i6o THE FROG chap. 



lateral ventricle (Fig. 50, Lat. li), which communicates 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. 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 their relations are different. In the olfac- 

 tory 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 at the neural canal through the intervertebral 

 foramina (p. 38). 



There are altogether ten pairs of spinal nerves in the adult 

 frog (Fig. 51, / — X), each of which on leaving the neural 

 canal divides into a smaller dorsal and a larger ventral 

 branch (Fig. 53, i sp — 5 sp). The first pair leave the cord 

 through the intervertebral foramina between the first and 

 second vertebra. They pass at first directly outwards, the 

 large ventral branch of each, known as the hypoglossal, 

 turning forwards, and going to the muscles of the tongue 

 (Fig. SI, 7, Fig. 53, I sp). 



The second pair (Fig. 51, //) are very large ; they emerge 



