292 INNERVATION. [CHAP. x. 



And admitting, For the present, that the hemispheres are the com- 

 mon centre of sensitive impressions, it is easy to understand how 

 nervous power, excited by the impulse of sound upon the ear, for 

 example, may be propagated along the auditory nerves to the olivary 

 columns in the fourth ventricle, and thence to the optic thalami, in 

 which are found many fibres which are continuous with those of 

 the hemispheres, and capable of propagating the nervous force to 

 the convolutions. 



To this it may, however, be objected, that perfect sensation is 

 frequently coexistent with a cerebral lesion, sufficient to produce 

 very complete paralysis of motion, and that an enduring paralysis 

 of sensation is a rare accompaniment of cerebral disease. But such 

 facts do not so much militate against these views, as they serve 

 to denote that the channels of sensation are more numerous than 

 those of motion ; and that, if one route be interrupted, another is 

 easily opened. It may be, that the commissures are valuable 

 instruments for this purpose ; and it is highly worthy of notice, 

 that no segment of the cerebrum has so many commissures, either 

 with the opposite or its own side, as the optic thalamus. 



It should be borne in mind that the foregoing remarks are partly 

 conjectural, and that they are introduced rather as a convenient 

 form of illustration, than as implying more than a probability of 

 their general correctness and accordance with the best established 

 views. 



Of the Circulation in the Brain. An organ of such great size, of 

 such high vital endowments, so active, and which exerts so consi- 

 derable an influence upon all other parts of the body, must neces- 

 sarily require a large supply of the vital fluid. Hence we find that 

 the blood-vessels of the brain are numerous and capacious. Four 

 large arteries carry blood to it ; namely, the two internal carotids, 

 and the two vertebrals. Each carotid penetrates the cranium at the 

 foramen on the side of the sella Turcica, and almost immediately 

 divides into three branches, the anterior and the middle cerebral 

 arteries, and the posterior communicating artery. 



The anterior cerebral arteries supply the inner sides of the ante- 

 rior lobes of the brain : they ascend through the great longitudinal 

 fissure, and pass along the upper surface of the corpus callosum, 

 giving off branches to the inner convolutions of both hemispheres 

 of the brain. These arteries anastomose with each other just 

 beneath the anterior margin of the corpus callosum by a transverse 

 branch, called the anterior communicating artery. The middle cere- 

 bral arteries, the largest branches of the carotids, pass outwards 



