THE LAMPREY. 23 



82. The two membranous semilunar aortic valves 

 between the ventricle and the ventral aorta. 



XVII. Turn the fish with the dorsal side upwards; dis- 

 sect away the muscles on the top of the head for 

 about an inch (P. fluviatilis) or two inches (P. 

 marinus) behind the nasal aperture : proceed 

 until the brain, the nasal sac (§ 89), and the audi- 

 tory capsules (§5) are exposed ; remove the roof 

 from the latter. Observe the following : — 



83. The flat band-like spinal cord or myelon (Figs. 4, 

 6 — 8, my, and 9), lying loosely in the neural canal (ii.cd), 

 and passing in front into the brain (Figs. 4 br, and 9), 

 which nearly fills the small cranial cavity. 



84. The medulla oblongata or myelencephalon 

 (Fig. 9, m.o\ the hindmost division of the brain, passing 

 insensibly into the spinal cord behind, and on its upper 

 surface presenting an escutcheon-shaped cavity, covered 

 only by pia mater, the fourth ventricle or myeloccele 

 {v. 4). 



85. The thin ledge of nervous matter overhanging the 

 anterior end of the fourth ventricle is all that represents the 

 cerebellum or epencephalon (Fig. 9, cb). 



86. The mid-brain, or mesencephalon (Fig. 9, o.T), 

 consisting on the upper surface of a pair of rounded eleva- 

 tions, the optic lobes, immediately anterior to the me- 

 dulla ; they contain a cavity, the aqueduct of Sylvius, 

 or mesocoele (aq.s), open above, save for pia mater, and 

 in communication behind with the fourth ventricle. 



87. The thalamencephalon, or twixt-brain (Fig. 9, 

 th), situated just in front of the mesencephalon ; it consists 

 of two paired masses, the thalami optici, between which 

 is a cavity, the third ventricle, or thalamoccele (?'.3), 

 continuous behind with the aqueduct of Sylvius. 



