CHAP, x.] CIRCULATION IN THE BRAIN. 297 



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 blood-vessels 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 con- 

 sequences ; yet, how frequently has the surgeon removed a large 

 piece of the cranium by the trephine without any untoward result ! 

 We have 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 imme- 

 diate pressure of the atmosphere; yet in this case no disturbance 

 of the cerebral circulation 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 under cerebral disease, the primary 

 source of irritation is in some distant organ, and not in the brain 

 itself. 



Neither can it be said that the brain is incompressible. 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 

 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. 



Majcndie's experiments, alluded to at p. 253, shew that the 

 brain and spinal cord are surrounded by fluid, the pressure of 

 which, probably, antagonises that which must be exerted through 

 the blood-vessels. The removal of this fluid disturbs the func- 

 tions of these centres apparently by allowing the vessels to be- 

 come too full. The pressure exerted by the former we shall call 

 the fluid-pressure from without the brain ; that by the blood, the 

 pressure from within. As long as these two are balanced, the 



