668 NERVOUS SYSTEM. 



in expiration and diminished in inspiration ; but it is not probable that the cerebro-spinal 

 axis undergoes any considerable movements. The important peculiarities in the cerebral 

 circulation have already been fully considered in connection with the circulation. It 

 has been shown that the encephalic capillaries are surrounded or nearly surrounded by 

 canals (perivascular canal-system), which exceed the blood-vessels in diameter by from 

 T ^_ to 4^-5- of an inch, and are connected with lymphatic trunks or reservoirs situated 

 under the pia mater. The system of canals may, by variations in its contents, serve to 

 equalize the amount of liquid in the brain as the blood-vessels are distended or contracted. 



Cephalo-rachidian Fluid. The greatest part of the fluid in the cranium and in the 

 spinal canal is contained in what is known as the subarachnoid space ; that is, between 

 the inner layer of the arachnoid and the pia mater, and not between the two layers of 

 the arachnoid. The ventricles of the encephalon are in communication with the central 

 canal of the cord, and are also connected with the general subarachnoid space, by a nar- 

 row, triangular orifice, situated at the inferior angle of the fourth ventricle. By this 

 arrangement, the liquid in the ventricles of the encephalon and in the central canal of the 

 cord communicates with the liquid surrounding the cerebro-spinal axis, and the pressure 

 upon these delicate parts is equalized. 



As far as we know, the function of the cephalo-rachidian fluid is simply mechanical, 

 and its properties and composition have no very definite physiological significance. Its 

 quantity was estimated by Magendie, in the human subject, at about two fluidounces ; 

 but this was the smallest amount obtained by placing the subject upright, making an 

 opening in the lumbar region and a counter-opening in the head to admit the pressure 

 of the atmosphere. The exact quantity in the living subject could hardly be estimated in 

 this way ; and it is difficult, indeed, to see how any. thing more than a roughly approx- 

 imative idea could be obtained. The quantity obtained by Magendie probably does not 

 represent the entire amount of liquid contained in the ventricles and in the subarachnoid 

 space, but it is the most definite estimate that has been given. 



The discharge of a certain quantity of the cephalo-rachidian fluid does not produce 

 any marked derangement in the action of the nervous system. When the liquid is 

 allowed to flow spontaneously through a small trocar introduced without division of the 

 muscles of the neck, there follows no serious nervous disturbance ; but, when the liquid 

 is drawn out forcibly with a syringe, the animal first becomes enfeebled and afterward 

 seems affected with general paralysis. These phenomena are probably due, not so much 

 to removal of the fluid, as to congestion of blood-vessels and even effusion of blood, 

 which follow sudden diminution in the pressure. Sudden increase in the quantity of 

 liquid surrounding the cerebro-spinal axis produces coma, probably from compression of 

 the centres. This fact was demonstrated by Magendie, by injecting water in animals, 

 and also by compressing the tumor, in cases of spina bifida in the human subject, by 

 which the fluid was pressed back into the spinal canal. In the cases of spina bifida, the 

 subject, during the compression, fell into coma, which was instantly relieved by remov- 

 ing the pressure. The cephalo-rachidian fluid is speedily reproduced after its evacuation. 

 In all probability it is secreted by the pia mater. 



The general properties and composition of the fluid under consideration are, in brief, 

 the following : It is perfectly transparent and colorless, free from viscidity, of a distinctly 

 saline taste, alkaline reaction, and it resists putrefaction for a long time. It is not affected 

 by heat or acids. As we should expect from its low specific gravity and purely mechani- 

 cal function, it contains a large proportion of water (981 to 985 parts per thousand). It 

 contains a considerable quantity of chloride of sodium, a trace of chloride of potassium, 

 sulphates, carbonates, and alkaline and earthy phosphates. In addition, it contains 

 traces of urea, glucose, lactate of soda, fatty matter, cholesterine, and albumen. 



As a summary of the function of the cephalo-rachidian fluid, it may be stated, in gen- 

 eral terms, that it serves to protect the cerebro-spinal axis, chiefly by equalization of the 



