426 THE NERVOUS SYSTEM. 



voluntary muscles. The intermediate process, between the sensation 

 and the volition, may be short and simple ; or it may be long and 

 complicated, involving the continued suggestion of many successive 

 ideas. There can be little doubt that, in either case, it is accompanied 

 by actions of some kind in the gray substance of the cerebral hemi- 

 spheres ; for if these organs are injured or defective, the mental opera- 

 tions are obstructed or disturbed. 



But the nature of the nervous process accompanying mental action 

 is unknown. Physiological research gives us no information with 

 regard to the brain as an organ of intelligence, beyond the fact that it 

 is, in some way, essential to its manifestation ; and all the modern 

 investigations into its structure and physiological properties have failed 

 to increase, in any essential particular, our knowledge of its office and 

 action in the operations of the mind. 



Localization of Function in different parts of the Hemispheres. 

 On the other hand, the most valuable information has been obtained of 

 late years from the study of the simpler nervous functions and their 

 localization in different parts of the hemispheres. The recent improve- 

 ments in our knowledge of cerebral physiology relate almost entirely 

 to the brain as an organ for combining and regulating the nervous 

 mechanism of conscious sensations and voluntary movements. They 

 show that certain parts of the cortex of the hemispheres are connected 

 with phenomena of motion, others with the power of sensation ; while 

 others still, so far as yet known, are indifferent to both these functions, 

 and are perhaps connected with nervous acts of a different kind. The 

 hemispheres, accordingly, do not act indiscriminately as a whole ; but 

 the convolutions of particular regions have a structure and properties 

 differing from those elsewhere. The knowledge thus far obtained 

 relates chiefly to three different points, namely, 1st. Centres of Motion ; 

 2d. Centres of Sensation ; 3d. The Centre of Language. 



I. The beginning of the present doctrine on this subject was the dis- 

 covery, by Fritsch and Hitzig* in 1810, of the centres of motion in 

 the hemispheres of the dog. They showed that galvanic currents, of 

 low intensity, applied to certain points on the surface of the convolu- 

 tions, give rise to definite movements of the head, body, or limbs ; 

 while no such effect is produced by galvanization of the cerebral sur- 

 face in other regions. These experiments were subsequently extended 

 to cats, guinea-pigs, rabbits, and monkeys. They have been confirmed 

 by many other observers in England, France, Italy, and the United 

 States, and we have repeatedly verified their main results, f 



The important features of these experiments are as follows. When 

 the animal is etherized, and the convexity of the hemisphere exposed 

 on one side by trephining the skull, the poles of a galvanic battery, 

 applied to many parts of the convoluted surface, produce no visible 



*Archiv fur Anatomie, Physiologic und wissenschaftliche Medicin. Leipzig, 

 1870, p. 300. Plitzig, Untersuchungen iiber das Gehirn. Berlin, 1874. 

 f New York Medical Journal, March, 1875, p. 225. 



