SKULL 317 



roughly spherical in shape. Their ventral surfaces are incomplete- 

 Running vertically between them is the internasal septum or 

 mesethmoid cartilage. Three slender rods, one from each capsule 

 and one from the mesethmoid, project forwards and are known 

 collectively as the rostrum. The auditory capsules are also 

 continuous with the cranium and, except for a pair of small 

 openings on the dorsal surface, completely enclose the ears ; 

 ridges on the capsules mark the positions of the semicircular 



ecf opVn 



I cp7 "fS- 



rest: 



Fig. 239. — Skull and branchial arches of the dogfish [Scyliorhinus). — From 

 Young, The Life of Vertebrates, 1950. Clarendon Press, Oxford. 



au.c, Auditory capsule ; b.b., basibranchial ; b.h., basihyal ; c, centrum ; cer.b., ceratobranchial ; cer.li. 

 ceratohyai ; d.r., foramen for dorsal root; e.b., extrabranchial ; e.c.f., external carotid foramen: 

 e.L, ethmoid ligament ; ep.b., epibranchial ; gr., groove for anterior cardinal sinus ; g.r., gill ray ; 

 hy.a., foramen for hyoid artery ; hvmd., hyomandibular ; i.d., interdorsal ; io.c, interorbital canal ; 

 I.e., labial cartilage ; M.c, Meckel's cartilage ; na., neural arch ; nas.c, nasal capsule ; o.n.f., orbitonasal 

 foramen ; op., foramen for ophthalmic nerve ; op.g., groove for ophthalmic branch of fifth nerve; 

 op.V., foramen for the same ; op. VII., foramen for ophthalmic branch of seventh nerve ; orb., orbit ; 

 ph.b., pharyngobranchial ; p.sp.L, prespiracular ligament ; r., rib ; rost., rostral cartilage ; spd., supra 

 dorsal ; tr., transverse process ; v.r., foramen for ventral root ; II-IX, foramina for cranial nerves. 



canals (p. 342). Between the auditory capsules is the foramen 

 magnum, through which the spinal cord passes into the brain, 

 and just below this the notochord may be seen going into the 

 floor of the cranium, in which it runs about half-way alon.^. 

 There are several other openings, called foramina (singular 

 foramen) through which nerves or blood vessels pass into or 

 out of the cranium ; the more important of them are shown in 

 Figs. 239 and 240. 



The visceral part of the skull consists of a series of half-hoops 

 of cartilage on each side, developed in the tissue between the 

 gill slits and behind the last gill slits, so that they correspond 



