628 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, and it is stated 

 that normally the amount increases with age, after puberty, as 

 the brain shrinks in size.* It is physiologically 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 physiological saline into the sub- 

 arachnoidal space under some pressure it is absorbed with sur- 

 prising rapidity. After death, also, the liquid present in the sub- 

 arachnoidal space is soon absorbed. Experimental work f indicates 

 that the cerebrospinal liquid is formed within the ventricles from 

 the choroid plexuses, and, indeed, there is some evidence that its 

 formation may be due to a process of active secretion on the part 

 of the epithelial cells covering these plexuses. However that may 

 be, the stream of liquid starts within the ventricles and passes out 

 through the foramen of Magendie and the foramina of Luschka 

 into the subarachnoidal spaces, from which in turn it is absorbed 

 into the cerebral veins. If the aqueduct of Sylvius or the for- 

 amina of exit are blocked, the continued formation of liquid within 

 the ventricles may lead to the production of internal hydro- 

 cephalus. The rapidity of formation of the secretion is increased 

 apparently by pituitary extracts and diminished by thyroid 

 extracts. { 



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. Most observers who have measured 

 this pressure state that it is 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 surrounded 

 by the cerebrospinal liquid, and consequently an equilibrium of 

 pressure may be established between them; any rise in the intra- 

 cranial pressure raises venous pressure by compression of the 

 veins, and probably by accelerating the flow of liquid from the 



* Stillman, "Archives of Internal Medicine," 8, 193, 1911. 



t Dandy and Blackfan, "Journal of the Amer. Med. Assoc.," December 

 20, 1913; also Dixon and Halliburton, "Journal of Physiology," 42, 215, 1913. 



JWeed and Gushing, "American Journal of Physiology." 36, 77, 1915; 

 also Frazier and Peet, ibid., 36, 464, 1915. 



