VISCERAL SKELETON. 233 



the skull. Within the capsules are seen the 

 large foramina, closed in the natural condition 

 by fenestrated membranes, through which the 

 olfactory nerves enter. 



5. The posterior end of the skull. 



To see this properly the skull must be separated from the 

 vertebral column. 



a. The foramen magnum is the large median hole 



through which the spinal cord enters the skull 

 before expanding into the brain. 



b. The notochord enters the base of the skull imme- 



diately below the foramen magnum. 



c. The condyles are a pair of round prominences 



at the sides of the notochord, and below the 

 foramen magnum : they articulate with the first 

 vertebra. 



d. The foramen for the pneumogastric nerve of each 



side is at the bottom of a pit-like depression at 

 the side of the foramen magnum, below and to 

 the inner side of the auditory capsule. 



C. The Visceral or Pharyngeal Skeleton. 



This consists of a series of cartilaginous hoops or arches 

 placed one behind another, and encircling the anterior part 

 of the alimentary canal. In conformity with the shape of 

 the mouth and pharynx, which they surround, the hoops are 

 wide from side to side, and compressed dorso-ventrally. 



The first arch gives rise to both upper and lower jaws. 



The second or hyoidean hoop is large, and plays an 

 important part in slinging the jaws to the skull. 



The third and succeeding hoops are more slender : they 

 surround the pharynx and support the gills. 



To see the visceral skeleton properly, a roll of paper should 

 be passed in at the mouth-opening, and backwards through the 

 several hoops. 



