2IO 



THE EVOLUTION OF MAN. 



marrow/' the peripheric nerve-system. Some, as sensory 

 nerve-fibres, convey the sensations of the skin and of other 

 sense-organs to the central medulla ; others, as motor nerve- 

 fibres, transmit the impulses from the central marrow to the 

 muscles. 



Fig. 217. — Human embryo 

 of three months, in natural 

 size, seen from the dorsal side ; 

 the brain and dorsal marrow 

 exposed (after Koelliker) : h, 

 hemispheres of the cerebrum 

 (fore-brain); m, "four-bulbs" 

 (mid-brain) ; c, small brain 

 (hind -brain, or cerebellum). 

 Below the latter is the 

 three-cornered "neck-medulla" 

 (after-brain) . 



'Fig. 218. — Central marrow 

 of a human embryo of four 

 months, in natural size, seen 

 from the dorsal side (after 

 Koelliker) : h, large hemi- 

 spheres; v, "four-bulbs;" c, 

 small brain ; mo, neck-medulla. 

 Below this the dorsal medulla 

 marrow). 



The central nervous system, or central marrow (medulla 

 centralis), is the actual organ of mental activities, in the 

 stricter sense. Whatever view is taken of the intimate 

 connection between this organ and its functions, it is, at 

 least, certain that those of its special activities which we 

 call sensation, volition, and thought, are in man, as in all 

 the higher animals, inseparably connected with the normal 

 development of this material organ. Hence we must neces- 

 sarily take a deep interest in the history of the development 



