I.] THE FROG. 25 



Ion, 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 median thalamencephalon, and the paired cerebral 

 hemispheres and 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, when laid open from its dorsal 

 aspect, a triangular cavity, the apex of which is directed 

 backwards. The cavity is the fourth ventricle ; it com- 

 municates behind with the central canal of the myelon, 

 while, in front, it narrows into a passage which connects the 

 fourth ventricle with the brain 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 optic lobes. When laid open, 

 each is seen to contain a cavity or ventricle which com- 

 municates with the tier a tertio ad quartum ventriculum, 

 as the ventricle of the mid-brain is termed. The floor 

 of this canal is formed by the thick principal mass of 

 the cerebro-spinal axis. It exhibits a median longitudinal 

 depression or raphe, and in this region represents the crura 

 cerebri. 



In front of the mid-brain comes the hinder division of 

 the fore-brain, or thalamencephalon, which is very distinct in 

 the Frog and contains a median cavity, the third ventricle. 



