556 CIRCULATION OF BLOOD AND LYMPH. 



these figures are correct it evidently does not form a thick envelope 

 to the nervous system. Under abnormal conditions (hydroceph- 

 alus, etc.) the quantity may be greatly increased. It is physio- 

 logically interesting to find that this liquid may be formed very 

 promptly from the blood and, when in excess, be absorbed quickly 

 by the blood. In fractures of the base of the skull, for instance, 

 the liquid has been observed to drain off steadily at the rate of 200 

 c.c. or more per day. On the other hand, when one injects physio- 

 logical saline into the subarachnoidal space under some pressure it 

 is absorbed with surprising rapidity. After death, also, the liquid 

 present in the subarachnoidal space is soon absorbed. 



Intracranial Pressure. By intracranial pressure is meant the 

 pressure in the space between the skull and the brain, therefore 

 the pressure in the subarachnoidal liquid and presumably also the 

 pressure in the ventricles of the brain, since the two spaces are in 

 communication. This pressure may be measured by boring a hole 

 through the skull, dividing the dura, and connecting the under- 

 lying space with a manometer. Observers who have measured this 

 pressure state that it is always the same as the venous pressure 

 within the sinuses. This we can understand when we remember 

 the close relations between the subarachnoidal liquid and the large 

 veins and sinuses. We may consider that the large veins are sur- 

 rounded by the cerebrospinal liquid, and consequently an equilib- 

 rium of pressure must be established between them ; any rise in the 

 intracranial pressure raises venous pressure by compression of the 

 veins. This statement holds true at least so far as the intracranial 

 pressure is due to the circulation. Variations of pressure from 

 pathological causes tumors, clots, abscesses, etc. may exercise 

 apparently a local effect. The intracranial pressure is caused 

 and controlled normally by the pressure within the arteries and 

 capillaries. This pressure, by enlarging these vessels, tends to 

 expand the brain against the skull, and exercises a pressure, there- 

 fore, upon the intervening cerebrospinal liquid. This pressure, 

 however, can not exceed that in the veins, since, as said, an ex- 

 cess will be equalized by a corresponding compression of the veins. 

 The venous pressure in the end determines, therefore, the actual 

 amount of intracranial pressure. Conditions which alter the 

 pressure in the cerebral veins affect the intracranial pressure 

 correspondingly. Thus, compression of the veins of the neck raises 

 the pressure in the cerebral veins and also intracranial pressure, 

 and a higher general arterial pressure also results finally in a higher 

 pressure in the cerebral veins and therefore in the subarachnoidal 

 space. 



Reduced to its simplest form, the conditions may be represented by a 

 schema such as is given in Fig. 232. A system with an artery, capillary area, 



