314 ELEMENTARY PHYSIOLOGY. [XXX. 



which crosses the olfactory lohe, its main branch 

 running in the fissure of Sylvius, cp. C, § 6, 

 giving off numerous branches, and the anterior 

 cerebral artery which passes forwards between 

 the front lobes of the cerebrum, having a trans- 

 verse communicating branch with its fellow of 

 the opposite side. The anastomoses between 

 the branches of the posterior cerebral and the 

 internal carotid arteries and those between the 

 two anterior cerebral arteries complete the 

 circle of Willis. 



9. Now carefully tear away the pia mater and 

 observe the points of exit of the foUwing nerve- 

 roots (cp. Lesson i. p. 12). 



a. The third pair of nerves, arising from the 

 inner surfaces of the crura cerebri in front of 

 the pons Varolii (cp. § 8). 



b. The fourth pair of nerves, which will be seen 

 curving round the front edge of the pons; 

 they originate a -little behind the corpora 

 quadrigemina (cp. § 4, c). 



c. The fifth pair of nerves, large and con- 

 spicuous, arising from the sides of the pons. 



d. The sixth pair of ner\'es, arising in front 

 of the olivary bodies and anterior pyramids 

 close behind the pons. 



e. The. seventh pair of nerves seen at the 

 lower edge of the pons nearly in a line with 

 the fifth pair, they arise from the trapezium 

 in a line with the outer border of the inferior 



