THE FROG 107 



lar body extending up to the skull. At its base and partly concealed 

 by it is the anterior choroid plexus, the roof of the diencephalon. 



Back of the optic lobes, and separated from them by a deep 

 groove, is a narrow, transverse ridge, the cerebellum ; and back of 

 that is the medulla oblongata, which is continuous with the spinal 

 cord. The dorsal wall of the medulla is a dark-colored, vascular 

 membrane called the posterior choroid plexus, beneath which is the 

 fourth ventricle of the brain. The triangular depressed area which 

 these structures form is called the fossa rhomboidalis. 



The spinal cord is the portion of the central nervous system 

 which lies in the neural canal of the spinal column. It is a thick, 

 white band, oval in cross section, from which the paired spinal 

 nerves spring. At two points it is swollen,— where the spinal nerves 

 forming the brachial plexus and those forming the sciatic plexus, 

 respectively, leave it. The hinder end of the cord tapers rapidly 

 until it becomes a fine thread which extends into the urostyle. 



Exercise 13. Draw the dorsal aspect of the brain and the spinal cord. 



Study the lateral surface of the brain, and the proximal por- 

 tions of the cranial nerves. Ten pairs of these nerves are present 

 in the frog; several pairs are so small, however, that they may 

 not be seen. 



The first cranial nerve is the olfactory, which extends forward 

 from the olfactory lobe. Cut away the roof of the anterior portion 

 of the skull and follow the two olfactory nerves forward. Each 

 will be seen to branch a short distance in front of the olfactory lobe 

 and be distributed to the walls of the nasal capsule. 



Cut the olfactory nerves. Dissect away the left side of the skull 

 and expose the left surface of the brain, preserving so far as pos- 

 sible the nerves which will be seen coming from it. 



Lying close to the inner wall of the skull, at the hinder 

 end of the orbit, is a yellowish body, often surrounded by a 

 calcareous sac. This is the Gasserian ganglion, and must not be 

 injured. Just behind the hemispheres the optic nerve, the second 

 cranial nerve, issues from the ventral surface of the diencephalon 

 and extends forward to the eye. The thickened side of the dien- 

 cephalon is called the optic thalamus. 



