Chap, ix.] SKULL. 325 



is the true cranium. With this there enters into 

 more or less complete union the cartilages or bones 

 that form the framework for the mouth, and give rise, 

 in higher forms, to the face. 



The notochord extends forwards below the two 

 hinder of the three primitive brain vesicles, and, on 

 either side, there appear masses of cartilage, homolo- 

 gous with those that form the arches of the vertebral 

 column. These parachordals unite with the noto- 

 chord to form a continuous basilar plate, which 

 serves as a floor for the two hind brain vesicles ; this 

 grows up on either side, and unites above to form a 

 ring of cartilage which embraces the hindermost part 

 of the brain. Posteriorly each half gives rise to a 

 cartilaginous coiidyle, which articulates with the 

 atlas. This hindermost portion of the cranium may 

 be distinguished as the occipital region. In front 

 of the parachordals there appear two bars, which unite 

 behind, where they embrace the anterior end of the 

 notochord, and in front also, leaving a space in the 

 middle. These are the trabeculre, and they form 

 the floor for the first brain vesicle, or fore-brain. As 

 the plates unite to form a solid floor they grow up at 

 the sides, but never form more than an imperfect roof 

 in this region, which, therefore, is not cartilaginous, 

 but membranous. This portion of the cranium may 

 be spoken of as the splieiioidal region. As the 

 cranium invests the brain, holes, or notches that will 

 be converted into holes, have to be left for the passage 

 of the cerebral nerves (Fig. 136 ; 5, 9). 



In addition to these, the whole architecture of the 

 skull is profoundly affected by another set of elements, 

 which enter into more or less close contact with it. 

 These are the capsules of the three higher senses, smell, 

 sight, and hearing. At the anterior end of the sphe- 

 noidal region the olfactory cartilaginous capsule 

 becomes connected with the cranium, the anterior wall 



