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 the 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 379 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 favorable to the delicacy of this sense, and enable us to appreciate with exactitude the qualities of the bodies we may feel.”
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?
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.
853. The healthy or unhealthy, active or inactive state of the brain, influences the action of the sensitive nerves. In 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 380 sleeping, when exposed to the warm rays of the sun, and the individual will not be aware of it at the time.
851. From what do the nerves proceed that supply this sense? 852–864. 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.