THE SKULL 209 



of the grooves in the middle line is an aperture 

 through which the internal carotid artery enters 

 the skull. 



b. The apertures of the olfactory capsules are large, 

 but are considerably reduced by the nasal car- 

 tilages, which are liable to be torn m cleaning 

 the skuU. "Within the capsules are seen the large 

 foramina, closed in the natural condition by fenes- 

 trated membranes, through which the olfactory 

 nerves enter. 



5. The posterior end of the skull. 



To see this properly the skull must he separated from the 

 vertebral colwmn. 



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. 



0. 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 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. 



