358 DISSECTION OF THE RABBIT 



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 pneumo- 

 gastric, 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 outgrowths ; 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 Hfe, 

 and are spoken of in the adult as the ventricles of the brain. 



1. Dissection of the brain. 



a. The cerebral hemispheres. 

 Gently press the hemispheres apa/rt, amd note the position 

 of the corpus callosum. 



Then, with the back of a knife, peel off the portion of the 

 right hemisphere Vyimg above the corpus callosum. Keep the 

 brcdn wet with spirit. 



i. The corpus callosum is a broad horizontal band 



of fibres connecting the two hemispheres, 

 ii. The structure of the hemisphere. 

 a. The outer layer or cortex is a superficial in- 

 vestment of grey matter, about an eighth 

 of an inch thick. 



