104 T^^ POPULAR SCIENCE MONTHLY. 



edly do not interfere with the passage of impulses along those paths, 

 we may put them aside, remembering that they probably are con- 

 cerned with low actions of the nervous system, such as eating, etc., 

 which are popularly termed automatic functions. 



In this photograph of a model made by Professor Aeby, of Berne, 

 you see represented from the front the two cerebral hemispheres with 

 the centers in the cortex as little masses on the surface, and the basal 

 ganglia as darker ones at the bottom, while leading from them down 

 into the spinal cord are wires to indicate the channels of communica- 

 tion. 



Note, in passing, that both hemispheres are connected by a thick 

 band of fibers called the " corpus callosum." It is, I believe, the close 

 tinion thus produced between the two halves that leads in a great 

 measure (though not wholly) to consonance of ideas. 



The arrangement of the fibers will be rendered still clearer by this 

 scheme, in which the cortex is represented by this concave mass, and 

 the fibers issuing from the same by these threads. The basal ganglia 

 would occupy this position, and they have their own system of fibers. 



I will now leave these generalizations, and explain at once the great 

 advance in our knowledge of the brain that has been made during the 

 last decade. The remarkable discovery that the cortex or surface of 

 the brain contained centers which governed definite groups of muscles, 

 was first made by the German observers Hitzig and Fritsch ; their 

 results were, however, very incomplete, and it was reserved for Pro- 

 fessor Ferrier to produce a masterly demonstration of the existence 

 and exact position of these centers, and to found an entirely new 

 scheme of cerebral physiology. 



The cortex of the brain, although it is convoluted in this exceed- 

 ingly complex manner, fortunately shows great constancy in the ar- 

 rangement of its convolutions, and we may therefore readily grasp the 

 main features of the same without much trouble. From this photo- 

 graph of the left side of an adult human brain you will see that its 

 outer surface or cortex is deeply fissured by a groove running back- 

 ward just below its middle, which groove is called the "fissure of Syl- 

 vius," after a distinguished mediaeval anatomist. This fissure, if car- 

 ried upward, would almost divide the brain into a motor half in front 

 and a sensory half behind. 



Of equal practical importance is another deep fissure which runs at 

 an open angle to the last, and which is called the " fissure of Rolando," 

 Rolando being another pioneer of cerebral topography. Now, it is 

 around this fissure of Rolando that the motor side of the centers for 

 voluntary movement is situated ; and when this portion of the cortex 

 is irritated by gentle electric currents, a constant movement follows 

 according to the part stimulated. 



Because of their upward direction, the convolutions bounding the 

 fissure of Rolando are called respectively the " ascending frontal " and 



