& 



BRAIN CURVATURE. 225 



with the head and neck curving of the whole body, owing 

 to the fact that the whole upper dorsal surface of the brain 

 grows miich faster than the lower ventral surface. The 

 result is that the brain is so curved that its parts are after- 

 wards situated thus : the fore-brain lies quite in front and 

 below, the twixt-brain somewhat higher and over it, while 

 the mid-brain lies highest of all and projects furthest for- 

 ward; the hind-brain is situated lower, the after-brain yet 

 further back and below. This disposition occurs only in 

 the three classes of the Amniota, in Reptiles, Birds, and 

 Mammals. (Cf Plates I, VI., and VII.) 



Though, in general features of growth, the brains of 

 Mammals correspond with those of Birds and Reptiles, yet 

 striking differences very soon appear between the two. 

 In Birds and Reptiles (Plate VI. Figs. H and C), the mid- 

 brain (m) and the central part of the hind-brain develop 

 considerably. In Mammals, on the other hand, these parts 

 remain small, and instead, the fore-brain begins to grow 

 so rapidly that it covers the other bladders from in front 

 and above. As it constantly grows further back, it even- 

 tually covers the whole of the rest of the brain above, 

 and also encloses the central part from the sides. This 

 process is of the greatest importance, because this fore-brain 

 is the organ of the higher mental activities, — because in it 

 are accomplished those functions of the nerve-cells, the sum 

 of which is generally designated as the mind, or the "spirit" 

 in the narrower sense. The highest activities of the animal 

 body, the wonderful manifestations of consciousness, the 

 complex phenomena of the activities of thought, have their 

 seat in the fore-brain. It is possible to remove the great 

 hemispheres of a Mammal, piece by piece, without killing 



