A FROG 179 



Exercise 11. Draw the dorsal aspect of the brain and the spinal cord. 



Study the lateral surface of the brain and the proximal portions 

 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 as far as 

 possible 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 third and fourth cranial nerves, the oculomotor and the 

 trochlear, are very small and can hardly be found ; they go to 

 muscles of the eyeball. The oculomotor springs from the ven- 

 tral surface of the midbrain, the trochlear from the dorsal sur- 

 face between the optic lobes and the cerebellum. 



The fifth, sixth, seventh, and eighth cranial nerves, which 

 are the trigeminal, abducens, facial, and auditory, respectively, 

 arise close together from the forward end of the medulla ob- 

 longata. The first three of these nerves, together with the ante- 

 rior end of the sympathetic nerve, are united in the Gasserian 

 ganglion. The trigeminal nerve is the largest of these three ; it 

 arises from the side of the brain just beneath the cerebellum and 

 passes forward to the ganglion. The abducens is a very slender 

 nerve which arises from the ventral surface of the medulla near 

 the median line. The facial and auditory nerves arise behind 

 the trigeminus. The auditory is the larger and passes directly 

 to the auditory capsule ; the facial is much smaller and passes 

 alongside the trigeminus to the Gasserian ganglion. 



