Function of nerves. The office of nerves as simple conveyors or conductors of nervous impressions is of a twofold kind: 1. They serve to convey to the nervous centers the impressions made upon the peripheral extremities or parts of their course; 2. They serve to transmit impressions from the brain and other nervous centers to the parts to which they are distributed. For this twofold office of the nerves two distinct sets of nerve fibers are provided, in both the cerebro-spinal and sympathetic systems. Those which convey impressions from the periphery to the center are classed together as centripetal or afferent nerves, or nerves of sensation--sensitive nerves. Those, on the other hand, which are employed to transmit central impulses to the periphery are classed as centrifugal or afferent nerves or motor nerves, conveying impulses to the voluntary and involuntary muscles, etc. Nerves are constructed of minute fibers or tubules full of nervous matter, arranged in parallel or interlacing bundles, which bundles are connected by intervening connective tissue in which their principal blood-vessels ramify. The size of nerve fibers varies, and the same fibers do not preserve the same diameter through their whole length, being largest in their course within their trunk and branches of nerves, in which the majority measure from 1/2000 to 1/3000 of an inch in diameter. As they approach the brain or spinal cord, and generally also in the tissue in which they are distributed, they gradually become smaller. In the gray or vesicular substance of the brain or spinal cord they generally do not measure more than from 1/10000 to 1/14000 of an inch. The chemical composition of nervous matter. Like most of the other tissues of the body, the nervous substance contains a large proportion of water (from three-fourths to four-fifths of its weight). Of the residue which remains after the removal of this by evaporation or other means, the larger part consists of a phosphuretted fat, which may be obtained crystallized, and in this condition was termed protagon. The crystalline substance, however, is in reality a mixture of two other substances, lecithin and neurin. Cerebrin is also described as being frequently met with in conjunction with lecithin. Lecithin. Neurin. Cerebrin. Cholestrin. Carbon, 44 5 17 26 Hydrogen, 90 15 33 44 Nitrogen, 1 1 1 Phosphorus, 1 Oxygen, 9 2 3 1 CHAPTER XVIII. FOOD AND FOOD-SUBSTANCES. There are two kinds of food: 1. Those food substances that are derived from the animal kingdom; and, 2. Food substances that are derived from the vegetable kingdom. Food is taken into the system to replace the material expended by the human body, or the waste products which are thrown off from the master tissues.