146 



THE MECHANISM OF LIFE 



The mediating mechanism is now greatly complicated, inas- 

 much as the receptor organs which are in primary connection 

 with the spinal ganglia are also in connection with the ganglia of 

 the organs of special sense in the head, and there are also motor 

 paths between the mid-brain :and the cord. Obviously actions 

 immediately carried out by the motor centres in the cord can 

 now become more complex, and may be co-ordinated to a greater 

 extent because of the additional stimuli given by the special 



c: ^ 



J4u5cles 

 of 



trunk 

 and 



limbs 



nef^es 



Fig. 44. — Main Paths between Spinal Cord and Lower Brain. 



sense organs. Still, the responses may largely be automatic and 

 inevitable ones, although there must be some subtle difference 

 in them when compared with the purely spinal reactions. 



Such a structure and relationships are, in general, those of the 

 fish, where the cortex is absent and the cerebellum is rudimentary. 

 Now consider the additional complexity which is brought about 

 by the development of the cerebral hemispheres and cerebellum, 

 organs which have evolved simultaneously, or nearly so. 



Neglecting the intermediate stages of this evolution (for we 



