the white substance of the brain and spinal cord. The axis-cylinder of the nerve appears to be the essential portion in the transmission of irritations received upon the peripheral terminations, and is composed of "protein

substance" and fine fibrillæ, so arranged as to be susceptible of isomeric transformation from one state to another. It is the axis portion of the nerve which becomes connected directly with the terminal vesicles, and also with the percep

tive cells and sensory ganglia of the grey matter at the base of the brain.

In order to indicate more fully the manner in which irritations are received upon and transmitted through the nerves to the basal ganglia, where they become converted into sensations, it becomes necessary to trace out more fully the peculiarity of their terminations. This varies to some extent in the different portions of the cutaneous surface.

In the skin of the fingers and toes are found small bodies called tactile corpuscles, imbedded in the immediate surface, which are filled with granular matter, easily compressible and mobile in character, to which the peripheral end of the nerve-fibre becomes united after passing around it. Whenever pressure or friction is effected on these surfaces and the corpuscles become affected, they communicate this motion to the nerve-fibres with a greater impulse than they could receive if they were only imbedded in the general mass composed of a much more stable tissue. In some other localities the surfaces of which are subject to frequent friction or pressure, such as the surfaces of the arms and legs, these minute corpuscular bodies have not been found. According to Mr. Herbert Spencer, on these surfaces each of the short hairs, which grow in greater or less abundance, acts as a "tactile multiplier," greatly increasing the effects of such pressures as are brought to bear upon them.

But it is more especially in connection with the other special organs of sense, that the peculiar connection of the peripheral termination of nerves with the external surface has been studied and demonstrated most perfectly. For instance, the vibrations of the atmosphere caused by the action of bodies upon one another are not heard at the perceptive cells at the basal ganglia as sound. These vibrations are supposed first to affect the cells which are imbedded in the liquid

of the internal chamber of the ear, and these cells communicate the impressions of irritations to the auditory nerve, which in turn conveys them to the perceptive ganglia at the base of the brain. Another theory is that the vibrations of the air, according to their rate, set in motion the different elements of the organ of Corti, which communicate the impulse to the special fibres with which they are connected.

These numerous cells are doubtless connected, either directly or indirectly, with the numerous nerve-fibres of the auditory nerve, thus causing the peculiarity of tone for which it is adapted by its construction; whereas, if all vibrations were to inpinge directly upon the auditory nerve, and be conveyed at once to the basal ganglia, they would eventuate in a multitude of undifferentiated sounds. That these cells and their normal connection with the cells of the nerve are essential to hearing becomes evident whenever they become injured by disease or force. Vibrations communicated through any other channel to the sensecentres are not perceived as sounds, though they may be as pain. It therefore becomes evident that we do not hear directly actually the vibrations of air, but simply the molecular motion of the perceptive cells. It is the activity of these cells which we hear as sound, when it is caused by irritations acting within the brain itself, as in hallucinations of hearing.

An arrangement of a similar nature, though differing in the detail of construction, exists in the eye. It is a familiar experience to every one that flashes of light are perceived in the cells of perception when in reality no light has reached the posterior portion of the eye; the flash of light has been caused by a sudden blow or pressure on the head, and the expression, "I will make you to see stars," has

passed into a proverb. Behind the retina, or rather as a part of it, a superficial layer of cells or rods is arranged, the axis of each one of which runs parallel with the rays of light as they impinge upon them. These minute lines or rods become at once excited to molecular activity by the effect of the particles of light, which activity communicates the influence and stimulates the fibres of the retina. It is, therefore, only in an indirect manner that the retina is affected by light, and it is, in fact, quite insensible to its direct influence. When the nerve-fibres of the retina have become thus indirectly excited, they communicate through the optic nerve the effect of the irritation in the form of molecular activity back to the cells or small ganglia of the optic thalami, where it becomes changed into sight sensations. In this case, as in that of hearing, it is only the molecular change which has passed through the nerve to the basal ganglia which is perceived by the individual. The image formed on the retina can pass back no further on the dark passage through which the impression proceeds. The eye may be in a perfectly normal condition; its lenses, muscles, nerves, and humors may all be intact, so that the image is reflected upon the fundus of the eye; but if the optic tract is invaded by disease, or if, for any cause, its ability to convey irritations and consequent impressions to the perceptive cells is destroyed or in abeyance, there can be no sight. Only, as in the sense of hearing, there may occur hallucinations which arise from centric irritations communicated to the ganglia of sight.

An arrangement, differing in some of its details from that above described as pertaining to general sensation, and yet serving a similar purpose, exists in the Schneiderian membrane, comprising the machinery for perceiving odors. The peculiar epithelial cells of this membrane, which may be

regarded as forming the termination of the nerve proper, do not lie immediately at the surface, but, on the contrary, are connected by means of fine nerve-fibres with corpuscles of granular material having a highly sensitive and unstable character which lie in and pervade the whole exposed surface, and whose office it is to receive and communicate the presence of such odors of various kinds as may reach them. back to the peculiar epithelial cells, which in turn convey the impression through the olfactory nerve to the basal ganglia, where they become differentiated as odors. The delicacy and sensitiveness of these corpuscular bodies, or of the communicating fibres, differ very largely in different persons, and still more largely in different species of animals. While some individuals are able to perceive only the more highly concentrated and pungent odors, others perceive the slightest and most delicate at once on approaching their presence. This is more conspicuous in respect to some varieties of game animals, especially the moose, whose organs of smell are so highly developed and enlarged as to enable the animal to perceive the approach of an enemy through long distances, and such approach is quite impracticable except from the windward.

Again, the nerve whose function is that of taste has a final termination peculiar to itself, which subserves a purpose similar to that above described as existing in the other special organs of sense. The small papillæ of the tongue are covered with cells which send out a fine, thread-like nerve fibril, which unites the nerve of taste with an epithelial cell. By means of this anatomical arrangement, highly concentrated and sharp, biting fluids do not come directly in contact with the nerve-fibre; and in the usual condition of the tongue, covered with the secretions of its mucous surface, do not reach even the corpuscular cells at