in relation with the roof of the pharynx, for the greater portion 

 of its course here ; but, in spite of this, the basiocciput and bas : sphenoid may be 

 looked on as developing in connection with the mesodermal somites that lie on each 

 side of the notochord. Four pairs of somites are taken up in this way into the skull 

 base : if there are any others cephalad to these they are lost in the condensation 

 round the end of the notochord, and give no separate indication of their existence 

 as distinct structures at any time in development, so far as has been observed 

 at present. 



Before chondrification begins the mesenchyme of the base forms a continuous 

 condensation in the future basioccipital and sphenoidal regions, and from the latter 

 extends out somewhat at the sides, while from the former situation the condensation 

 shows a tendency to extend back round the neural tube towards the region of the future 

 post-occipital. 



Cartilaginous change first occurs in the basiocciput and basisphenoid, spreading 

 later from this into the mesenchyme of the root of the alisphenoid, orbito-sphenoid, 

 and post-occipital. At the same time another element becomes apparent in the base : 

 this is the periotic capsule, developed as a chondrification round the otocyst, which has 

 come into position beside the parachordal part of the base, behind the alisphenoid. 

 Thus, when the process has gone on for a little time the base would present the various 

 distinct parts shown in the scheme in Fig. 211, where the periotic capsules are seen as 

 separate elements, the small and great wings as small projections from the sphenoidal 

 central mass, and the post-occipital as two cartilaginous sheets spreading round the 

 foramen magnum to meet behind it. A little later these different parts unite and 

 form a mass of cartilage continuous at the sides and in front and behind with the thinner 

 membranous covering of the sides of the brain. The cartilaginous portion or chondro- 

 cranium made in this way includes the basi-, ex-, and post- occipital, the basisphenoid 

 and basal parts of the great wings, the orbitosphenoids, and the petrous temporal. 

 The bones that correspond with these regions therefore ossify in cartilage : the mem- 

 brane bones in continuity with them include the orbital plates of the frontal, the greater 

 part of the alisphenoids, the squamous temporal, and the occipital bone above the 

 superior curved line. The nasal capsule (cartilaginous) is also present, in front of the 

 sphenoid, but it is possibly not in the same morphological plane as the rest of the base 

 and will be considered separately later. 



\\~hile these modifications are proceeding in the immediate coverings of the brain, 

 additional visceral elements are being added on the under aspect of the base. These 

 are derived from the visceral arches that constitute the floor of the primitive pharynx : 

 it is evident (Fig. 211) that the outer and upper parts of the mesoderm of these arches 

 must come into relation with the ventro-lateral part of the covering of the brain, 



