xii PHYLUM CHORDATA 353 



The central canal of the spinal cord expands in the 

 medulla oblongata into a wide shallow cavity, roofed over only 

 by a thin membrane : this is known as the fourth ventricle 

 (F. rho). From this runs forwards a narrow passage, the iter 

 or aqueduct of Sylvius, expanding in front in the diencephalon 

 into a laterally compressed cavity, the third ventricle. From 

 this are given off a pair of lateral ventricles, passing into the 

 prosencephalon, each giving off a prolongation into the 

 corresponding olfactory lobe. 



The roof of the third ventricle is very thin : it is pro- 

 duced into a slender process the epiphysis or pineal body. 

 Its side walls are formed of two masses, the optic thalanii ; 

 its floor is produced into a hollow prolongation, the infundi- 

 bulurn, with the end of which a vascular body, the hypophysis 

 or pituitary body is connected. 



In the brain of the Lizard the same parts are recognis- 

 able as in the Dog-fish, 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 

 recognise the same parts : but the whole brain is larger 

 in proportion to the bulk of the body, the cerebral hemi- 

 spheres are much more highly developed, and the cere- 

 bellum 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. 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 nerve, i.e., those fibres 

 which are concerned in carrying impulses from the various 



Man. Zool. A A 



