THE LAMPREY. 25 



the under side of the mesencephalon, and passing outwards and slightly 

 forwards to the orbit, where they supply most of the eye muscles. 



93. The fourth pair of nerves (Fig. 9, IV), arising from the dorsal 

 side of the anterior end of the medulla. They supply the superior 

 oblique muscles of the eyes. 



94. The large nerve cord formed by the united roots of the fifth and 

 seventh nerves (Fig. 9, V, VII), leaving the brain just in front of 

 the auditory capsule, and passing forwards and outwards. 



95. The auditory sac (Fig. 9, v), exposed by the 

 removal of the dorsal wall of the auditory capsule ; it con- 

 sists of an ovoidal membranous sac, the vestibule (7;), 

 presenting on its upper surface, towards the outer side, two 

 semicircular canals {a.s.c, p.s.c), corresponding , to 

 the anterior and posterior canals of the higher vertebrata, 

 the horizontal canal being absent. 



96. The auditory nerve (Fig. 9, VIII), passing directly outwards 

 from the medulla oblongata to the auditory sac. 



.97. The glossopharyngeal (ninth) and vagus (tenth) nerves (Fig. 

 9, IX, X), arising from the medulla oblongata, just posterior to the 

 auditory nerve, and passing backwards and outwards. 



XVIII. With a guarded bristle find an aperture lying 

 in the anterior ventral region of the nasal sac, close 

 under the entrance of the external nasal canal (see 

 Fig. 4) ; pass the bristle carefully into the opening ; 

 it will be found to take a course backwards and 

 downwards. Without removing the bristle, turn the 

 fish once more with its left side upwards, and dissect 

 away the tissues between the anterior end of the 

 notochord and the roof of the mouth, until the 

 bristle is found. It will be found to have passed 

 into 

 98. The nasal caecum (Fig. 4, na'), a considerable 



pouch, lying close under the anterior end of the notochord. 



opening anteriorly, as already seen, into the ventral side of 



