THE SKULL 207 



b. The under surface of the skull is a broad flat plate 

 of cartilage, crossed almost transversely a little 

 behind its middle by a pair of grooves in which 

 the carotid arteries lie. At the point of meeting 

 of the grooves in the middle line is an aperture 

 through which the internal carotid artery enters 

 the skull. 



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 to 

 expand into the brain. 



b. The notochord lies immediately below the foramen 



magnum. 



c. The condyles are a pair of rounded prominences 



at the sides of the notochord, and below the fora- 

 men 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 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 accordance 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 hoop is very large and stout : it surrounds the 

 mouth-opening, and is divided about its middle into dorsal 

 and ventral halves, forming the upper and lower jaws respec- 

 tively. Its resemblance to the hinder arches is best seen 

 when the mouth is opened : it is still uncertain, however, 

 whether the upper and lower jaws belong to the same arch 

 or to separate ones. 



