x BRAIN 459 



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 continuous posteriorly with a spinal 

 cord, contained 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, and it is not 

 swollen opposite the paired appendages. 



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

 oblongata (NH} broadens out anteriorly to form lateral 

 swellings, and its contained fourth ventricle (F. rho) is roofed 

 over by the " pia mater." The cerebellum (HH\ 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 (MH) are hollow, their cavities communicating with 

 the median ventricle or iter, and ventrally to them are the 

 crura cerebri. The diencephalon (Zff] is relatively narrower 

 than in the frog. From its thin roof, which covers over the 

 tMrd-Wntridei. .is a delicate tube-like structure (Gp), 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 either side, and to it 

 is attached the pituitary body* with a vascular sac on either 

 side of it. In front of the infundibulum is the optic chiasma 

 (compare Figs. 49 and 50). 



Apart from the large size of the cerebellum, the most 



