CEPHALOKACHIDIAN FLUID. 261 



as these points are chiefly of anatomical interest. The circu- 

 lation in these parts presents certain peculiarities. In the 

 first place, the encephalon being contained in an air-tight 

 case of invariable capacity, it has been a question whether 

 or not the vessels be capable of contraction and dilatation, 

 or whether the quantity of blood in the brain be subject to 

 modification in health or disease. This question may cer- 

 tainly be answered in the affirmative. In infancy and in the 

 adult, when an opening has been made in the skull, the volume 

 of the encephalon is evidently increased during expiration and 

 is diminished in inspiration. Under normal conditions, in the 

 adult, it is probable that the amount of blood is increased in 

 expiration and diminished in inspiration ; but it is not prob- 

 able that the cerebro-spinal axis undergoes any considera- 

 ble movements. The important peculiarities in the cerebral 

 circulation have already been folly considered in another 

 volume. 1 



An important fact was pointed out by Hobin, and after- 

 ward by His, with regard to the arrangement of the lym- 

 phatic vessels of the brain. It was shown by these observers, 

 that the encephalic capillaries are surrounded or nearly sur- 

 rounded by canals (perivascular canal-system) which exceed 

 the blood-vessels in diameter by from y^Vo to T J- - of an 

 inch, and are connected with lymphatic trunks or reservoirs 

 situated under the pia mater. 8 The system of canals may, 

 by variations in its contents, serve to equalize the amount 

 of liquid in the brain as its blood-vessels are distended or 

 contracted. 



Cephalo-rachidian Fluid. The older writers referred 

 to in works upon physiology, as giving the most accurate 

 description of the cephalo-rachidian fluid, are Haller 3 and 

 Cotugno; 4 but it remained for Magendie, in 1825, to de- 



1 See vol. i., Circulation, p. 332. * See vol. ii., Absorption, p. 433. 



3 HALLER, Elemento Physiologies, Lausannae, 1762, tomus iv., p. 87. 



4 Extraii de la dissertation de COTUGXO, de Ischiade Nervosa, content dans le 



