CRANIUM. 



61 



The hinder ends of the trabeculae embrace the front end of the notochord, 

 so that the posterior cUnoid ridge must be regarded as being formed by 

 the hind end of tlie trabeculae. Moreover it must not be forgotten that the 

 internal carotid artery enters the skull in the embryo through the space 

 between the trabeculae before they fuse, so that the carotid canal also 

 belongs to the hind end of the trabecular region. 



Though the roof of the cranium is largely cartilaginous in fishes 

 even when membrane bones are present on it, there is always a 

 considerable fontanelle in which cartilage is absent. The carti- 

 laginous cranium so constituted becomes in the Teleosts, 

 Ganoids and Dipnoi replaced by bone to a varying extent, and 

 reinforced by the development of osseous tissue in the adjacent 

 connective tissue. The membrane bones, formed in the latter 



Fig. 36. — Median section of the cranium of Hexanchus, inner view (after Gegenbaur). 1 Fora- 

 men for vagus, 2 glossopiiaryngeal, 3 auditory, 4 facial, 5 trigeminal nerves ; 6 posterior 

 clinoid ridge ; 7 foramen for oculomotor, 8 trochlear, 9 optic nerves ; 10 fontanelle ; 

 11 rostrum ; 12 lateral process of ethmoid region ; 13 foramen for carotid ; 14 transverse 

 canal in skull base ; 15 notochord ; 16 foramina for spino-occipital nerves ; 17 neural 

 arch of the first vertebra with nerve foramina. 



manner, apply themselves to the subjacent cartilage and help in 

 forming the cranial wall. The occipital region of the cranium 

 is attached to the anterior end of the vertebral column, usually 

 without any special articulation (except in Batoidei and Chima- 

 era, etc.), the basioccipital region having the conical depression 

 and form of a vertebral body. The cranial part of the noto- 

 chord persists in the adult in some forms, but it more usually 

 undergoes atroph}-. It occasionally happens, as will be men- 

 tioned in the special accounts, that a few of the anterior 

 vertebrae are fused with the occipital region of the cranium. 



Visceral Skeleton. — The walls of the anterior part of the 

 alimentary canal (mouth and pharynx) are supported and 



