THE NERVOUS SYSTEM 387 



a hundred feet per second, or nearly at the rate of twenty-two miles 

 per hour. 



The most careful researches into the elements which enter into the 

 chemical composition of nerve tissue afford no insight into the extra- 

 ordinary properties it possesses. In the living state it may be regarded 

 as a kind or form of protoplasm, but when dead and submitted to analysis 

 it only presents those elements with which we are familiar in the proteids 

 carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus, with salts 

 of calcium, sodium, potassium, and magnesium. Complex substances 

 known as protagon and neurokeratin may be obtained by analysis, but 

 these are but the caput mortuum of the active, living, sentient material, 

 which receives impressions, retains and reproduces them, and can liberate 

 impulses. We are yet far from being able to predict function from 

 chemical composition or molecular arrangement. It is interesting to 

 notice that the gray substance of the cord and brain, which by common 

 consent is acknowledged to be the active part, contains no less than 

 from eighty-five to ninety per cent of water, whilst the white part, chiefly 

 composed of fibres, contains only about seventy per cent. The reaction 

 of nervous tissue is alkaline to test-paper. 



Speaking generally, three parts are recognizable in every nerve-fibre: 

 the origin, which is usually from a cell in one of the nerve centres; the 

 course, which is longer or shorter in correspondence with the part of the 

 body supplied; and the termination, which presents special modifications, 

 in accordance with the special organ of sense to which the nerve is dis- 

 tributed, if it be a sensory nerve, or the muscle or gland, if it be a 

 motor nerve. 



The agent exciting a nerve to action is named a stimulus. Some 

 stimuli, as electricity and mechanical irritation, seem to be able to excite 

 all nerves to action, but, as a rule, each nerve responds, or responds 

 best, to its own proper stimulus. Thus the undulations of light excite 

 specifically the nerve terminations in the retina of the eye, the vibrations 

 of sound those in the ear. The change, or information of the change, 

 exciting the nerve endings, say, from blue to red in the case of light, 

 is propagated along the nerve -fibre till it reaches a special sensory cell 

 in the nerve centre. The cell, if well - nourished and not already ex- 

 hausted, is excited, and a wave of force is liberated from it which may 

 be propagated to a neighbouring cell, and may expand itself through it 

 in producing ideas, or movement, or secretion. The stimuli with which 

 we are most familiar, besides those of electricity, light, and sound already 

 mentioned, are those of contact or of a mechanical nature, those of a 

 chemical nature, as odours and tastes, those of temperature, and those 



