862 



THE NERVE SYSTEM 



of lower centres, as in the way of motor responses to external impressions; such 

 reactions may be delayed or immediate according to the exercise of the will power 

 residing in the cerebral cortex. 



This control by the will is intensified the higher we ascend the animal scale: 

 the pyramidal tract, which originates in the cerebral cortex and threads its way 

 to the motor centres of the spinal cord without interruption, along the brain axis, 

 is better developed in man than in any other animal. In the course of evolution 

 the lower or more automatic ganglia and tracts remain relatively about the same 

 in mass as in other mammalia, but the higher, more intellectual ganglia surpass 

 these in growth, so that there is an apparent but not real diminution of the auto- 

 matic systems observed in the human brain. 



-3i.fc-'-',,-' ~V-'~-*^^i.:ail- " -^^*-^^" 



BASAL GANGLIA 

 OF FORE-BRAIN 



CORPORA 



OUADRIGEMINA 



RETICULAR GANGLIONIC 



MASS WITH CRANIAL 



NERVE NUCLEI 



CENTRAL GRAY (FLOOR OF 

 FOURTH VENTRICLE AND 

 AROUND AQUEDUCT) 



CENTRAL GRAY OF 

 SPINAL COAO 



FIG. 630. Schematic representation of the chief ganglionic categories. Accurate topographical relations and 



interconnections are shown in other figures. 



It has been seen from the foregoing brief accounts of the development of the 

 nerve system that the most prominent feature is the redundant growth of the 

 cephalic or brain end of the neural tube. Comparative neuroanatomic re- 

 searches have thrown much light upon the probable genesis of this remarkable 

 characteristic. The ancestral vertebrate, built upon the segmental type, was a 

 swimming animal, and its locomotion took place in the direction of its long axis. 

 In its progress through the water the cephalic (or anterior) segments were those 

 which first encountered the foreign objects floating in the same medium. It was 

 for these segments to determine the quality of the objects encountered whether 



