262 NERVOUS SYSTEM. 



scribe its exact situation, with the communications between 

 the different cavities of the brain and the subaraclmoid 

 space. 1 By a series of ingeniously-contrived experiments 

 upon the cadaver and in living animals, Magendie showed 

 that the greater part of the fluid in the cranium and the 

 spinal canal is contained in what is known as the sub- 

 arachnoid space; that is, between the inner layer of the 

 arachnoid and the pia mater, and not between the two lay- 

 ers of the arachnoid. The ventricles of the encephalon are 

 in communication with the central canal of the cord, and, as 

 was shown by Magendie, they are also in communication 

 with the general subaraclmoid space, by a narrow, triangu- 

 lar orifice, situated at the inferior angle of the fourth ventri- 

 cle. By this arrangement, the liquid in the ventricles of the 

 encephalon and 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 composi- 

 tion have no very definite physiological significance. Its 

 quantity was estimated by Magendie at about two fluid- 

 ounces ; a but this was the smallest amount obtained by 

 placing the subject upright, making an opening in the lum- 

 bar region and a counter-opening in the head to admit the 

 pressure of the atmosphere. The exact quantity in the liv- 

 ing subject could hardly be estimated in this way ; and it is 

 difficult, indeed, to see how any thing more than a roughly 

 approximative idea could be obtained. The quantity ob- 

 tained by Magendie probably does not represent the entire 

 amount of liquid contained in the ventricles and in the sub- 



TJicmurus Dissertationum de SANDIFERT, tome ii., p. 411, Koterdara, 1769. 

 Journal de physiologic, Paris, 1827, tome vii., p. 83. 



1 MAGENDIE, Memoire sur un liquids qui se trouve dans le crane et le canal 

 vertebral de Vhomme et des animaux mammiferes. Journal de physiolcgie, Paris, 

 1825, tome v., p. 27; Ibid., 1827, tome vii., pp. 1, 66; and, Recherches phy'siolo- 

 giques el d'miques sur le liquide cephalo-rachidien, Paris, 1842. 



2 MAGENDIE, Liquide cephalo-rachidien, Paris, 1842, p. 36. 



