CHAPTER XLI.

THE SENSE OF TOUCH.

846. SENSATION is the perception of external objects by means of the senses. There are five senses, namely, Touch, Taste, Smell, Hearing, and Vision.

847. TOUCH is the sense by which the mind becomes acquainted with some of the properties of bodies, and enables us to determine whether their surfaces are smooth or roug, their relative temperature, and, to a certain degree, their form and weight.

848. Some physiologists make a distinction between the sense of touch and tact. Tact, or feeling, is more general, extending over the whole surface of the skin and mucous membranes, while touch exists chiefly in the fingers of man and in the noses of certain quadrupeds.

849. "In the exercise of these functions, tact is considered passive; as, when any part of the system comes into contact with another body, a sensation of its presence is given, without the exercise of volition. On the contrary, touch is active, and is exercised voluntarily, for the purpose of conveying to he mind a knowledge of the qualities or properties of the surfaces of bodies; as when we feel of a piece of cloth to ascertain its qualities, or a polished surface, to prove its smoothness."

850. In man, the hand is admirably adapted to the exercise

846. Define sensation. How many senses have we? 847-851. What is said of the sense of touch? 847. Define touch. 848. What is the difference between touch and tact? 849. In the exercise of these functions, which is active, and which passive? 850. Why is the hand so admirably adapted to the exercise of the sense of touch?

of touch.

"The fineness of the skin, its great sensibility, the species of cushion formed by the sub-cutaneous fat at the extremities of the fingers, the length and flexibility of these organs, and the capability of opposing the thumb to the fingers, like a pair of forceps, are so many conditions essentially favor able to the delicacy of this sense, and enable us to appreciate with exactitude the qualities of the bodies we may feel."

851. The nerves that supply the sense of touch, proceed from the anterior half of the spinal cord. Where this sense is most acute and delicate, we find the greatest number of sensitive nervous filaments, and those of the largest size.

Observation. In amputating limbs, and other surgical operations, the division of the skin causes more pain than all the subsequent steps of the operation, however protracted. The muscles, cellular membrane, and fat have but little sensibility; while the bones, tendons, and ligaments are insensible when not diseased, and may be cut without causing pain.

HYGIENE OF THE SENSE OF TOUCH.

852. The sense of touch varies in different persons, and also in individuals of different ages. Thus the sensibilities of the child are more acute than those of the aged. Although there is an original difference of sensibility from organization, still, the function of the nerves of sensation is modified by certain influences.

In

853. The healthy or unhealthy, active or inactive state of the brain, influences the action of the sensitive nerves. sound and perfect sleep, the brain is inactive. In this state, ordinary impressions made upon the skin are not observed by the sleeping person. Thus the arm may be blistered while

851. From what do the nerves proceed that supply this sense? 852864. Give the hygiene of the sense of touch. 852. Does this sense vary in different persons? 853. Mention a condition of the brain that influences the nerves of sensation.

sleeping, when exposed to the warm rays of the sun, and the individual will not be aware of it at the time.

854. If there is compression of the brain, as when the skull-bones are depressed, or disease of this organ exists, as in severe typhus fever, impressions made upon the nerves of the skin will not be noticed. The same is true when the mind is engaged in intense thought or study; heat or cold may be so intense as to disorganize the skin, and not to be noticed.

855. The varying health or condition of the brain usually depresses or increases the sensitiveness of the skin. This is seen in grief and fear, which diminish, while hope and joy increase the impressibility of this tissue. It is not uncommon to see the unfortunate insane endure exposure to heat and cold with seeming impunity; whereas it would induce almost insupportable suffering to the sane man. Diseases of the heart, stomach, and lungs, alter the condition of the brain, and modify, to a greater or less degree, the sensitiveness of the skin.

856. The state of the conducting nervous trunks influences the nerves of sensation. If a nervous trunk is compressed or divided, the parts supplied by nervous filaments from this branch, will be insensible to the impressions made upon them, and consequently such impressions are not transmitted to the brain.

Observation. When the inside of the arm or lower extremities rests upon a hard surface, the nerves may be compressed so as to deprive the parts of sensibility. This condition is called "numbness."

857. The quantity of blood supplied to the skin modifies

854. Mention other conditions that affect these nerves. 855. What is the effect of the varying health or condition of the brain upon the sensitiveness of the skin? Give instances of this effect. 856. What is the result if a nervous trunk is divided or compressed? How may "numb. ness" in the limbs be produced? 857 Does the quantity of blood supplied to the skin affect its sensibility?

its sensitiveness. If the quantity of blood is diminished, the sensibility of the skin will be impaired. This is demonstrated by noting the effects of cold upon the cutaneous tissue, the application of which contracts the blood-vessels, and drives the circulating fluid from this membrane, which is shown by the paleness, as well as by the shrivelled appearance of the skin. And, if this tissue is wounded while under the influence of cold, but little pain will be felt, and this chilling influence may be carried so far as not only to deprive the part of sensation, but of vitality.

858. The influence of the blood upon the sensibility of the skin, is further demonstrated by the pain experienced when chilled extremities are suddenly exposed to heat. The nerves, by the sudden dilatation of the contracted blood-vessels, are put in vivid and rapid motion, which causes the painful and tingling sensation that we experience. In every part of the system, sudden changes produce unpleasant sensations, and frequently a diseased condition of the organs.

Observation. When the hands, or other portions of the body, are frozen, or severely chilled, safety and comfort demand that circulation be restored to the parts by moderate exercise in a cool room. Not unfrequently, the vitality of the limb is destroyed by immersing it in hot water or holding it near the fire.

859. The quality of the blood also influences sensation. If the brain and other parts of the nervous system receive impure blood, their energy is depressed, and the sensibility of the skin rendered more or less obtuse.

860. The condition of the cuticle modifies the impression made upon the cutaneous nerves. 1st. When the cuticle has

How is it demonstrated? 858. How is the influence of the blood upon the skin further demonstrated? How should circulation be restored to limbs frozen or severely chilled? What should be avoided? 859. Show how the quality of the blood influences sensation. 860. Give the 1st condi tion of the cuticle that influences the impressions made on the cutaneous nerves.

sleeping, when exposed to the warm rays of the sun, individual will not be aware of it at the time.

and the

854. If there is compression of the brain, as when the skull-bones are depressed, or disease of this organ exists, as in severe typhus fever, impressions made upon the nerves of the skin will not be noticed. The same is true when the mind is engaged in intense thought or study; heat or cold may be so intense as to disorganize the skin, and not to be noticed.

855. The varying health or condition of the brain usually depresses or increases the sensitiveness of the skin. This is seen in grief and fear, which diminish, while hope and joy increase the impressibility of this tissue. It is not uncommon to see the unfortunate insane endure exposure to heat and cold with seeming impunity; whereas it would induce almost insupportable suffering to the sane man. Diseases of the heart, stomach, and lungs, alter the condition of the brain, and modify, to a greater or less degree, the sensitiveness of the skin.

856. The state of the conducting nervous trunks influences the nerves of sensation. If a nervous trunk is compressed or divided, the parts supplied by nervous filaments from this branch, will be insensible to the impressions made upon them, and consequently such impressions are not transmitted to the brain.

Observation. When the inside of the arm or lower extremities rests upon a hard surface, the nerves may be compressed so as to deprive the parts of sensibility. This condition is called "numbness."

857. The quantity of blood supplied to the skin modifies

854. Mention other conditions that affect these nerves. 855. What is the effect of the varying health or condition of the brain upon the sensitiveness of the skin? Give instances of this effect. 856. What is the result if a nervous trunk is divided or compressed? How may numb. ness" in the limbs be produced? 857 Does the quantity of blood supplied to the skin affect its sensibility?

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