XIIL] THE FROG. 185 



are found in the viscera and vessels. An account of the 

 disposition of the muscles in the hind-limb will be found in 

 the Laboratory work. 



The nervous system is conveniently divisible into two 

 parts, the cerebro-spinal and the sympathetic. The cerebro- 

 spinal nervous system again consists of the brain, or encepha- 

 lon, with its nerves, and the spinal cord, or myelon, with its 

 nerves. 



The encephalon lies in the cranial cavity, which it nearly 

 fills, and is divisible into the hind-brain, the mid-brain and 

 the fore-brain, which last again comprises three divisions ; 

 the thalamencephalon, the cerebral hemispheres, and the 

 olfactory lobes. 



The greater part of the hind-brain is formed by the 

 medulla oblongata, which is the continuation of the myelon 

 forwards and presents, on its dorsal aspect, a triangular 

 cavity, the apex of which is directed backwards. It is roofed 

 over by a thick and very vascular membrane (choroid plexus), 

 the inner surface of which presents transverse folds on either 

 side of a median longitudinal ridge. The cavity is the 

 fourth ventricle; it communicates behind with the central 

 canal of the myelon, while, in front, it narrows into a pas- 

 sage which connects the fourth ventricle with the cavities 

 anterior to it. The thick lateral ridges of nervous substance 

 at the sides of the fourth ventricle, which represent the 

 restiform bodies, pass, in front, into the outer extremities of 

 a short broad tongue-shaped plate, convex ventrally and 

 concave dorsally, which overhangs the anterior part of the 

 fourth ventricle, and is the cerebellum. 



In front of this, the dorsal moiety of the mid-brain is 

 formed by two oval bodies, the long axes of which are 

 directed inwards and backwards. These are the optic lobes. 

 When laid open, each is seen to contain a cavity or ventricle 



