62 THE NERVOUS SYSTEM 



in physiology, as in psychology, depend, for example, on the nature of the 

 nervous impulse, and this depends on the chemical composition and on 

 the metabolism of the neural tissue. 



The amount of water in the nervous system and in the white and gray 

 matter varies greatly according to age. In a fetus the white matter 

 appears to contain about 87 per cent, of water and the gray matter 

 92 per cent. In adults the percentage of water in the white matter is 

 about 69, -and in the gray matter about 83; while in old age the 

 percentages of both are somewhat higher. From this it will be seen 

 that nervous tissue is about four-fifths water, and that the gray matter 

 (cells and unmedullated fibers) contains more water than white matter, 

 especially in the prime of life. The gray matter contains less than 17 

 per cent, of solid materials, and we see herein the physical basis of its 

 great activity. Among the solids of the nervous system may be noted 

 proteids, nuclein, neurokeratin, cerebrins, cholesterin, various extrac- 

 tives (such as creatin, xanthin, lactic acid, uric acid), gelatin from the 

 adherent connective tissue, and inorganic salts. Of the proteids, the 

 gray matter has the largest proportion, 51 per cent., according to 

 Halliburton. These are a nucleoproteid and two cell-globulins, one 

 of which coagulates at a temperature as low as 47 C. (a possible cause of 

 death from sunstroke and from high fever). The cerebrospinal liquid is 

 much like lymph in the matter of inorganic salts, but it contains only 

 a trace of proteid. One of its constituents is a substance (perhaps 

 dextrose) that will reduce Fehling's solution. 



THE BLOOD-SUPPLY OF THE NERVOUS SYSTEM. 



The nervous system receives proportionally more blood than do most 

 of the organs of the body, as the unique circle of Willis at the base of the 

 brain and the ample sinuses about the hemispheres indicate. At present 

 little is definitely known about the vasomotor system of the nerves and 

 the brain, but that it is elaborate is becoming more and more evident. 

 Recent researches by Weber, Bourgery, and Cavazani have shown by 

 anatomical and physiological experiments that a vasomotor mechanism 

 exists in the brain, while Obersteiner has worked out somewhat systemati- 

 cally its nerve-fibers. The ventricles of the cerebrum are portions of an 

 hydraulic system, parts of which surround the brain and the cord. About 

 the whole brain is the pia mater, lining the skull is the dura mater, while 

 between these is the arachnoid, which is probably an important osmotic 

 membrane. The hydraulic cavity in question is the space beteen the pia 

 mater and this membrane. The liquid filling it (a very thin lymph) 

 communicates freely with the ventricles in the brain through many 

 canals, the largest of which are the foramens of Luschka and Majendie. 

 By osmosis the lymph of this subarachnoid space communicates with 

 the venous blood in the subdural sinus. Thus, there is a set of large 

 vascular cavities in the interior and about the periphery of the brain, and 



