160 THE FROG CHAP. 



In front of the 'tween-brain comes a pair of long, 

 oval bodies, wider behind and narrower in front. These 

 are the cerebral hemispheres (Cer. H). Each contains 

 a cavity, the lateral ventricle (Lat. v), which communi- 

 cates with the third ventricle by a small aperture, the 

 foramen of Monro (for. M). 



Lastly, each cerebral hemisphere is continued forwards 

 by a rounded olfactory lobe (Olf. I), which is fused with 

 its fellow of the opposite side, the single mass lying in 

 the posterior compartment of the girdle-bone (p. 42). 

 The lateral ventricles are continued forwards into the 

 olfactory lobes, forming the small olfactory ventricles 

 (Fig. 50, Olf. v). 



The brain, like the spinal cord, is formed of grey and 

 white matter, but differently arranged. In the olfactory 

 lobes, cerebral hemispheres, and 'tween-brain the white 

 matter is internal, and the grey forms a thin outer layer 

 or cortex. In the optic lobes and medulla the grey 

 matter is mainly around the ventricles, and the white 

 matter more external. 



Like the spinal cord, the whole brain is covered with 

 pia mater, densely pigmented in the region of the optic 

 lobes, and the cranial cavity in which it is contained is 

 lined with dura mater. 



The Spinal Nerves. The spinal nerves arise symmetri- 

 cally from the spinal cord on the two sides of the body, 

 and pass out from the neural canal through the inter- 

 vertebral foramina (p. 38). 



There are altogether ten pairs of spinal nerves in the 

 adult frog (Fig. 51, 7 X), each of which on leaving the 

 neural canal divides into a smaller dorsal and a larger 

 ventral branch (Figs. 52 and 53). The first pair leaves 

 the cord through the intervertebral foramina between the 

 first and second vertebra. Each passes at first directly 

 outwards, its large ventral branch, known as the hypo- 



