THE EVOLUTION OF THE BRAIN 10 1 



all Other parts of the brain that they have covered not only 

 the stem but also the cerebellum so that the external view 

 of the human brain is almost entirely that of its hemispheres. 

 Thus the brain of man differs from that of the frog chiefly 

 in the disproportionate growth of two of its parts, the 

 cerebellum and especially the hemispheres. How dispro- 

 portionate this growth is may be judged from the fact that 

 in man the stem of the brain represents about 2 per cent of 

 its total weight, the cerebellum about 1 1 per cent, while the 

 hemispheres account for 87 per cent. The distinguishing 

 feature of man as an organism is his inteUigence and the 

 distinguishing feature of his brain is the relatively enormous 

 size of the hemispheres. Hence it is natural to conclude 

 that the hemispheres are that part of his brain concerned 

 with his intelhgence, a conclusion confirmed by many 

 other hues of evidence. 



In the earher part of this chapter the statement was made 

 that the brain was the most comphcated organ in the verte- 

 brate body. This compHcation, which is as apparent in the 

 spinal cord as in the brain itself, is due to the enormously 

 intricate system of centers and connections that go to make 

 up the structure of these parts. The essential element in 

 this structural complexity is the nerve cell or neurone. 

 The structure of these nervous units is discussed in chapterviii. 

 The cell bodies of neurones are commonly concerned in the 

 formation of nerve centers and their processes, the nerve 

 fibers, are the means of connection between these centers. A 

 true nerve cell or neurone consists not only of a cell body 

 but also of such nerve fibers as grow out from that body 

 and since in some instances these fibers are extremely long 

 the spread of a single neurone is sometimes very considerable. 

 Thus in the hemispheres of the human brain are certain 

 cell bodies whose nerve fibers extend downward not only 

 through the brain but through the spinal cord almost to its 

 lower end. Here they terminate in contact with a second 

 set of neurones whose nerve fibers may extend as compo- 

 nents of one of the nerves of the leg to the muscles of the 

 toes. Thus the first neurone has its origin in the head and 

 the second terminates in the toe; together they represent 

 a length about equal to that of the human body. When it is 



