THE

FIVE SENSES OF MAN.

INTRODUCTION.

IN the animal kingdom a number of organs have been developed which possess the property of making each organism acquainted with occurrences in the outer world, and which are therefore called the 'sensory organs.' They are found in their highest perfection in man, whose mental power surpasses in the same degree that of the rest of organised beings.

Every sensory organ may be shown to be anatomically connected with the nervous system by means of nerve-trunks and nerve-fibres. Touch, sight, hearing, smell, and taste are inconceivable without the presence of a nervous system, even if the sensory organs were in their present full development. An eye, of which the optic nerve has been destroyed, can present to us no picture of the outer world; an ear, of which the auditory nerve has been severed, conducts no sound to us; an arm, of which the nerve is injured, can feel nothing. Such an

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eye may have all the appearance of a sound one, it may
receive the rays of light and form an image of objects
on its background, and nevertheless no sensation can be
produced, for the connection with the brain, the centre
of the nervous system, is wanting. The case is the same
with the deaf ear, and with the arm devoid of feeling.
The sensory organs are, therefore, only instruments of
the mind, which has its seat in the brain, and by means
of nerves makes use of these instruments to obtain
information of external objects. The forces which
operate in the outer world—namely, light, heat, sound,
motion, and chemical affinity-produce in the sensory
organs an irritation of the sensory nerves connected
with them, and these convey the irritation which is
there received throughout their entire length to the
brain. Each organ of sense has its own specific
irritation by which it is excited. The terminations of
the optic nerve in the eye can only be excited by
light-waves, not by sound-waves, and the latter can
only excite the terminations of the auditory nerve in
the ear.
For the tactile nerves of the skin mechanical
pressure and heat are specific excitements; for the
nerves of taste and smell some chemical substance is
necessary.

The sensation itself evidently first takes place in the brain. The sensation of light does not take place in the eye, where there is only an impression of light upon the expanded surface of the optic nerve; the sensation of light cannot, however, take place in the optic nerve itself, for it merely conveys the fact of the existence of the irritation to the brain. The sensation of light, a process to us most obscure, begins rather in the brain, which is I, where newe is in wrenschon with orgleen

irritated by the excited nerve; and since we can follow the optic nerve up to its origin in the brain, we therefore conclude that this process occurs in the central organ of the optic nerve.

The eye, therefore, is nothing more than an optical instrument which receives the light, and the optic nerve nothing more than an apparatus for conveying the intelligence of an irritation to the brain. It has been observed in operations that if the optic nerve is either torn, crushed, or even severed, at the moment when it is broken a strong flash of light is observed by the patient. This light is not real, for it is only perceived by the person under operation. The sensation of light arises merely from the mechanical irritation of the optic nerve, and from the extension of the irritation to the nervecentre, where it awakens the process of the sensation of light, just as if the excitement had proceeded from the eye. In such cases the sensation of light occurs without any external objective light, and always takes place if the optic nerve is irritated in any way whatever by those influences which have an irritating effect on other nerves, such as electricity, heat, and chemical action. Objective light, that is to say, the light-waves of the ether, takes no part in this action; it may therefore be accepted as a fact that in ordinary vision no trace of the light which enters the eye finds its way to the brain, but only a process of irritation peculiar to the nerve, and which can be produced in the nerve-trunk by pressure, electricity, heat, and chemical action, just as well as in the eye by light. In whatever way the irritation may have been caused, the process in the optic nerve is always the same, and the action on the nerve-centre always produces

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the sensation of light. It must be exactly the same with the other sensory organs and their nerves. sound does not extend beyond the end of the auditory nerve, and none of it is conveyed to the brain by the auditory nerve. The nerve, which is excited at its termination, communicates its condition to the brain, and causes in the centre of the auditory nerve the sensation of sound.

The sensation of sound, therefore, can take place without a sound-wave reaching the ear, if only the auditory nerve is in any way excited, whether it be by pressure, rupture, electricity, etc. Thus the irritation, which in the centre of the auditory nerve causes the sensation of sound, always takes place in the nerve.

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It is clear that these ideas must be extended to the other sensory organs of taste, smell, and touch. sensory nerves are only intended to communicate the fact of an excitement of the nerve from the terminations of the nerves to their centre in the brain-the sensorium. This irritation of the nerve is by no means similar to the first irritation. It is neither light nor sound, nor is it pressure nor warmth, nor a current of liquid which can be tasted, nor of a gas which can be smelt. It is rather a process of a peculiar kind, about which we may conclude that in all the nerves of the body the irritation is one and the same, since, in the muscular as well as in the sensory nerves, it exhibits the same phenomena and obeys the same laws.

The nerve, again, can no more appropriate a trace of the excitement than possess a trace of sensation. If we have at any place divided a sensitive nerve we can excite the divided part as strongly as we please;

nerve centres

but all sensation is gone. The central nerve-trunk, however, is sensitive throughout. Sensation can only take place in the sensorium.reade 2 The excited condition of the sensorium is the material fact which corresponds to a sensation; and it is unnecessary that the sensory nerve concerned should have caused the irritation; for in dreams we have distinct sensations which are not caused by the specific excitement of the nerves, but only by the action of some internal excitement within the sensorium. Abnormal excitements also, which occur in the case of lunatics, or abnormal states of the blood in febrile diseases, cause subjective perceptions, which are called phantoms and hallucinations.

From these remarks it is clear that we really have no sensations of objects of the external world themselves, but only of the changes which occur in the sensorium.

How is it then that we, nevertheless, transfer our inward sensations to the outer world, that we consider as external to ourselves all that we see, hear, or feel? This fact, which to the healthy human mind seems so simple and natural, requires consideration.

The above question can be answered shortly, as follows:-From our very birth we learn by experience how to explain the sensations of our senses; and by a thousand experiments, which we make with eye, ear, and limbs in every-day life, arrive at the conclusion that the object of sensations, that is, their ultimate cause, is external to ourselves. The newly-born child, of course, experiences sensations. The light which enters its eye acts indisputably on its brain, for the

Proof.

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