x BRAIN 459 



Nervous System. The nervous system is constructed 

 on a similar plan to that of the frog (compare Part I, 

 Chapter X) and of Vertebrates in general. The central 

 nervous system is dorsal in position and consists of a 

 brain, contained within the cranial cavity, and con- 

 tinuous posteriorly with a spinal cord, enclosed in the 

 neural canal of the vertebral column : it consists of 

 grey and white matter, and its cavity or neuroccele, lined 

 with epithelium, gives rise to the ventricles of the brain 

 and to the central canal of the spinal cord. 



In correspondence with the form of the body, the 

 spinal cord is relatively much longer than in the frog; 

 it is not swollen opposite the paired appendages. 



In the brain (Figs. 117 and 124) the bulb or medulla 

 oblongata (Med. obi) broadens out anteriorly to form 

 lateral swellings, and its contained fourth ventricle (w 4 ) is 

 roofed over by the " pia mater/' The cerebellum (Cb), 

 which is very small in the frog, is here relatively 

 enormous, and its surface is marked by slight grooves : 

 it overlaps the bulb behind and the optic lobes in front, 

 and contains a ventricle communicating with the fourth 

 ventricle. The oval optic lobes (Opt. I) are hollow, their 

 cavities communicating with the median ventricle or 

 iter, and ventrally to them are the crura cerebri. The 

 diencephalon (Di) is relatively narrower than in the frog. 

 From its thin roof, which covers over the third ventricle, 

 is a delicate tube-like structure (pin), which extends 

 upwards and forwards and ends in a small knob attached 

 to the roof of the skull : this is the pineal body. From 

 the ventral surface of the diencephalon arises the 

 infundibulum, with an oval swelling on each side, and 

 to it is attached the pituitary body (Fig. 117, pty), with 

 a vascular sac on either side of it. In front of the 

 infundibulum is the optic chiasma (compare Figs. 49 

 and 50). 



