STRUCTURE OF THE ENCEPHALON. 



199 



interval, on each side lies a deep fissure, fissure of Sylvius, 

 separating the middle from the anterior lobe of the hemi- 

 sphere, and in this is concealed a group of convolutions, the 

 island of Reil, corresponding exactly in position with the 

 corpora striata seen from the interior of the brain. On the 

 under surface of the anterior lobes are the olfactory bulbs, 

 from which the nerves of smell take origin. 



148. There are many other complications in the structure of 

 the brain which have not been alluded to; but probably the 

 best general conception of the whole organ will be obtained 

 by looking now at some of the simpler forms seen in the lower 

 animals, and in development. 



If we examine the brain of a codfish, we see at the back 

 part the " posterior" columns of the cord separate one from 

 the other, so as to leave a hollow between them, with a 

 mesial groove, which is continuous with the central canal of 

 the cord, and is the fourth ventricle. Overlying this is a 

 mesial pouch, the cerebellum; in front of the cerebellum are 

 two bodies, the optic lobes, likewise hollow; and in front of the 

 optic lobes is another pair of small bodies, which may be called 



Fig. 103. BRAIN OF A COD. A, From above. B, From below, 

 a, Medulla ohlongata; 6, cerebellum; c, optic lobe; d, hemi- 

 sphere -vesicle ; e, olfactory bulb; /, /, optic nerves; g, g, hypo- 

 aria ; h, pituitary body. 



