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SCIENCE FROM AN EASY CHAIR.

THE SENSES AND SENSE-ORGANS

(Br Sir Rat Lankester, X.C.8.,

F.R.S.)

[Special rights secured by "The, Press.")

In the skin aad underlying the surfaces of deep-lying organs inside the body, there is an enormous number of minute root-It ke fibrils or filaments of extreme tenuity penetrating in every

direction. They are the finest nerve filaments, threads., or fibres. They gather together into skeins or "strauds," and these again into larger bundles called "nerves/ and thus pass from the surface and other parts of the body where they commence, joining to form larger and larger nerves until they reaim the brain, euner entering it aiiectly or by way oi the great spinal cord, which lies in tlio bony aremvay lornied by the backbono oi" vertebral column. There are ncrvo fibres, the business of wnicn is to or.ng "inipuistM" —Ai6 ic.'were, messages —iivjui tuo oouy and its suriace to vie brain anu spinal coid. Tney are caaea "ahercnt" fibres. And there are other nerve hores, uncuscinguisiiaoie in appearance from these, ana often mixed in with them m one uanuie or .nerve, whose business it is to convey impulses or messages from tho brain ana spinal cord to muscies and gland ceiis. They are called "unerent'' fibres.

. I am.concerned to-day with the afferent jiervo fibres, often cailed "sensory nerve ribres," whilst the efferent ones aro called "motor nerve fibres." Tlje endings of the finest afferent nerve fibres or threads near tho surface of the body or of its internal cavities are of such a nature tnat they can be acted upon by various external agencies, such as pressure, change of temperature, light, chemical, and electrical disturbance; that is to say, the "state" of condition of tho living nerve fibre can bo definitely altered by the impact of these agencies. The action of these agencies on the nerve fibres is- spoken of "stimulation" or "excitation" of the nerve fibre. The immediate fact by which this "stimulation" is made evident is the setting up of rapid changes, either chemical or electrical, in the substance of the nerve fibre. . Theso rapid changes are called "impulses" or "nerve impulses," and are • transmitted or propogated along a nervo fibre with a quickness less than but resembling that of an electric current until they reach the nerve cell to which it belongs, one of millions contained in tho brain and spinal cord. The nerve cell is a plump, granular lump of protoplasmic substsahce, with a large spherical kernel or nucleus, and and with many branching fibrils reaching out from its substance and joining it to other nerve cells. The nerve cells with which we are here concerned are those which exist by millions in the brain, and form what is called ."the grey substance of the brain." here the impulse, or a whole series of such impulses coming to many»brain cells, produce further changes, -which give rise'to those mental conditions which we call "sensations." I do not propose*, at this moment, to .go further into the relation of the structure of the brain to mental activities; but I will say—-what is the ' unshaken and unanimous conviction of all tho great physiologists—-namely. ' that it is by "sensations," T and - only by senations, that we arrive at of tne world around us and of our own bodies.

In order that external agencies may thus act on the fine terminal twigs of the nerves- it is necessary that the endings of these delicate filaments be connected with a receptive npuaratus, an "end-organ," as it is termed, which is adapted to recoivo tho special action inf. one or other of the- external agencies;! have named above, and so set up the stimulation 'of'the fine nervethreads which end in it. There is one kind of end-organ which is specially fitted to the action of light—or, as we say, is specially "sensitive" to light; another kind which is specially sensitive to the vibrations of sound; •another which •is so for tbe chemical actions causing taste; another for those : causing smell; others for those set up by mechanical pressure, for rise of-temperature (heat), for. fall of temperature (cold); another for the changes of pressure in 'liquid-holding tubes \cn used by alterations of balance and equilibrium;.another for the muscular contractions which enable us to estimato weight; and another for those violent and destructive changes in our tissues which cause the sensation which we call, "pain." There thus appear to be some ten distinct groups of sensations, requiring and associated with distinct • end-organs specially fitted to receive the stimulating influence <of ten different kinds of actions or changes which occur as we live and move in relation to other existing things and as they move and change around us. '

The more or.loss elaborate mechanisms formed at the free ends'of sensory verves are called "sense-organs." The capacity and working of a sense-organ is called a "sense." Thus there are ten "senses," each of a different kind, appropriate each to one of the ten different kinds of sense-organs. The four first mentioned in my list above given are often called "special senses," and their end-argahs are called the organs of special sense. They are: (1) Sight, (2) hearing, (3) smell, and (4) taste! The organs of these senses are separate parts, distinctly .and easily recognised, and in the case of the first two greatly elaborated and brought to perfection by obvious "accessory" apparatus, which, assists or helps to render the sense-organ responsive to small quantities of tho stimulating agent and to other features connected with it, such as tho direction from which it comes and the variation (often very. great) in its special qualities.

Thus we have the eye (a pair or more in different animals), which consists not merely of the light-sensitive retina, built up by most peculiar and varied end-cells (the retinal cells), .in which the nerve threads of the optic nerve terminate, but of the eyeball, provided with lenses, which can be "focussod" so as to produce a picture in its dark chamber, where the retina or sensitive plate is spread, also provided with the series of muscles which turn the axis of the eyeball in different directions, and the circular curtain »of the iris and.the eyelids by which tho amount oK light entering the eyeball can bo regulated. There are also minute elaborations of- great importance in the retinal-cells, such as those connected with the - discrimination of colours.

The second organ of sense," the ear' (usually as, in man, a pair), is less obvious than the eye ; For what we commonly call "the ear is only an external "hearing trumpet," tho real organ of hearing being sunk in the bones of the skull and called "the internal ear.'' It is an elaborately-con-structed membranous sac, containing liquid. It is of the shape of a coiled snail shell, with three looplike tubes— the semi-circular canals—growing out of it. On its walls are distributed _an immense profusion of bunches of fine nerves, which gather together,to con-

stituto the auditory nerve. The ends oi these fine nerves penetrate the wall of the snail-shaped sac. and are connected with peculiar end-cells and hairlike rods, forming in its inside a complex apparatus, of which different element* aro excited "by the vibrations of notes of different pitch, resulting m a difference of sensation for all the immense vanetv of sounds and musical combinations which assail it in the form of sound-vibrations. The organ of the sense of spiell s placed on the passage by which* air t-nters tho lungs—the nasal pa-ssage— and tho nerve-threads of the two olfactory nerves, which pass directly from it to tho brain, end (or, to out it the other way, originate) in the nasnl cavity in a multitude of rodlike cells, which cover the walk of the mucrfolded and deep recesses into - whi-.li ultra-microscopic odoriferous particles are carried by the inspired air. The perfection of this sense-organ consists m its sensibility to extremely minute quantities ot odoriferous matter, anu its prooerty of being differently affected "by odoriferous particles wiucn differ only in tho minutest degree, chomicallv, from one another. 1» '» the dog "is differently affected by tl;e odoriferous particles given off by afferent human beings, and can thus recogn.se an individual, or even Ins me.t. footprints, by his smell, a Pf we J,™s to judge by tho size and structure of his nn*al caviv.es. man a'&o. "», . so . m =: remote i>eriod. possessed, but ha* 1»k« som.s rare exceptions) now lost.

The sense-organs of taste are found in the tongue; they, lUe- the organ of smell, are not so complex as the ?igans of sight and hearing, but are limited to tumishing a different sensation according to the chemical compos.tion or substances presented to them. Pho substances which can be tasted _ must be soluble in water and the different sensations which they produce arc only distinguishable by us as sweet, bitter, sour, and salt, of greater or loss intensity. Very usually people call by the name "taste" what is really due to the sense of smell. All "flavours" of foods aud drinks are really odours which reach the olfactory organs from substances held in the mouth. The endorgans of the nerve of taste are little bulb-likejgroups of cells which are set on tuft-like projections of tho tonguo or in circular grooves far back on itssurface. Bitter and sweet taste are dependent on end-organs distinct from those which are concerned with acid and salt taste, and there seem also to be separate end-bulbs for sweet taste distract from those for bitter taste. The nerve-fibres -connected with the taste-organs do not unite to form a pure nerve of taste. The optic nerve is formed purely by nerves from the retina, the auditory nervo purely by nerves' from tho auditory sac, and the olfactory nerve purely by nerves from the olfactory recesses of the nasal cavity. But the taste nerve-fibres pass to the brain in the glossopharyngeal toorve. and ateo in the lingual branch of the fifth nerve in company with fibres of other kinds. In this respect the sense of taste resembles the less^ elaborated senses, namely, those of "touch, heat, cold, the muscular sense, the sense of pain, and apparently also the sense of eqnilibrial pressure, which though not concerned with sound-vibra-tions is conveyed to the brain by fibres which form part of the auditory nerve.

These less elaborated and less specialised "senses" have until recent years been classed vaguely with the sense of touch, and the term "general sensibility" has also been used so as to include them. Thus, it was usual to spietfk of tho five senses,. or five gateways of knowledge—sight, hearing, smell, taste, and touch. But really, ,it seems that there is a distinct apparatus and distinct nerve threads, and a distinct sensation, for (1) the sonse of heat, (2) the sense of cold, (3) the muscular sense, (4) the equilibrial sense, (s).the sense of pain, as well as for (6) the sense- of touch. So that wo recognise ten distinct senses. The nervefibres of the sense of touch are distributed in the skin all over the body, and pars by nerve-bundles, containing other kinds of nerves," to the spinal cord and brain. The same is true of the sense of heat, the sense of cold, the muscular sense, and the sense, of pain; but we have hot, as yet, discovered any distinctive end-organs of peculiar character for these last four; tho nerves appear to end in or between the surface-cells of. the skin in the first two, and in tho muscular cells in the third. Some special characters are present in those terminations related to the operation of a particular kind of physical agent in each case, but they have not yet been described. The sense of pain is excited by the stimulation of nerves (in many", but not all, parts of the body) by destructive! rlrocesses, s)uc3i as cutting or cltishiug, and also especially by the condition called inflammation.

It has been shown experimentally that tho sensation caused by contact of the stria of the hand or other part with a surface which is quickly raised in temperature, or, as we 3ay "warmed," may be brought about in a person who has, by disease or injury, lost the sense of cold in that part that is to say, is unable to recognise a sudden fall in the temperature to which it'is exposed, and that a fall of temperature in other eases, is recognised where rise of temperature produces iio sensation. Hence it is inferred that there are separate nervefibres and nerve-end-organs concerned in the heat-sense and the coid-senso.

On the other hand, in many parts of the surface of the, body the sense of touch is more acutely developed than it is in others, and there tve find special end-organs called taste-bulbs. These are minute spherical or oblong groups of compressed cells in which the terminations of nerve fibres aro enclosed. They are particularly abundant' in the fine ridges separated from one another by grooves, which mark the finger-tips with whorls and. loop-like patterns. The sense of touch is the sense of mechanical pressure, rough and smooth surfaces causing difference of pressure when the finger, or whatever part of the body is used as the exploring instrument, is moved over such surface in contact with it.. Great tactile discrimination is thus possible, and we see the extent to which it can be carried by the wonderful skill obtained by the blind, who can not only read "by touch" the embossed printing of the Braille books prepared for them, but can distinguish and recognise a great variety of surfaces of different kinds, which ordinary men who are not dependent on their sense of touch, and so have not cultivated it, cannot distinguish in that way.

With regard to the muscular sense, it seems that the colls of tho muscular tissue may be the end-organ of tho sensory nerves, which certainly exist and enable us to estimate "weight" by the amount of muscular effort necessary to hold up a given body, and prevent it from falling to the ground.

Tho sense of equilibrium has its sense-organ in the three semi-circular canals of the internal ear. These are membranous tubes filled with liquid and lying in liquid. They are set in the three planes of a cube, one horizontal and two vertical. A new movement in any plane, or the sudden cessation of previous movement in any plane will cause more or less of what is ordinarily called a "jerk" or "chuck" in the liquid-holding membranous tube, and as the three ultimate planes of space of three dimensions are represented by three corresponding semi-cimdar tubes each will he affected according to the direction of the movement, and a corresponding pressure on the end-organs of the nerves distributed to its walls will result. Sinco there is a set of these canals in the inner ear on each side' of the head the apparatus furnishes tko necessary nerve-impulses aud seusa-

tions for a comparison of the relation of the two sides of the head to any movement, and consequently a resultant sensation which is indicative of the equilibrium and poise of the head and of the direction of movement. Birds in which one of the canals is injured cannot fly; they cannot "feel their balance or want of balance and adapt the movement of the wings accordincly. Rats and rabbits so injured cannot walk straight. When the canals of the internal ear of on 8 side are diseased in man giddiness and a tendency to fall m tho attempt to walk are consequences. The pair of liquid-holdi"" vesicles containing ono or more solid particles suspended within, which aro found in snails, mussels, aud other molluscs and many worms, are usually called "internal ears" or "auditory vesicles," because they are affected by the vibrations of sound. But it has been reasonably contended that they must serve "if not exclusively, vet ako as organs of the sense of balance—and hence m place of the name "otoevsts" or 'ear-vesicles" they hav 0 been called "statocysts"' or "balance vesicles." The walls of these sacs in the lower animals are supplied each by a large nerve from tho brain, th c fine" fibres of which end in peculiar "cells" which line the vesicle. A few years ago similar vesicles were discovered in the leaves of nlants. especially iv those which climb and twist round tho stems of other plants for support. It appears that they may serve as directive organs in the movements of these plants, though such a notion involvos the supposition that the living protoplasm of plant tissues can act as a nervous system does in animals and transmit "impulses." Mr Darwin did not shrink from such a supposition, and in his last work—that on the movement of plants—he established 1 the existence of such "transmission" in several cases.

By this brief review I have placed beforo the reader an outline of what is I meant by a "sense." It is always dependent on the excitation of a demon- | strable and appropriate apparatus —a ! sense-organ—and through it of connected nerve-fibres, which transmit "impulses" to the brain. Wo have^ seen what are the agencies which can be distinguished as definitely stimulating verve-fibres in this way, and that there are ten different kinds of such agencies acting on ten differently constructed appropriate sense-organs (some few of them not yet fully investigated), in which the nerves terminate, or perhaps we should rather say "take origin." in view of these facts, tho absurdity of talking about "a sixth-senso" (there are already ten), or a "spider-sense," or a "cat-sense" becomes obvious. It is, of course, conceivable and possible that a spider or a cat may act with some unusual intensity in some people on ono of the ten "senses" which have been distinguished by investigators of the human nervous system. But we must require experimental demonstration of the fact before accepting assertions on the subject, whilst, those who invoke a new special "sense" to bolster up their untested beliefs in theso stories of detection of concealed spiders and cats must,' it seems, be using tho word "sense" in a misleading and illegitimate manner. So, too. it seems to ,mc that there is a fundamental misunderstanding as to the nature and operation of the only channels by which man is known to receive impressions, and those highly complicated groups of impressions which result in knowledge of the world around him —on the part of those who are of the opinion that one human brain can communicate, not merely signs, but detailed information to another at a distance without operating through the sense-organs of the recipient. Of tliis I will say more on a future occasion.

Permanent link to this item
Hononga pūmau ki tēnei tūemi

https://paperspast.natlib.govt.nz/newspapers/CHP19140530.2.46

Bibliographic details
Ngā taipitopito pukapuka

Press, Volume L, Issue 149814, 30 May 1914, Page 9

Word count
Tapeke kupu
3,120

SCIENCE FROM AN EASY CHAIR. Press, Volume L, Issue 149814, 30 May 1914, Page 9

SCIENCE FROM AN EASY CHAIR. Press, Volume L, Issue 149814, 30 May 1914, Page 9

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