62 VERTEBRATE SKELETON 



commissure) of the sphenolateral cartilage. The rest of the wall 

 of the nasal capsule arises by outgrowths from septum, cornu, 

 sphenethmoid and a separate cartilage (antorbital process) which 

 forms the posterior wall of the capsule and soon joins the rest of the 

 cranium. The cavity of each nasal capsule is connected with the 

 cranial cavity by an olfactory foramen through which the olfactory 

 nerve passes from the sensory epithelium to the brain. On the 

 anterior or antero-inferior wall is a large opening, the naris (nostril) , 

 for the entrance of water to the nasal cavity. Nasal capsules, and 

 ethmoid plate form a trough for the olfactory nerves and the anterior 

 end of the brain. 



The eye is surrounded by a similar skeletal capsule, the sclera, but as the 

 eye is in continual motion, moved by its muscles, this sclera never unites with the 

 rest of the cranium and so is not usually considered as part of the skull. That it 

 is skeletal is shown by the facts that it is usually of cartilage and in many Saurop- 

 sida and a few other forms sclerotic bones are formed in, these being true carti- 

 lage bones. 



Each otic (ear) vesicle is similarly enclosed in a cartilage otic 

 capsule, which begins, in sharks, as an upgrowth from the lateral part 

 of the basal plate, but in other groups may arise independently, the 

 capsules of the two sides in either case forming the side walls of the 

 chondrocranium in the otic region. By gradual additions the cap- 

 sule is completed by the formation of ends (cupulas), a lateral wall 

 and a roof, while the medial wall (towards the brain) is last to be 

 enclosed and always has foramina for nerves and for the perilymph 

 and endolymph ducts of the ear. The interior of the capsule 

 becomes compKcated to form the otic labyrinth which will not be 

 described here. 



The posterior part of the cranium is formed by vertebral arches, 

 varying in number in different groups and separated by the trunks 

 of the postotic nerves. These arches gradually extend upwards, the 

 most anterior pair joining the capsule above, completing the jugular 

 foramen through which usually the ninth and tenth nerves and the 

 jugular vein pass. These occipital vertebra?, by continuous growth 

 and by contributions from the otic capsule, form a roof (synotic 

 tectum) over the hinder part of the brain, and from this tectum, 

 except in Elasmobranchs, ex- and supraoccipital bones will ossify. 



In the higher vertebrates this chondrocranium develops as a 

 cartilage brain case but little farther. In Elasmobranchs (lig. 87) 



