THE NERVOUS SYSTEM. 129 



Unilateral action. Respecting the mode in which the brain dis- 

 charges its functions, there is no evidence whatever. But it appears 

 that, for all but its highest intellectual acts, one of the cerebral hemi- 

 spheres is sufficient. For numerous cases are recorded in which no 

 mental defect was observed, although one cerebral hemisphere was 

 so disorganized or atrophied that it could not be supposed capable of 

 discharging its functions. The remaining hemisphere was, in these cases, 

 adequate to the functions generally discharged by both; but the mind does 

 not seem in any of these cases to have been tested in very high intellectual 

 exercises; so that it is not certain that one hemisphere will suffice for 

 these. In general, the mind combines, as one sensation, the impressions 

 which it derives from one object through both hemispheres, and the ideas 

 to which the two such impressions give rise are single. In relation to 

 common sensation and the effort of the will, fhe impressions to and from 

 the hemispheres of the brain are carried across the middle line; so that in 

 destruction or compression of either hemisphere, whatever effects are pro 

 duced in loss of sensation or voluntary motion, are observed on the side 

 of the body opposite to that on which the brain is injured. 



Localization of Functions. In speaking of the cerebral hemi- 

 spheres as the so-called organs of the mind, they have been regarded as if 

 they were single organs, of which all parts are equally appropriate for the 

 exercise of each of the mental faculties. But it is possible that each 

 faculty has a special portion of the brain appropriated to it as its proper 

 organ. For this theory the principal evidences are as follows: 1. That 

 it is in accordance with the physiology of the compound organs or systems 

 in the 'body, in which each part has its special function; as, for example, 

 of the digestive system, in which the stomach, liver, and other organs 

 perform each their separate share in the general process of the digestion 

 of the food. 2. That in different individuals the several mental func- 

 tions are manifested in very different degrees. Even in early childhood, 

 before education can be imagined to have exercised any influence on the 

 mind, children exhibit various dispositions each presents some predom- 

 inant propensity, or evinces a singular aptness in some study or pursuit; 

 and it is a matter of daily observation that every one has his peculiar 

 talent or propensity. But it is difficult to imagine how this could be the 

 case, if the manifestation of each faculty depended on the whole of the 

 brain; different conditions of the whole mass might affect the mind gen- 

 erally, depressing or exalting all its functions in an equal degree, but 

 could not permit one faculty to be strongly and another weakly mani- 

 fested. 3. The plurality of organs in the brain is supported by the 

 phenomena of some forms of mental derangement. It is not usual for 

 all the mental faculties in an insane person to be equally disordered ; it 

 often happens that the strength of some is increased, while that of others 

 is diminished; and in many cases one function only of the brain is 

 VOL. II. 9. 



