346 DISSECTION OF THE BABBIT 



8. The auditory nerve is large, and arises from the side 

 of the medulla, immediately behind and to the outer 

 side of the facial nerve. 



9 and 10. The glosso-pharyngeal and pneumogastric nerves 

 arise close together, by many slender rootlets, from 

 the side of the medulla, close behind, and a little 

 above the root of the auditory nerve. 



11. The spinal accessory nerve arises from the side of 



the medulla and spinal cord by about ten rootlets, 

 the most anterior of which is just behind the 

 pneumogastric, and the hindmost as far back as the 

 fifth spinal nerve. 



12. The hypoglossal nerve arises by a number of rootlets 



from the ventral surface of the medulla, close to the 

 middle line, and to the outer side of the ventral 

 pyramid. 



D. The Structure of the Brain. 



The brain is, from the mode of its formation, tubular; 

 its cavity being directly continuous with the central canal 

 of the spinal cord. From a very early period it presents a 

 series of dilatations or vesicles, partially separated from one 

 another by constrictions. Of these vesicles, the first or most 

 anterior one is the fore-brain, or thalamencephalon, from 

 which the cerebral hemispheres arise as hollow out-growths ; 

 the second vesicle is the mid-brain or mesencephalon ; and 

 the succeeding ones, of which there are at least four or five, 

 are spoken of collectively as the hind-brain. The cavities 

 of the vesicles remain in communication with one another 

 throughout life, and are spoken of in the adult as the ven- 

 tricles of the brain. 



1. Dissection of the brain, 



a. The cerebral hemispheres. 



Gently press the hemispheres apart, and note the position 

 of the corpus callosum. 



Then, with a razor or sharp knife, cut off successive hori- 

 zontal slices from the top of the right hemisphere until the 



