CHAPTER VIII.

THE SENSES.

THE senses are the endowments by which we perceive the physical properties of external objects and the phenomena produced by their various reactions, such as solidity, pressure, smoothness or inequality of surface, temperature, light, sound, and sapid and odoriferous qualities. All our information with regard to the objects of nature is obtained through these channels, which are consequently the primitive source of all conscious relation with the external world. Sensation alone indicates merely the perception of some impression derived from without, whatever may be its nature. The senses, on the other hand, form so many subdivisions of the main function, each of which is devoted to the perception of a particular class of physical properties or reactions. They are divided into five different groups, namely: 1. General sensibility. 2. The sense of taste. 3. The sense of smell. 4. The sense of sight. 5. The sense of hearing.

General Sensibility.

General sensibility is that by which we appreciate the simpler physical properties of external objects, such as their consistency, roughness or smoothness of surface, temperature, and mass. It is so called because it is generally diffused over the external integument, beside being present in most of the mucous membranes near the surface. Notwithstanding that this endowment includes the power of perceiving several different kinds of impression, they are all, so far as we know, communicated to the perceptive centres by the same nerves; and the grade of sensibility for all varies, as a general rule, in the same direction and to the same degree in different parts of the body. The sensations thus produced, though presenting some peculiarities by which they may be distinguished from each other, are therefore naturally comprised under the single head of general sensibility.

Sensations of Touch.-This is, perhaps, the least complicated form of sensory impression, and is known as "tactile sensibility." It is produced by the simple contact of a foreign body with the sensitive surface, and gives information as to its solidity, its external configuration, and its indifferent or irritating qualities. Although there is a certain variety in these impressions, yet they evidently belong to the same group, and there is no essential difference in the effect produced by the contact of sharp-pointed instruments, and that caused by irritating substances, like mustard, applied to the skin, the continuous of

interrupted galvanic current, pungent liquids placed upon the tongue, or pungent vapors in the nasal passages. These are all impressions of tactile sensibility, and depend upon a similar irritation of the peripheral nervous extremities.

In

The structures especially devoted to the exercise of tactile sensibility are minute bulbous organs developed upon the terminal extremities of the nerve fibres in the papillæ of the skin and adjacent mucous membranes, in each of which two situations they present certain distinguishing features, though their essential character is the same in both. the skin, these organs are known as the tactile corpuscles. They are elongated oval bodies, measuring, according to Kölliker, about of a millimetre in length by of a millimetre in thickness. They are situated in the substance of certain of the papillæ, with their long axes placed longitudinally, and extending nearly to the free extremity of the organ. They are not to be found in all of the papillæ, since even at the end of the index finger, where they are most abundant, according to the observations of Meissner, not more than one papilla in four is provided with a tactile corpuscle. The papillæ containing the corpuscles are not supplied with blood vessels; while the remainder, constituting

Fig. 186.

the large majority, contain capillary bloodvessels, but no tactile corpuscles. The tactile corpuscle itself consists, 1st, of a sheath, exhibiting a number of transverse nuclei, and considered as representing a form of connective tissue; 2d, of an inclosed mass of transparent, homogeneous material; and, 3d, of one or two medullated nerve fibres, which pass upward from the superficial plexus of the skin through the substance of the papilla, reach the tactile corpuscle, wind round it in a spiral direction toward its apex, and finally, losing their medullary layer, terminate in some manPAPILLA OF THE HUMAN ner not yet distinctly ascertained. Tactile SKIN, containing tactile corpuscle and nerve fibres. (Köl- corpuscles have been found, in man, upon the dorsal and palmar surfaces of the hand and foot, upon the nipple, and upon the anterior part of the forearm. their abundance in these different regions corresponds with the local acuteness of sensibity, they are undoubtedly to be regarded as the special organs of touch, though not perhaps the only form of nerve structure capable of exercising this function.

liker.)

As

In the conjunctiva, the red portion of the lips, the tongue, the sublingual mucous membrane, and the glans penis, the organs of touch are constituted by the terminal bulbs of the nerve fibres in these regions. These organs differ from the tactile corpuscles mainly in their smaller size and the greater simplicity of their structure. In man, according to Kölliker, they are for the most part nearly spherical in form, though in the inferior animals they are often elongated and club-shaped. They

consist of a very thin, external envelope of connective tissue, inclosing, as in the tactile corpuscle, a mass of homogeneous or finely granular substance. The medullated nerve fibre which penetrates the bulb, loses its medullary layer at its entrance, and runs through the central homogeneous substance, to terminate by a free extremity near its apex. Both the tactile corpuscles and the terminal bulbs are therefore anatomical forms, in which the axis cylinder of the sensitive nerve fibre terminates, after divesting itself of its medullary layer.

The tactile sensibility varies considerably in different regions of the integument. The best method of appreciating this variation is that adopted by Weber and Valentin. It consists in applying to different parts the points of a pair of compasses, tipped with suitable pieces of cork. If these points be applied to the skin when fixed at very short distances apart, the two sensations cannot be accurately distinguished from each other but are blended into one; and the impression thus produced is that of a single contact. The minimum distance at which the two points can be distinguished by the integument thus becomes a measure of its sensibility at that spot. The observations of Valentin,' which are the most varied and complete in this respect, give the following as the limits of distinct perception in different regions:

DISTANCE AT WHICH TWO POINTS MAY BE SEPARATELY DISTINGUISHED.
At the tip of tongue.

1.00 millimetre.

66

palmar surface of tips of fingers

1.50

66

This method does not necessarily give an absolute measure of th› acuteness of sensibility in the different regions, since the two points might be less easily distinguished from each other in any one region, and yet the absolute amount of sensation produced might be as great as in the surrounding parts; but it undoubtedly affords an accurate estimate of the delicacy of tactile sensation, by which we distinguish slight inequalities in the surface of solid bodies. There is every reason to believe that the two qualities of delicacy and acuteness of local sensibility correspond with each other in their degree of development in various localities; since the regions where tactile sensibility is most delicate are frequently found to be also those where the amount of sensation is the greatest. A feeble galvanic current may be perceived

In Todd's Cyclopædia of Anatomy and Physiology, vol. iv., article on Touch.

when applied to the tips of the fingers, though it will produce no impression on the rest of the limbs or trunk; and one which is too faint to be distinguished by the fingers may be perceptible at the tip of the tongue. Certain parts of the body, furthermore, are especially well adapted for use as organs of touch, not only on account of their acute sensibility, but also owing to their conformation and mobility. In man, the hands are the most favorably constructed for this purpose, by the numerous articulations and varied power of movement of the fingers, by which they may be applied to solid surfaces of any form, and brought successively in contact with all their irregularities and depressions. We are thus enabled to obtain the most precise information as to the texture. consistency, and configuration of foreign bodies.

But the hands are not the exclusive organs of touch, even in man and in the lower animals the function is mainly performed by other parts. In the cat and in the seal, the long bristles seated upon the lips are used for this purpose, each bristle being connected at its base with a nervous papilla; and in the elephant the end of the nose, which is developed into a flexible and sensitive proboscis, is employed as the principal organ of touch. This function, therefore, may be performed by one part of the body or another, provided the accessory organs be developed in a favorable manner.

About the head and face, the sensibility of the skin is principally dependent upon branches of the fifth pair. In the neck, trunk, and extre mities it is due to the sensitive fibres of the cervical, dorsal, and lumbar spinal nerves. It exists, to a considerable extent, in the mucous membranes of the mouth and nose, and of other passages leading to the interior. The sensibility of the mucous membranes is most acute in parts supplied by branches of the fifth pair, namely, the conjunctiva, anterior part of the nares, inside of the lips and cheeks, and the anterior twothirds of the tongue. The tactile sensibility, which is resident in the skin and in a certain portion of the mucous membranes, diminishes in degree from without inward, and disappears altogether in the internal organs which are not abundantly supplied with nerves from the cerebrospinal system.

While the general sensibility of the skin, and of the mucous membranes, varies in acuteness in different parts of the body, it is everywhere the same in kind. The tactile sensations produced by the contact of a foreign body are of the same nature, whether they be felt by the tips of the fingers, the dorsal or palmar surfaces of the hands, the ips, cheeks, or any other part of the integument. Their only difference is in the intensity and distinctness of the impressions produced.

The appreciation of the weight or mass of a foreign body is obtained from the degree of pressure which it causes upon the integument, when supported by the hand or other part of the body. It does not appear that other kind of sensation is necessary for this purpose, although we generally also employ, in estimating a weight, the degree of muscular effort required to sustain it. If the hand, however, be rested upon some

any

soiid support, and the foreign body placed upon it, its weight is then appreciated solely by the amount of pressure which it causes. The sensation of muscular contraction is itself a result, so far as we can judge, of the physical impression produced upon the sensitive nerve fibres in the muscular tissue; and there is nothing to indicate that it differs essentially from that caused by pressure upon the nerves of the integument.

Sensations of Temperature.-The appreciation of temperature is also most highly developed, as a general rule, in those parts which have the greatest share of tactile sensibility. The difference in this respect between the integument of the face and that of the scalp is very marked; since hot applications may be readily borne upon the scalp, which would be nearly or quite intolerable upon the face. The extent of surface exposed to a given temperature has also an influence upon the effect produced, and a moderate degree of either warmth or cold applied over a considerable portion of the skin is much more readily perceived than if confined to a limited region. There is evidence that the impressions of temperature and those of touch, if transmitted by the same fibres, depend upon two different forms of nervous excitation, or are received by different peripheral nervous structures; since abundant instances have been observed in which one of these two kinds of sensibility was impaired independently of the other. In various forms of paralysis, tactile sensibility may be lost while that of temperature remains; or, on the other hand, the power of appreciating temperatures may disappear while impressions of contact continue to be perceived.'

Sensations of Pain.-The sense of pain is different in character from that caused by tactile impressions or by variations in temperature. It is caused by any exaggerated mechanical irritation or by the application of excessive heat or cold; but in all these instances, when the intensity of the impression rises above a certain point, the ordinary perceptions produced by it disappear, and that of pain takes their place. Thus if the blade of a knife or the point of a needle be placed gently in contact with the skin, we perceive, by means of tactile sensibility, the character and form of its surface. But if the pressure be increased beyond a certain degree, or if the integument be actually wounded, we obtain no precise information of the physical qualities of the foreign body, and are only conscious of the pain which results. The appreciation of cold or warmth, in like manner, is only possible within moderate limits; and when the variations are so excessive as to produce pain, all accurate perception of the degree of temperature is lost. The contact of a redhot iron and that of one much below the freezing point of water produce sensations which are not essentially different from each other, and which are marked only by their painful character.

It is not known whether the sensation of pain be confined to nerve

1 Brown-Séquard, Physiology and Pathology of the Central Nervous System. Philadelphia, 1860, pp. 84, 98, 125.