XIII.] THE FKOG. 177 



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

 position 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 mydon, 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 passage 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 

 with an opening on its inner face, These openings lead into 

 M. 12 



