RESPONSIVE LIFE OF ORGANISMS 



463 



which extend across from one side to the other joining like 

 parts of the two halves of the brain; 2) association fibers, 

 which wholly within the hemispheres and, extending 

 between lobes, connect one part of a hemisphere with 

 another part; and 3) projection fibers, which connect the 

 cortex of the hemispheres with underlying parts of the brain 

 and with the cord. Only the main tracts of projection 

 fibers are indicated in the diagram. Some of these fibers 

 will be seen to extend directly down the cord (as the fibers 

 of the motor tract B, through which the movements of the 

 body are controlled at will), some end in the medulla 

 (tract C) , others in the mid-brain (tract D) , others in the 

 potts (tract A), etc. The diagram also shows the three 

 main tracts that go out from the cerebellum, to the mid- 

 brain (tract F) , to the pons (tract G) , and into the cord 

 (tract H). 



We have seen in the salamander that the principal "dis- 

 tance receptors", eyes and ears 

 and olfactories, are connected 

 directly with the brain. Figure 

 262 shows the nerves of these 

 organs, and also the other cranial 

 nerves as they occur in our- 

 selves. 



The structure of the cortex of 

 the hemispheres is of very great 

 interest to us because this is the 

 highest control-center of the 

 body. An intelligent animal 

 with its cerebral cortex removed 

 becomes a mere automaton, 

 without volition or spontaneity 

 of action. The nutritive process 

 may go on : consciousness is lost. 

 Five layers of cells are recog- 



FIG. 262. Diagram of the relations 

 of brain and cranial nerves in 

 man. M, medulla, C, cerebellum, 

 cer, cerebral hemisphere. 1, 

 olfactory nerve. 2, optic nerve. 

 3 4, and 6 oculo-motor nerves. 

 5 sensory nerve of the face, 7 

 motor nerve of the face. 8, 

 auditory nerve. 9, the glosso- 

 pharyngeal nerve. 10, the vagus 

 nerve, s, spinal nerves. 



