THE FROG 35 



of the body wall to the skin. Next remove the skin of the 

 body, the roof of the cranial cavity, the muscles of the back, 

 and with sharp pointed scissors cut away the arches of the 

 vertebrae. This will expose the brain and spinal cord which 

 will be found to be covered with a delicate blackish membrane, 

 the pia mater. In addition to this inner covering there is 

 an outer one, the dura mater, which lies close to the skull 

 and vertebrae, and has been removed with the bone. 



The following parts may now be identified, beginning 

 with the anterior end : 



(a) Olfactory Lobes. These are at the extreme anterior 

 end of the brain. They are not distinct lobes but 

 are separated by a shallow groove from the rest of 

 the brain. Are the two lobes separate or fused? 

 Olfactory nerves pass forward from these lobes to 

 the nostrils. 



(6) Cerebral Hemispheres. Two large ovoid bodies im- 

 mediately posterior to the olfactory lobes. 



(c) Thalamencephalon, a narrow, cylindrical portion 

 connecting the cerebral hemispheres with the optic 

 lobes. Upon it, occupying a median position, is 

 a small body, the pineal gland. 



(d) Optic Lobes, posterior to the thalamencephalon. 

 What is the size and shape of these parts as com- 

 pared with the cerebral hemispheres? 



(e) The Cerebellum is a small transverse fold posterior 

 to the optic lobes. 



(/) Medulla Oblongata, following the cerebellum. In it is 

 a triangular cavity, the fourth ventricle. 



(</) Spinal Cord. The medulla tapers gradually into the 

 cord without any sharp demarcation. Is the cord 

 as long as the body? Is it of equal width through- 

 out its length? If not, in what regions is it widest? 



