706 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



lation through the same channel as that of the 

 dura inater, of the cranial bones, and of the 

 eyeball : the internal jugular veins are the 

 channel towards which the venous blood of the 

 cranium tends. An obstacle, therefore, in both 

 or either of these vessels must affect the entire 

 venous system ot the brain, or at least that of 

 the corresponding hemisphere. A ligature tied 

 tightly round the neck impedes the circulation, 

 and may cause congestion of the brain. The 

 bodies of criminals who have died by hanging 

 exhibit great venous congestion, both of the 

 walls and the contents of the cranium, in con- 

 sequence of the strong compression to which 

 the veins have been submitted. 



We have seen that, when the blood of one 

 carotid artery is cut off, the parts usually sup- 

 plied by it are apt to become exsangueous and 

 softened ; and this is more especially the case 

 if the vertebral be stopped up, or the circula- 

 tion in it impeded. And it has been remarked, 

 that these effects will follow the application of 

 a ligature to either common carotid artery. 



Notwithstanding these facts, a doctrine has 

 received very general assent, and the support 

 of men of high reputation, which affirms that 

 the absolute quantity of blood in the brain 

 cannot vary, because that organ is incom- 

 pressible, and is enclosed in a spheroidal case 

 of bone, by which it is completely exempted 

 from the pressure of the atmosphere. 



The cranium, however, although spheroidal, 

 is not a perfectly solid case, but is perforated 

 by very numerous foramina, both external and 

 internal, by which large venous canals in the 

 diploe of the bones communicate with the cir- 

 culation of the integuments of the head as well 

 as with that of the brain ; so that the one can- 

 not be materially affected without the other 

 suffering likewise. And as the circulation in 

 the integuments is not removed from atmo- 

 spheric pressure, neither can that which is so 

 closely connected and continuous with it, be 

 said to be free from the same influence. Still 

 it must be admitted, that the deep position of 

 the central vessels, and the complicated series 

 of channels through which they communicate 

 with the superficial ones, protect them in some 

 degree from the pressure of the air, and render 

 them less amenable to its influence than the 

 vascular system of the surface. 



If it were essential to the integrity of the 

 brain that the fluid in its bloodvessels should 

 be protected from atmospheric pressure (as the 

 advocates of this doctrine would have us to 

 believe), a breach in the cranial wall would 

 necessarily lead to the most injurious conse- 

 quences ; yet, how frequently has the surgeon 

 removed a large piece of the cranium by the 

 trephine without any untoward result ! Some 

 years ago I watched for several weeks a case in 

 which nearly the whole of the upper part of 

 the cranium had been removed by a process of 

 necrosis, exposing a very large surface to the 

 immediate pressure of the atmosphere ; yet in 

 this case no disturbance of the cerebral circula- 

 tion existed. In the large and open fonta- 

 nelles of infants we have a state analogous to 

 that which art or disease produces in the adult : 



yet the vast majority of infants are free from 

 cerebral disease for the whole period during 

 which their crania remain incomplete ; and in 

 infinitely the greatest number of cases in which 

 children suffer from cerebral disease, the pri- 

 mary source of irritation is in some distant 

 organ, and not in the brain itself. 



It cannot be said that the brain is incom- 

 pressible. That only is incompressible, the 

 particles of which will not admit of being more 

 closely packed together under the influence of 

 pressure. That the brain is not a substance of 

 this kind is proved by the fact that, while it is 

 always undergoing a certain degree of pressure 

 as essential to the integrity of its functions, a 

 slight increase of that pressure is sufficient to 

 produce such an amount of physical change in 

 it as at once to interfere with its healthy action. 

 Too much blood distributed among its elements, 

 and too much serum effused upon its surface, 

 are equally capable of producing such an effect. 



Majendie's experiments, described in a for- 

 mer part of this article, show that the brain 

 and spinal cord are surrounded by fluid, the 

 pressure of which must antagonise that which 

 is exerted through the bloodvessels. The re- 

 moval of this fluid disturbs the functions of 

 these centres, apparently by allowing the vessels 

 to become too full. The pressure exerted by 

 the former may be called the fluid pressure from 

 without the brain ; that by the blood, the pres- 

 sure from within. As long as these two are 

 balanced, the 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 sur- 

 rounding 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 antagonizing forces 

 may be taken as a proof that either of them 

 may prevail ; and, therefore, from the presence 

 of the cerebro-spinal fluid, we may infer that 

 the actual quantity of circulating blood in the 

 brain is liable to variation. 



The cerebro-spinal fluid is a valuable regu- 

 lator of vascular fullness 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. 



When the brain receives too little blood, the 

 requisite degree of pressure will be maintained, 

 and the healthy cerebral action preserved, if the 

 surrounding fluid do not increase too rapidly. 

 But if the brain be deprived of its due propor- 

 tion of blood by some sudden depression of the 

 heart's power, there is neither time nor source 

 for the pouring out of a new fluid, and a state 

 of syncope or of delirium will ensue. Such 

 seems to be the explanation of those cases of 



