164 Mammalia, 



lobes of the Cerebrum), but not covered (as it is in Man) by 

 tbe anterior development of the middle lobe; tbe second 

 branch Q anterior Cerebral') of each, side coming into close 

 relation in front of the Optic Commissure, and sending off 

 anastomosing twigs to each other as they pursue their parallel 

 course along either side of the base of the longitudinal fissure 

 between*the Cerebral hemispheres. These remarkable anas- 

 tomoses, first between the branches of the vertebral and inter- 

 nal Carotid Arteries ; and secondly between the anterior 

 Cerebral Arteries inter se, constitute the Circle of Willis, and 

 form a means for equalising the circulation in the brain. 



Removal of Membranes. The student should now care- 

 fully remove the adherent membranes from the surface of one 

 half* of the brain, being especially careful at the followiDg 

 points, viz. the origin of the third pair of nervesf; the division 

 between the Cerebrum and Cerebellum, especially the external 

 part of that division in which the fourth nerve lies ; the 

 lateral extremities of the Pons where is the origin of the fifth 

 pair ; the anterior end of the Medulla adjoining the Pons, 

 where are the origins of the seventh pair (laterally), and of 

 the sixth pair (near the median line) ; the lateral border of 

 the Medulla oblongata for the eighth pair ; and a little inter- 

 nal thereto for the ninth pair and lastly, at the posterior end 

 of the longitudinal fissure of the Cerebrum superiorly, so as not 

 to tear away the pineal gland or velum interpositum. 



In all cases it is best to leave the membrane round the roots of 

 the nerves f cutting round them with the scissors : and in clean- 

 ing the fissures, to open them out with the handle of the 

 scalpel. 



This done, the following structures present themselves for 

 notice at the base of the Brain. 



* In order to be able to compare tbe two sides after removal of tbe membrane, 

 t Supra, p. 163. 



