THE SKULL. 217 



upwards from the sides of each centrum opposite 

 its middle. 



b. The neural plates are a series of thin hexagonal 



plates of cartilage which form the sides of the 

 neural canal (figs. 45 and 49). On each Bide 

 there are twice as many neural plates as there a,re 

 vertebrae, the neural plates being placed alternately 

 opposite the middles of the vertebrae, and opposite 

 the intervertebral intervals. The former or 

 vertebral neural plates are fused in the adult 

 with the tops of the neural processes, and Lave 

 their posterior borders notched for the passage 

 of the ventral roots of the spinal nerves. The 

 dorsal roots of the nerves pass out through notches 

 in the posterior borders of the intervertebral 

 neural plates. 



c. The neural spines are a series of median nodules 



of cartilage, wedged in between the dorsal ends of 

 the neural plates so as to complete the neural 

 arches above. 



Along the greater part of the length of the 

 column there are two neural spines to ea&h 

 vertebra, but in places the arrangement is less 

 regular. 



B. The SkuU. 



The skull of the dog-fish remains cartilaginous throughout 

 life, thereby escaping the compHcations that result from the 

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

 the skull is made up are more easily determined than in the 

 higher forms. For these reasons it is peculiarly instructive, 

 affording an important clue to the complex condition met 

 with in higher 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- 



