CHAPTEE IX. 



The Midbrain, oa Mesekcephalon. 



Theee is no part of the brain into which such large tracts enter, none 

 from which so many tracts emerge for distribution to remote parts of the 

 nervous system, and none within which are furnished so many connections 

 between right and left side as the midbeain: j?he mesencephalon of the 

 lower vertebrates. Only in mammals, where the cerebrum is developed into 

 the great organ peculiar to that class, does there arise a brain-segment 

 which contains still more extended connections and still greater commissural 

 tracts. 



Fig. 63. — Brain of the cod: Gadus wglefmis. 



Even the external appearance is suggestive of the significance of the 

 structure. The accompanying figure (Fig. 63) of the brain of the cod shows 

 at once how relatively great is the development of the mesencephalon, and 

 that it is approached in size only by the medulla, which is the origin of the 

 large cranial nerves. The cerebrum and, indeed, even the cerebellum — 

 always unusually large in teleosts — are hardly to be compared in size with 

 the midbrain and medulla. 



In the description of the midbrain it is advisable to differentiate at once 

 a roof-segment and a basal segment. Throughout the entire animal king- 

 dom the roof exhibits fewer changes than any other part of the brain. The 

 (112) 



