xiii.] THE FROG. 239 



over the spinal cord it is usually concealed by a 

 quantity of soft material ; gently remove this with a 

 pair of forceps or wash it away with a syringe. 



2. The brain. 



On the dorsal aspect of the brain, which is 'now ex- 

 posed, the following parts will be seen ; 



a. In front, two elongated masses forming about 

 the anterior half of the brain: a slight trans- 

 verse depression divides each into an anterior 

 smaller and a posterior larger portion. The 

 inner faces of the anterior portions are closely 

 united together; those of the posterior portions 

 are separated by a cleft. The posterior por- 

 tions are the cerebral hemispheres {prosencepha- 

 lon}; the anterior, the bases of the olfactory 

 lobes (rhinencephalon). 



a. The olfactory lobes become narrowed into two 

 rounded trunks (commonly termed the olfactory 

 nerves), which leave the skull, and applying 

 themselves to the outer face of the lining mem- 

 brane of the nasal chamber, give off a number 

 of branches which are distributed on that mem- 

 brane. 



b. The thalamencephalon: lying between the pos- 

 terior ends of the cerebral hemispheres: on it 

 are to be noticed 



a. The pineal gland ' : a very small mass in front; 

 not composed of nervous tissue. 



(3. The thalami optici: the nervous masses seen 

 below the pineal gland : between them lies a 

 narrow cavity, the third ventricle. 



