298 INNERVATION. [CHAP. x. 



brain enjoys a healthy state of function, supposing its texture 

 to be normal. If either prevail, more or less of disturbance 

 will ensue. Their relative quantities, if not in just proportion, will 

 bear an inverse ratio to each other. If there be much blood, the 

 surrounding fluid will be totally, or in a great measure, deficient ; 

 if the brain be anaemic, the quantity of surrounding fluid will be 

 large. 



The existence of these two antagonising forces may be taken as 

 a proof that either of them may prevail; and therefore, from 

 the existence of the cerebro- spinal fluid we may infer that the 

 actual quantity of blood circulating in the brain is liable to 

 variation. 



This fluid is a valuable regulator of vascular fulness within the 

 cranium, and a protector of the brain against too much pressure 

 from within. So long as it exists in normal quantity, it resists 

 the entrance of more than a certain proportion of blood into the 

 vessels. Under the influence of an unusual force of the heart, 

 an undue quantity of blood may be forced into the brain; the 

 effects of which will be, first, the displacement of a part, or of the 

 whole surrounding fluid, and, secondly, the compression of the 

 brain. 



On the other hand, the brain may receive two little blood. In 

 such a case, if the surrounding fluid do not increase too rapidly, 

 the requisite degree of pressure will be maintained, and the healthy 

 action of the brain preserved. But, if the brain be deprived of 

 its due proportion of blood by some sudden depression of the 

 heart's power, there is no time nor source for the pouring out of 

 new fluid, and a state of syncope, or of delirium, will ensue. Such 

 seems to be the explanation of those cases of delirium which ensue 

 upon haemorrhages, large bleedings, or the sudden supervention of 

 inflammation of the pericardium or endocardium. In many of these 

 cases, however, it is important to notice, that the blood is more or 

 less damaged in quality, deficient in soine of its staminal prin- 

 ciples, or charged with some morbid matter; and this vitiated 

 state of the vital fluid has, no doubt, a considerable share in the 

 production of the morbid phenomena. 



The following inferences, which are of practical application, will 

 form a suitable conclusion to these remarks on the circulation 

 within the cranium. 



1. That the brain, although not so amenable to the influence of 

 atmospheric pressure as more superficial parts, is sufficiently so to 

 admit of variations in the quantity of its circulating fluid. 



