142 OX THE STRUCTURE OF THE SKULL. 



In front, the coalesced trabecular become changed into a plate 

 of cartilage, compressed from side to side, which occupies the 

 middle of the gradually-narrowing fronto-nasal process, as the 

 ethmo-vomerine, or internasal, cartilage. 



From the sides of the basi-sphenoid cartilaginous plates are 

 developed, which foreshadow the form and relations of the 

 alisphenoids ; at the sides of the presphenoidal region of the 

 cartilage, similar plates represent the orbito-sphenoicls. In 

 front of these the upper part of the internasal, or ethmoidal, car- 

 tilage passes laterally into broad deflected cartilaginous lamellar, 

 which curve round the olfactory sacs, and occupy the places of 

 the lateral masses of the ethmoid and the inferior turbinal bones. 



Thus far the terms of my description are almost as appli- 

 cable to the embryonic cranium of Man as to that of the chick. 



The human cranium has been observed forming part of an 

 open groove ; it undergoes a flexure, and develops visceral 

 arches altogether similar to those of the chick, nor is there any 

 reason to doubt that the organs of sense are developed in the 

 same manner. The very earliest condition of the cartilaginous 

 cranium of the human embryo has not been observed ; but, at 

 the beginning of the second month, it consists wholly of carti- 

 lage and of membrane, disposed in a manner which differs only 

 in detail from that seen in the chick. Thus the occipital fora- 

 men is surrounded by cartilage, continuous with that which 

 extends through the basi-sphenoidal, presphenoidal, and ethmoidal 

 regions to the anterior end of the face. The alisphenoids and 

 orbito-sphenoids are represented by cartilage, and cartilaginous 

 plates arch clown from the summit of the internasal cartilage, 

 on each side, to form the substratum on which, the nasal bom ss 

 and, in which, the spongy bones will be developed. That part 

 of the cranial cartilage which lodges the auditory organ is 

 exceedingly large, and constitutes, not only an oval capsule for 

 the membranous labyrinth, but sends back a continuation which 

 fills the space corresponding to the pars mastoidea, and extends 

 somewhat higher than it beneath the parietal region of the 

 skull. All the upper part of the cranium is and remains simply 

 membranous. 



