Hitherto, we have only considered the anatomy and functions of the organs employed in Digestion, Absorption, Circulation, Respiration, Secretion and Excretion. We have found the vital process of nutrition to be, in all its essential features, a result of physical and chemical forces; in each instance we have presupposed the existence and activity of the nerves. There is not an inch of bodily tissue into which their delicate filaments do not penetrate, and form a multitude of conductors, over which are sent the impulses of motion and sensation. [Illustration: Fig. 54. The Nervous System.] Two elements, _nerve-fibers_ and _ganglionic corpuscles_, enter into the composition of nervous tissue. Ordinary nerve-fibers in the living subject, or when fresh, are cylindrical-shaped filaments of a clear, but somewhat oily appearance. But soon after death the matter contained in the fiber coagulates, and then the fiber is seen to consist of an extremely delicate, structureless, outer membrane, which forms a tube through the center of which runs the _axis-cylinder_. Interposed between the axis-cylinder and this tube, there is a fluid, containing a considerable quantity of fatty matter, from which is deposited a highly refracting substance which lines the tube. There are two sets of nerve-fibers, those which transmit sensory impulses, called _afferent_ or _sensory_ nerves, and those which transmit motor impulses, called _efferent_ or _motor_ nerves. The fibers when collected in bundles are termed nerve trunks. All the larger nerve-fibers lie side by side in the nerve-trunks, and are bound together by delicate connective tissue, enclosed in a sheath of the same material, termed the _neurilemma_. The nerve-fibers in the trunks of the nerves remain perfectly distinct and disconnected from one another, and seldom, or never, divide throughout their entire length. However, where the nerves enter the nerve-centers, and near their outer terminations, the nerve-fibres often divide into branches, or at least gradually diminish in size, until, finally, the axis-cylinder, and the sheath with its fluid contents, are no longer distinguishable. The investing membrane is continuous from the origin to the termination of the nerve-trunk. [Illustration: Fig. 55. Division of a nerve, showing a portion of a nervous trunk (_a_) and separation of its filaments (_b, c, d, e_.)] In the brain and spinal cord the nerve-fibers often terminate in minute masses of a gray or ash-colored granular substance, termed _ganglia_, or _ganglionic corpuscles_. The ganglia are cellular corpuscles of irregular form, and possess fibrous appendages, which serve to connect them with one another. These ganglia form the cortical covering of the brain, and are also found in the interior of the spinal cord. According to Kölliker, the larger of these nerve-cells measure only 1/200 of an inch in diameter. The brain is chiefly composed of nervous ganglia.