xii PHYLUM CHORDATA 355 



In the brain of the lizard the same parts are recognisa- 

 ble as in the dogfish, the chief differences being that the 

 prosencephalon is deeply divided by a median longitudinal 

 fissure into two lobes, the cerebral hemispheres, and that 

 the cerebellum is very small. In the rabbit also we rec- 

 ognise the same parts. But the whole brain is larger in pro- 

 portion to the bulk of the body ; the cerebral hemispheres 

 are much more highly developed, and the cerebellum is not 

 only of large relative size, but is of complicated structure. 



The peripheral nervous system consists of the spinal and 

 cerebral nerves given off from the spinal cord and the brain 

 respectively, with their ramifications through all parts of the 

 body. A pair of spinal nerves emerge from the neural 

 canal between each adjoining pair of vertebrae. Each 

 spinal nerve arises from the spinal cord by two roots a 

 dorsal and a ventral ; the former is dilated into a ganglion. 

 Experiments prove that the dorsal root contains the sensory 

 fibres of the nerves, i.e., those fibres which are concerned 

 in carrying impulses from the various parts to the nerve 

 centres to be translated in consciousness into sensations. 

 When, for example, the skin of some part of the body is 

 touched, the impulse by means of which we become con- 

 scious of the contact passes from the surface through 

 branches of the spinal nerves, and enters the spinal cord 

 through the dorsal root, in order to be transmitted to the 

 brain. The ventral root, on the other hand, contains the 

 motor fibres ; the fibres through which impulses which lead 

 to the contraction of muscles pass outwards from the central 

 nervous system. 



More or less extensive intercommunications take place 

 between the spinal nerves that are situated opposite the 

 origin of the limbs ; these spinal nerve plexuses give off the 

 nerves to the limbs. 



