144 ZOOLOGY SECT. 



is continuous throughout, and not composed, like the skulls of 

 higher Vertebrates, of a number of distinct bony elements fitting 

 in together. At the anterior end is a rostrum, consisting in Scyllium 

 of three cartilaginous rods converging as they extend forwards and 

 the lateral ones meeting anteriorly. At the sides of the base of this 

 are the olfactory capsules (olf.) thin rounded cartilaginous sacs open- 

 ing widely below, the cavities of the two capsules being separated 

 from one another by a thin septum. The part of the roof of the 

 cranial cavity behind and between the olfactory capsules is formed 

 not of cartilage, but of a tough fibrous membrane, and the space 

 thus filled in is termed the anterior fontanelle : in contact with the 

 lower surface of the membrane is the pineal body, to be afterwards 

 mentioned in the account of the brain. Each side-wall of this 

 part of the skull presents a deep concavity the orbit over which 

 is a ridge-like prominence, the supra-orbital crest, terminating 

 anteriorly and posteriorly in obscure processes termed respectively 

 the pre-orbital and post-orbital processes. Below the orbit is a 

 longitudinal infra-orbital ridge. 



Behind the orbit is the auditory region of the skull a mass of 

 cartilage in which the parts of the membranous labyrinth of the 

 internal ear are embedded. On the upper surface of this posterior 

 portion of the skull are two small apertures situated in a mesial 

 depression. These are the openings of the aqueductus vestibuli 

 (endolymphatic ducts), leading into the vestibule of the membranous 

 labyrinth. Behind this again is the occipital region, forming the 

 posterior boundary of the cranial cavity, and having in the middle 

 a large rounded aperture the foramen magnum through which 

 the spinal cord, contained in the neural canal and protected by the 

 neural arches of the vertebrae, becomes continuous with the brain, 

 lodged in the cranial cavity. Below this, on either side is an 

 articular surface the occipital condyle for articulation with the 

 spinal column, and between the two condyles is a concavity, like 

 that of the vertebral centra, containing notochordal tissue. 



A number of smaller apertures, or foramina, chiefly for the 

 passage of nerves, perforate the wall of the skull. Behind and to the 

 outer side of the anterior fontanelle are apertures (oph.%,1) through 

 which the ophthalmic branches of the fifth and seventh nerves 

 leave the skull. Piercing the inner wall of the orbit are foramina 

 through which the optic or second pair of cerebral nerves (opt.}, the 

 oculomotor (oc.), or third, the pathetic, or fourth (path.), the tri- 

 geminal, or fifth (tri.), the abducent, or sixth, and the facial, or 

 seventh, gain an exit from the interior of the cranial cavity. Just 

 behind the auditory region is the foramen for the glosso- 

 pharyngeal (gl), and in the posterior wall of the skull, near the 

 foramen magnum, is the foramen for the vagus. 



In close connection with the cranium are a number of cartilages 

 composing the visceral arches (Figs. 823 and 824). These are in- 



