xrr PHYLUM CfiTORDATA 369- 



processes termed respectively the prce-orbital and post-orbital 

 processes. Below the orbit is a longitudinal infra-orbital ridge. 



Behind the orbit is the auditory region of the skull (and. cp.) 

 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 aqueduct us 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. 

 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. 198). These 

 are incomplete segmented hoops of cartilage, which lie in 

 the sides and floor of the pharynx. The first of these 

 forms the upper and lower jaws. The upper jaw, or palato- 

 quadrate (///./.), consists of two stout rods of cartilage firmly 

 bound together in the middle line and bearing the upper (or 

 anterior) series of teeth. The lower jaw, or MeckePs cartilage 

 (/./.), likewise consists of two stout cartilaginous rods bear- 

 ing teeth, firmly united together in the middle line, the union 

 being termed the symphysis. At their outer ends the upper 

 and lower jaws articulate with one another by a movable 

 joint. In front the upper jaw is connected by a ligament 

 with the base of the skull. 

 Man. Zool. B B 



