748 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



spinal, and as it is no less readily absorbed than reproduced, it evidently serves 

 as an equalizer of the amount of pressure within the cranial cavity ; admitting 

 the distension or contraction of the vessels to take place, within certain limits, 

 without any considerable change in the degree of compression to which the 

 nervous matter is subjected. That this uniformity is of the greatest importance 

 to the functional exercise of the brain, is evident from a few well-known facts. 

 If an aperture be made in the skull, and the protruding portion of the brain be 

 subjected to pressure, the immediate suspension of the activity of the whole or- 

 gan is the result ; in this manner, a state resembling profound sleep can be in- 

 duced in a moment, the normal activity being renewed as momentarily so soon 

 as the pressure is withdrawn. This phenomenon has often been observed in the 

 Human subject, in cases in which a portion of the cranial envelop has been lost 

 by disease or injury. The various symptoms of Cerebral disturbance which are 

 due to a state of general Plethora, are evidently owing to an excess of pressure 

 within the vessels ; but an undue diminution of pressure is no less injurious, 

 as appears from the disturbance in the Cerebral functions which results from the 

 very opposite cause, namely, a depression of the power of the heart, or a defi- 

 ciency of blood in the vessels. It is of peculiar importance to bear in mind the 

 disturbance of the Cerebral functions, which is occasioned by internal pressure, 

 when we are endeavoring to draw inferences from the phenomena presented by 

 disease. 



779. We shall now proceed with our physiological inquiry into the functions 

 of the Cerebrum ; and shall, as before, apply to Human and Comparative Ana- 

 tomy, to Experiment, and to Pathology, for our chief data. The anatomical 

 relations of the Cerebrum to the other Encephalic centres clearly demonstrate 

 that it is not one of the essential or fundamental portions of the Nervous system ; 

 but a superadded organ, receiving all its impulses to action from the parts below, 

 and operating upon the body at large through them. And its great bulk, joined 

 to its position at the summit of the whole apparatus the vesicular substance 

 of its convolutions affording a termination to the fibres in connection with it, 

 and not being merely traversed by them, as is the case with the vesicular sub- 

 stance of all the lower centres clearly mark it out as the highest in its func- 

 tional relations, and as ministering, so far as any material instrument may do, 

 to the exercise of those psychical powers which, in Man, exhibit so remarkable 

 a predominance over the mere animal instincts. This conclusion is fully borne 

 out, when we extend our inquiries from Human to Comparative Anatomy ; for 

 with some apparent exceptions, which there would probably be no great diffi- 

 culty in explaining if we were in possession of all the requisite data, there is a 

 very close correspondence between the relative development of the Cerebrum in 

 the several tribes of Vertebrata, and the degree of Intelligence they respectively 

 possess using the latter term as a comprehensive expression of that series of 

 mental actions, which consists in the intentional adaptation of means to ends, 

 based on definite ideas as to the nature of both. It is the essential character 

 of Instinctive actions, on the other hand, that they are performed automatically, 

 in obedience to internal impulses, without even the perception of their adapt- 

 iveness on the part of the being who is the agent in them ; these impulses being 

 called into play by impressions on the nervous system, which are made either 

 by external objects, or by changes in the individual organism. The justness of 

 this distinction becomes obvious when we analyze our own consciousness, and 

 distinguish our own Instinctive actions from those which involve Intelligence ; 

 for we are thus led to perceive that, in regard to those operations which are most 

 closely concerned in the maintenance of our own lives and in the continuance 

 of the race, provision has been made in the mechanism of the Automatic por- 

 tion of our nervous system, so as to render them independent of the exercise of 

 Intelligence or the exertion of Will on our own parts. Thus the infant seeks the 



