342 CIRCULATION OF NUTRITIVE FLUID. 



ducing dropsy. Thus it happens, that such effusions may often be 

 traced to that state of deficient vigour of the system, which peculiarly 

 manifests itself in want of tone of the blood-vessels ; and that it is relieved 

 by remedies which tend to restore this. In many young females of 

 leuco-phlegmatic temperament, for example, there is a tendency to 

 swelling of the feet, by cedematous effusion into the areolar tissue, in 

 consequence of the depending position of the limbs ; the oedema disap- 

 pears during the night, but returns during the day, and is at its maxi- 

 mum in the evening. And the congestion which frequently manifests 

 itself in the posterior parts of the body, towards the close of exhausting 

 diseases, in which the patient has lain much upon his back, is attributable 

 to a similar cause ; of such congestion, effusions into the various serous 

 cavities are frequent results ; and such effusions, taking place during 

 the last hours of life, are often erroneously regarded as the cause of 

 death. To the same cause we are to attribute the varicose state of the 

 veins of the leg, which is so common amongst persons of relaxed fibre, 

 and especially in those whose habits require them to be much in the erect 

 posture ; and this distension occasionally proceeds to complete rupture, 

 the causes of which are fully elucidated by the experiments just cited. 



611. It has been thought that the circulation within the Cranium 

 takes place under different conditions from that of other parts of the 

 body. For as the cranium is a closed cavity, a certain part of which 

 is occupied by the cerebral substance and its membranes, the remain- 

 der being filled up with blood, it has been argued that the amount 

 of blood in the vessels of the brain must be always the same; and 

 that any disturbance of its circulation must be due to a difference in 

 the relative quantity of blood in the arteries and the veins. This 

 idea appeared to derive support from the results of experiments, which 

 showed that the blood is retained in the vessels within the cranium of 

 animals bled to death, unless an opening be made in the skull, so as 

 to allow the air to exert the same pressure upon these vessels, as upon 

 those of other parts. But such experiments do not at all sanction the 

 assertion, that the quantity of blood within the cranium is constant ; 

 on the contrary, we have reason to believe that it undergoes as much 

 change as in other parts. For although the cerebral substance be 

 incompressible, yet its bulk is subject to constant variation, according 

 to the quantity of fluid it contains ; and the presence of the cerebro- 

 spinal fluid in the sub-arachnoid cavity of the brain and spinal cord, 

 appears to be peculiarly destined to favour this continual change, the 

 proportions of it contained in the spinal and the cerebral cavities, re- 

 spectively, being governed by the bulk of the other contents of the 

 cranium. Thus if the vessels of the cerebrum be in their ordinary state 

 of fulness, a certain amount of fluid is present in the sub-arachnoid 

 cavity of the brain ; this will be pressed out into the spinal portion of 

 the cavity, if the cerebral vessels be unusually distended with blood ; 

 whilst it will be increased from the latter source, so as to fill up the 

 vacant space within the cranium, if the cerebral vessels be unusually 

 empty. 



