SYSTEM.] PHYSIOLOGY 27 the change may be, however, it does not appear to pass from one nerve-fibre to another running alongside of it. Each fibre con ducts only its own impression, and there is nothing analogous to the inductive effect of one electrical conductor upon an adjacent one. Another question much debated is whether sensory and motor nerves act in the same way ; or, in other words, is there any essential difference between them? There appears to be no difference in mode of action ; the difference in the effect produced depends on the apparatus in which the nerve ends. Thus there may be con traction of a muscle if the nerve terminates in a muscle, change of the calibre of a blood-vessel if the nerve ends in that structure, secretion from a gland if the nerve is in connexion either with the vessel or the nerve-cells of a gland, an electrical discharge if the nerve ends in the electrical organ of a Torpedo or Gymuotiis, and a feeling or sensation if the nerve-fibres go to a sentient brain. In all these instances the nature of the change in the nerve and the mode of its transmission are the same, and the results are differ ent because the nerves terminate in different kinds of structure. It would appear from experimental evidence that, when a nerve- fibre is irritated, say about the middle of its length, a change is simultaneously propagated towards each end ; but, as only one end is in connexion with an apparatus capable of responding, the effect at this end is the only one observed. Thus, if a motor nerve be irritated, there is muscular contraction, in consequence of the stimulus rousing the muscular substance into activity, probably through the agency of the end -plates ; but there will be at the same time a backward wave along the nerve to the motor centres in the cord or brain. It is doubtful whether the nerve-energy be comes weaker or gathers intensity as it passes along a nerve ; but the balance of evidence is in favour of the view that the so-called "avalanche theory" of Pfliiger, according to which the energy gathers intensity as it passes along, is incorrect. Classification of Nerves. Functionally, nerves may be classified into motor, sensory, vascular, secretory, and inhibitory. The original meaning attached to the term " motor " nerve was a nerve entirely composed of fibres by the excitation of which influences were conveyed to a muscle which caused the muscle to contract. As these influences passed outwards from a nerve-centre towards the periphery of the body they were also termed " efferent " nerves. On the other hand, nerves were found which, when stimulated, gave rise to sensations of pleasure or of pain, and these were called " sensory " nerves. Finally, it was shown that a thii d class of nerves were composed both of sensory and of motor fibres, and they were called " senso-motor " nerves. Sensory nerves were also sub divided into those of general and those of special sensibility. This was an artificial classification based on the fact that when a nerve of so-called special sensibility, such as the optic, was stimulated in any way the same kind of sensation followed. Thus stimulation of the optic nerve by cutting, pricking, pressure, or electricity is always followed by a luminous sensation. But the progress of research showed that when certain nerve-fibres were stimulated the result was not necessarily a muscular contraction : it might be contraction of a blood-vessel, modified secretion of a gland, or a diminution or arrest of some kind of nervous action. These facts demand another classification of nerves such as the following. 1. Motor, sometimes termed efferent, to muscles, exciting contraction. 2. Secretory, to the cells of glands, causing secre tion, possibly a particular kind of secretion. 3. Vascular, or vaso-motor, to the walls of blood vessels, so as to cause contraction (vaso-motor) or dilatation (vaso-dilators, or vaso-inhibitors). 4. Inhibitory, so affecting other centres of nervous activity as to moderate or neutralize their action. 5. Electrical, so affecting a special organ as to call forth electrical discharges, as in electric ftshes, V. Torpedo, Gymnotus, Malapterurus, &c. fa. General, conveying to nerve- centres in brain influences which cause sensations of a vague character, scarcely perceptible to conscious ness, and not permanent, as from lungs, heart, stom ach, &c. b. Special, conveying to nerve- centres in brain influences which cause visual, audit ory, gustatory, olfactory, V or tactile sensations. j 2. Afferent, or reflex, conveying to nerve-centres in fluences which usually cause no sensation, and which may or may not be followed by move ments, secretions, changes in calibre of vessels, V <fcc. In addition there are nerve-fibres connecting nerve-cells in the great centres, to which no special functions can be attributed. 2. TERMINAL ORGANS. Terminal Although, as has been shown, a nerve may be stimulated in any organs, part of its course, the stimulus is usually applied to a special structure adapted physiologically for the reception of the particular kind of stimulus. Such a special structure may be termed a CENTRIFUGAL, or EFFER ENT, or MOTOR, convey ing influences outwards from a nerve-centre. CENTRIPETAL, or AFFER ENT, or SENSORY, con veying influences in-- wards towards a nerve- centre. 1. Sensory, caus ing more or less acute * sensations. "terminal organ." For example, in the mechanism of vision (see EYE, vol. viii. p. 821 sq.) there are the retina or terminal organ, the optic nerve or conductor, and the brain or a portion of it, the recipient of the impression. The fibres of the optic nerve are not affected by light, but when they are mechanically or electrically irritated the result is a luminous sensation, because the action of the fibres of the optic nerve is to call forth in the brain the mechanism connected with luminous sensations. But light has a specific action on the retina, and in turn the activity of the retina stimulates the fibres of the optic nerve. The retina is therefore the terminal organ adapted for the reception of rays of light. In like manner, each sense has its appropriate terminal apparatus, and these are described under the headings of the various senses, EAR, EYE, SMELL, TASTE, TOUCH. To understand the true nature of nervous action it is necessary to be clear as to the functions of the terminal organs. They are liberating mechanisms. They do not transform the outer energy into the physiological energy, nervous action ; but they call it into action. Thus light acting on the retina is not directly transformed into nervous energy, but it excites changes in the retina, which in turn produce activity of the optic nerve. The structure of each of these terminal organs need not be here described, but it may be stated that they all essentially consist of modified epithelium-cells, or what may be called "nerve- epithelium." In tracing their development throughout the animal kingdom it will be found that the simplest terminal organs are epithelium-cells on the surface of the body ; but during evolutionary progress from lower to higher forms these cells become more and more modified and more and more protected by descending deeper into the structure of the animal, until we meet with the complicated organs of special sense in the higher animals. Another class of ter minal organs is that comprehending the forms at the ends of motor nerves. Such are the end -plates found in muscle, and described in vol. i. pp. 861, 862. The different modes of nerve-termination may be here briefly classified. Organ. Skin (see TOUCH) Effect. Ear (see vol. i. p. 894, and vol. vii. p. 591). Eye (see vol. i. pp. 886 and 888, and vol. viii. p. 816). Nose (see SMELL) .... Tongue (see TASTE) . . Muscles (vol. i. p. 862) Terminal Organ. Tactile cells of Merkel, in the epi dermis. Tactile corpuscles of Wagner and Meissner, in papilla of the skin. End-bulbs of Krause, in conjunc tiva, penis, and clitoris. Facinian bodies, attached to nerves of hand or foot, or in the mesentery. Corpuscles of Grandry, found in bills of birds. Network of fibres, as in cornea. Hair -cells, supported by arches Hearing, of Corti, and connected with the basilar membrane. Rods and cones of retina. . . . Vision. Touch, pressure, or temperature. Rods and olfactory cells Smell. Taste-buds and gustatory cells. . Taste. Motorial end -plates of Doyere, Motion. Kiihne, Krause, Ranvier, &c. Glands Nerve-endings in secreting cells Secretion. Pfliiger and Kupffer. Electric organs (see vol. Lamina; with free cilia-like pro- Electric discharge. xii. pp. 649, 650). cesses. 3. CENTRAL ORGANS. A. General Physiology of Central Organs. General Structure. The central organs consist of a special kind of cells called "nerve-cells," of nerve-fibres, both medullated and non-medullated, and of a variety of connective tissue, termed "neuroglia." On cutting into any central nervous organ, such as the spinal cord or brain, two kinds of nervous matter are seen, the white and the grey. The grey consists of nerve - cells, nerve - fibres, and neuroglia, whilst the white is com posed chiefly of nerve -fibres with a small amount of neuroglia and no nerve-cells. Nerve cells vary much in FIG. 5. Various forms of nerve -cells, a, miiltipolar, Shapes form as will be seen from & rey matter of spinal cord ; , 6> d> bl > tar > fro ! of nerve- lurai, us wu ueai-cu gang ij a on posterior roots of spinal nerves ; c, g, u ni- ., by referring to fig. 5. polar, from cerebellum ; g shows indications of a celi! > They may be spher- process coming off at lower end ; e, union of three ni.l-i"! nvniMal nr ir multi polar cells in spinal cord; /, union of three U ,, , " . x , " " tripolar cells in grey matter of cerebral hemispheres, regularly triangular. The cells of the spinal ganglia are usually rounded ; those of the sympathetic more angular ; those of the spinal cord multipolar, that is, having many processes or poles connected with them ; those
of the cerebrum triangular or pyramidal ; and those of the cere-