THE SKULL 205 



they have very thin walls and are readily torn. 

 Their long axes are placed somewhat obliquely, 

 and they are separated from each other by a thin 

 median internasal septum. 



b. The rostrum is formed by three cartilaginous bars, 



of which two run forwards from the upper surface 

 of the olfactory capsule, converging in front ; 

 while the third is an anterior prolongation of the 

 floor of the cranium, or brain-case. 



c. The anterior fontanelle is a large, almost circular 



hole in the roof of the skuU, between and behind 

 the olfactory capsules. During life it is closed by 

 a connective-tissue membrane, in connection with 

 the inner surface of which is the pineal body. 



d. The supra- orbital crests are prominent curved ridges 



of cartilage running along the sides of the skull 

 from the olfactory to the auditory capsules, and 

 forming the dorsal boundaries of the orbits. 

 Along the inner side of each crest, between it 

 and the cranium, is a shallow longitudinal groove 

 in which lies a bundle of mucous canals. 

 b. The foramina for the ophthalmic nerves are a pair 

 of small apertures behind and to the outer sides 

 of the anterior fontanelle, and to the inner sides 

 of the supra-orbital crests. Through them the 



Fig. 45.— ScyUium canicula. The skull and visceral skeleton with the 

 anterior part of the vertebral column, seen from the right side. 

 The labial cartilages and gill-rays are omitted, (a. m. m.) 



A, auditory capsule. B, post-orbital groove. C, inter-orbital canal. 

 D, pre-spiracular ligament : the white spot below the reference line is the 

 pre-sptracular cartilage. E, upper jaw. F, lower jaw. Gr, hyo-mandibular 

 cartilage. H, cerato-hyal. I, pharyngo-branchial. K, epi-branchial. L, cerato- 

 branohial. M, extra-branchial. IT, vertebral neural plate. NO, olfactory 

 capsule. O, centrum of vertebra. P, intervertebral neural plate. B, neural 

 spine. S, foramen for the ventral root of a spinal nerve. T, foramen for the 

 dorsal root of the same nerve. TT, orbital grooves, lodging the ophthalmic 

 branches of the fifth and seventh nerves. "W, aperture at end of orbital 

 gi'oove through which the ophthalmic branches of the fifth and seventh nerves 

 leave the orbit. Z, ethmo-palatine ligament. 



II, optic foramen. Ill, foramen for third nerve. IV, foramen for fourth 

 nerve. V, foramen for the main branches of the fifth and seventh nerves, and 

 for the sixth nerve. Va, foramen for the ophthalmic branch of the fifth nerve. 

 Vila, foramen for the ophthalmic branch of the seventh nerve. IX foramen 

 for the ninth or glosso-pharyngeal nerve. 



