The sense that enables an individual to assess the physical characteristics of objects – for example, their size, shape, temperature and texture. The sense of touch is considered here along with other senses associated with the skin and muscles. The cutaneous senses comprise:
Touch sense proper, by which we perceive a touch or stroke and estimate the size and shape of bodies with which we come into contact, but which we do not see.
Pressure sense, by which we judge the heaviness of weights laid upon the skin, or appreciate the hardness of objects by pressing against them.
Heat sense, by which we perceive that an object is warmer than the skin.
Cold sense, by which we perceive that an object touching the skin is cold.
Pain sense, by which we appreciate pricks, pinches and other painful impressions.
Muscular sensitiveness, by which the painfulness of a squeeze is perceived. It is produced probably by direct pressure upon the nerve-?bres in the muscles.
Muscular sense, by which we test the weight of an object held in the hand, or gauge the amount of energy expended on an e?ort.
Sense of locality, by which we can, without looking, tell the position and attitude of any part of the body.
Common sensation, which is a vague term used to mean composite sensations produced by several of the foregoing, like tickling, or creeping, and the vague sense of well-being or the reverse that the mind receives from internal organs. (See the entry on PAIN.)
The structure of the end-organs situated in the skin, which receive impressions from the outer world, and of the nerve-?bres which conduct these impressions to the central nervous system, have been described under NERVOUS SYSTEM. (See also SKIN.)
Touch affects the Meissner’s or touch corpuscles placed beneath the epidermis; as these di?er in closeness in di?erent parts of the skin, the delicacy of the sense of touch varies greatly. Thus the points of a pair of compasses can be felt as two on the tip of the tongue when separated by only 1 mm; on the tips of the ?ngers they must be separated to twice that distance, whilst on the arm or leg they cannot be felt as two points unless separated by over 25 mm, and on the back they must be separated by more than 50 mm. On the parts covered by hair, the nerves ending around the roots of the hairs also take up impressions of touch.
Pressure is estimated probably through the same nerve-endings and nerves that have to do with touch, but it depends upon a di?erence in the sensations of parts pressed on and those of surrounding parts. Heat-sense, cold-sense and pain-sense all depend upon di?erent nerve-endings in the skin; by using various tests, the skin may be mapped out into a mosaic of little areas where the di?erent kinds of impressions are registered. Whilst the tongue and ?nger-tips are the parts most sensitive to touch, they are comparatively insensitive to heat, and can easily bear temperatures which the cheek or elbow could not tolerate. The muscular sense depends upon the sensory organs known as muscle-spindles, which are scattered through the substance of the muscles, and the sense of locality is dependent partly upon these and partly upon the nerves which end in tendons, ligaments and joints.
Disorders of the sense of touch occur in various diseases. HYPERAESTHESIA is a condition in which there is excessive sensitiveness to any stimulus, such as touch. When this reaches the stage when a mere touch or gentle handling causes acute pain, it is known as hyperalgesia. It is found in various diseases of the SPINAL CORD immediately above the level of the disease, combined often with loss of sensation below the diseased part. It is also present in NEURALGIA, the skin of the neuralgic area becoming excessively tender to touch, heat or cold. Heightened sensibility to temperature is a common symptom of NEURITIS. ANAESTHESIA, or diminution of the sense of touch, causing often a feeling of numbness, is present in many diseases affecting the nerves of sensation or their continuations up the posterior part of the spinal cord. The condition of dissociated analgesia, in which a touch is quite well felt, although there is complete insensibility to pain, is present in the disease of the spinal cord known as SYRINGOMYELIA, and a?ords a proof that the nerve-?bres for pain and those for touch are quite separate. In tabes dorsalis (see SYPHILIS) there is sometimes loss of the sense of touch on feet or arms; but in other cases of this disease there is no loss of the sense of touch, although there is a complete loss of the sense of locality in the lower limbs, thus proving that these two senses are quite distinct. PARAESTHESIAE are abnormal sensations such as creeping, tingling, pricking or hot ?ushes.
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