bipolar, sending one into the motor root, the other into the sensitive.

Nerve-tubes are of two kinds, gelatinous and cylindrical-here figured. The first, so named by Henle, occur in the sympathetic and special sense nerves. They are grey, the white matter with which the next kind is coated being absent. Oval nuclei give them a varicose outline, and they much resemble unstriped muscular fibres; their diameter averages o.

nerve-tube. cal nerve

tube.

2. Cylindrical nerve-tubes form the great mass of the white substance of the brain, spinal cord, and nerves. They are placed parallel, and in nerves Gelatinous Cylindriare connected by areolar tissue. Their average width is 3000. With a high power and transmitted light, three cylinders can be seen to constitute the tube:-1. A homogeneous membrane, analogous to myolemma, and to it the term neurilemma (which is now applied to the connecting areolar tissue of a nerve) appears suitable. 2. The white substance of Schwann, an opaque tube, defined by an inner and outer dark boundary line. 3. The rest of the tube is termed the axis cylinder of Purkinje and Rosenthal, and forms from one-third to one-half its entire diameter. It may be that during life, and at a temperature of 98°, these matters are combined, and only separate by kind of coagulation after death. The white substance consists mainly of fat, and seems to have insulating properties, the axis cylinder being probably the true conductor. The continuance of the white matter, even to the termination of many nerves, throws some doubt on its insulating properties. It varies in thickness remarkably in nerves, those of young animals and of the organs of sense having so thin a layer of it, that mechanical injury

often ruptures it, and produces a nodulated or varicose appearance. The white substance is rendered laminated and brown by chromic acid, which does not affect the axis cylinder; carmine, on the other hand, stains the latter, not the former, and hence there is supposed to be some chemical difference between these two constituents of nerve-tubes.

A nerve-tube runs uninterruptedly from the nervous centre to some peripheral point without any anastomosis whatever; there are thus inconceivably numerous points on the centres, each connected, throughout a special nerve-tube, with a corresponding point in a muscle or sensory surface, as the case may be. Their relation to nerve-vesicles has been alluded to, and there is also reason to believe they form loops in the nervous centres, as in cases where various portions of the cerebrospinal axis were absent, the nerve-tubes have been observed to turn upon themselves in the cranio-spinal cavity, and run back to the same nerve-trunk. At the periphery they end in, 1, branching lashes, as in the retina, olfactory and auditory expansions, and in the electric organs of the ray; white substance is absent or very scanty, and vesicles are often added; 2, in loops, as in muscles, in skin, mucous and serous surfaces, which being plane, render this arrangement necessary, also in the papillæ of the tongue and tooth-pulp; 3, by free extremities, as in Pacinian corpuscles and tactile papillæ, which will be hereafter described.

A Nerve is a bundle of such tubes as we have described, bound together by areolar tissue, which is termed the neurilemma, and which serves to support the abundant capillaries. An artery occasionally runs through the interior of a nerve, as in the optic and sciatic.

A Ganglion is a mesh of nerve-tubes, in the interstices of which vesicles lie, as figured on the following page. We shall treat of them in describing the nervous

centres.

Branches come from a nerve usually at an acute angle,

and they may join those of other nerves, forming an intricate web we call a plexus.

Plexuses insure more harmonious action, and prevent loss of motion or sensation if one nerve should be destroyed. That the nerve-tubes of any one nerve emerge distinctly from the plexus, and are distributed to the tissue for which they were destined, is shown by irritating any of them which go to muscular tissue, when only those muscles under the influence of the original nerve are affected.

b.

This brief account of the histology of nervous a, tissue will be concluded with a few remarks on its development and repair. Schwann found the vesicles in the early fœtus closely resembled their adult state, save in being smaller and lighter coloured. He regarded the tube as merely a number of cells which had coalesced in the same way as muscle is supposed to be developed. It is at first a pale, nucleated band, which can be split into shreds, and it then resembles the permanent state of gelatinous nerve-tube. The white substance is next gradually deposited on the inner surface of the tubular membrane, in the same way as the sarcous element is laid down in the muscle-cell, and thus the

A Sympathetic Ganglion and its Roots. Vertical sympathetic nerve tubes. c, d. Spinal nerve tubes which interlace, oval nerve vesicles being deposited in the meshes.

cylindrical nerve-tube is perfected. Unlike many other tissues, nerves, when divided, become repaired by new tubes being formed between the cut ends. Dr. Haighton, by the following experiment, discovered that their function was restored. He divided both pneumogastrics high in the neck of an animal, and death at once ensued. He then divided one, and the other in 8 weeks after; this animal survived-indicating that the first nerve had united by conducting medium. When the supra-orbital nerve is merely divided for the cure of brow-ache, the nerve often re-unites and the disease is reproduced. The frequent occurrence of amaurosis after it, has now made this operation unadvisable. The conducting power of a divided median nerve is said to have been rapidly restored by suture in some experiments before the French Academy. Schiff states that a divided motor nerve will not unite with a sensitive one, but will do so with any other motor nerve. It has been proved in the dog that portions of nerve wholly excised will unite with the subcutaneous areolar tissue, losing, however, everything but their neurilemma.

Nervous Force is that agent which vesicular neurine generates—hence called dynamic, and which nerves conduct-hence they are termed internuncient. We only know it by its effect on the parts where nerves are distributed, for it produces no visible change on nervous tissue. Impressions are either carried to the nervous centres along nerves, which are therefore termed afferent or centripetal, or from the centre to periphery along nerves which are termed efferent or centrifugal. These kinds of nerves correspond respectively to sensitive and motor, and no characteristic appearances distinguish them, save that the tubes of the former are much smaller than those of the latter. Impressions are sometimes carried from centre to periphery along sensitive nerves, as in cerebral disease, producing painful sensations in the extremities, and in irritation of a nerve,

producing pain where its filaments are distributed, as so often seen in hip-disease, in which the sensation is carried by the obturator and long saphenous nerves to the knee. The conduction of an impression from a part which has been altered in position, is shown in an interesting way after rhinoplastic operations, where an impression on the new nose is referred to the forehead, from which the skin had been abstracted.

Special-sense nerves, we have seen, differ histologically from those of common sensibility; and they differ still more remarkably in function, for irritation of them does not produce pain, but gives rise to subjective phenomena peculiar to each-for example, muscæ volitantes, perverted smell, morbid tastes, tinnitus aurium, &c., symptoms which every physician knows are often due to centric disease. Those who suppose that nerves generate force when irritated by mechanical or other stimuli call this inherent power "vis nervosa" or neurility. Division of a nerve, a tight ligature or pressure such as tumors often produce, will interrupt the conduction of a sensation centrad, or of a motor influence peripherad. These facts can be well illustrated by tying two ligatures on a nerve which contains both sensitive and motor fibres; irritation between the ligatures will produce no effect-but if the lower one be removed, motions will occur; or if the upper one, pain will be felt. The rate of transmission of nervous force is about 86 feet per second in the frog, and 200 in man; whereas electricity travels 462,000,000 feet in the same time.

Theories of Nervous Force have been numerous. For many years Descartes' supposition of a nervous fluid was accepted, till Hartley maintained that the influence was transmitted by vibrations. The resemblance between nervous force and electricity has been often dwelt on since the discovery of Galvani, and especially by Wilson Philip. Both are transmitted with wonderful rapidity along conductors, but the velocity of electricity is far greater-both require two dissimilar substances to excite