THE SKULL 201 



Along the greater part of the length of the 

 column there are two neural spines to each 

 vertebra, but in places the arrangement is less 

 regular. 



B. The Skull. 



The skull of the dog-fish remains cartilaginous throughout 

 life, thereby escaping the complications that result from the 

 development of bone : in it, also, the several factors of which 

 the skull is made up are more readily determined than in 

 the higher forms. For these reasons it is peculiarly instruc- 

 tive, affording an important clue to the complex condition 

 met with in other vertebrates. 



The skull consists of, (1) an axial tube, the cranium 

 proper, open at both ends, which lodges the brain, and is to 

 be regarded as an anterior unsegmented continuation of the 

 vertebral column ; and (2) the olfactory and auditory sense 

 capsules, which are cartilaginous capsules investing the nose 

 and ear respectively. These latter are at first independent 

 of the cranium, but in the adult fuse with it, forming lateral 

 expansions of its anterior and posterior ends. This fusion of 

 the sense-capsules and cranium is so complete that they will be 

 described together. 



1. General form of the skull. 



The skull is a somewhat oblong box of cartilage, 

 deeply hollowed at the sides to form the orbits for 

 lodgment of the eyes, and ending in front in a short 

 pointed rostrum formed by three converging rods. 



The olfactory capsules are thin-walled lateral 

 expansions at the anterior end, in front of the 

 orbits ; and the auditory capsules are more massive 

 projections behind the orbits. 



Examine the several surfaces of the skull in succession, 

 identifying the parts described below. 



2. The dorsal surface of the skull, 



a, The olfactory capsules are a pair of large oval 

 cartilaginous sacs at the anterior end of the skull : 



