370 MANUAL OF ZOOLOGY sect. 



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 fontanclte ; 

 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 (aud. 

 cp), a mass of cartilage in which the parts of the mem- 

 branous 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 laby- 

 rinth. 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 vertebra? becomes 

 continuous with the brain, lodged in the cranial cavity. On 

 either side of this is an articular surface, the occipital 

 condyle, for articulation with the spinal column. 



A number of smaller apertures or foramina, chiefly for 

 the passage of nerves, perforate the wall of the skull. 



In close connection with the cranium are a number of 

 cartilages composing the visceral arches (Fig. 224). These 

 are incomplete hoops of cartilage, mostly segmented, which 



