70 



ZOOLOGY 



SECT. 



nerves of smell, situated at the anterior end of the cerebral cavity, 

 one on each side of the mesethmoid ; the optic foramina (nv. 2) 

 for the nerves of sight, in the interorbital region ; the trigeminal 

 foramina (nv. 5) for the fifth nerves, just in front of the auditory 

 capsule ; the auditory foramina (nv. 8) for the nerves of hearing, 

 in the inner wall of the auditory capsules : and the vagus foramina 

 (Nv. 10); for the tenth nerves, immediately posterior to the auditory 

 capsules. 



In addition to the elements of the brain-case parachordals, 

 trabeculse, and auditory capsules there enter into the composition 



m.cth 



B 



b.br.s 



FIG. 719. A, diagram of cartilaginous skull ; B, cranium in sagittal section, au. cp. auditory 

 capsule ; b. br. 1 5, basi-braiichials ; b. cr. basis cranii ; b. //>/. basi-hyal ; c. br. cerato- 

 branchial ; c. hy. cerato-hyal ; ep. br. epi-branchial ; ep. hy. epi-hyal ; fon. fontanelle ; for. mag. 

 foramen magnum; h. br. hypo-branchial; h. liy. hypo-hyal ; hy. m. hyomandibular ; lb. 1 It, 

 labial cartilages; mck. c. Meckel's cartilage; m. ctk. mesethmoid; nv. 110, foramina 

 for cerebral nerves ; olf. cp. olfactory capsule ; pal. qv. palato-quadrate ; ph. br. pharyngo- 

 branchial ; r. rostrum ; s. t. pituitary fossa or sella turcica. 



of the skull another set of elements called visceral bars. These are 

 cartilaginous rods formed in the walls of the pharynx between the 

 gill-slits, and thus encircling the pharynx like a series of paired 

 half-hoops (Fig. 715, B,rs. &.). The corresponding right and left 

 bars become united with one another below by an unpaired cartilage 

 (Fig. 719, A, 1}. br.), forming a visceral arch, and the unpaired ventral 

 pieces unite successive arches with one another in the middle ventral 

 line, thus giving rise to a more or less basket-like visceral skeleton. 

 It will be noticed that the visceral skeleton has a segmental 

 arrangement, being formed of parts arranged in an antero-posterior 

 series, whereas in the cranium there is no indication whatever of 

 segmentation. There is, however, no exact correspondence between 



