620 



CIRCULATION OF BLOOD AND LYMPH. 



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 Hquid present in the sub- 

 arachnoidal space is soon absorbed. Experimental work f indicates 

 that the cerebrospinal hquid is formed within the ventricles from 



the choroid plexuses, and, in- 

 deed, 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 Ma- 

 gendie and the foramina of 

 Luschka into the subarach- 

 noidal spaces, from which in 

 turn it is absorbed into the 

 cerebral veins (Fig. 260a). If 

 the aqueduct of Sylvius or the 

 foramina of exit are blocked, 

 the continued formation of 

 liquid within the ventricles 

 may lead to the production of 

 internal hydrocephalus. The rapidity of formation of the secre- 

 tion is increased apparently by pituitary extracts and diminished 

 by thyroid extracts, t 



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 



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



t Dandy and Blackfan, "Jour, of the Amer. Med. Assoc," December 20, 

 1913; also "Amer. Jour, of Dis. of Children," 14, 424, 1917 (second paper). 



J Weed and Gushing, "American Journal of Physiology," 36, 77, 1915; 

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



Fig. 260a.— Diagram by Dandy and Black- 

 fan, slightly changed, to show the circulation of 

 the cerebrospinal liquid: I, II, lateral ventri- 

 cles; III, the third ventricle; IV, the fourth 

 ventricle; A, foramina of Monroe; L, L, foram- 

 ina of Luschka; M, foramen of Magendie; S, 

 aqueduct of Sylvius. The arrows show the direc- 

 tion of flow of the cerebrospinal liquid. 



