SENSATIONS OF TEMPERATURE. - The skin is not merely the seat of tactile impressions, but also of impressions of temperature. This depends on thermic irritation of the terminal organs, as proved by the following experiment of E. H. Weber : - " If the elbow be dipped into a very cold fluid, the cold is only felt at the immersed part of the body (where the fibres terminate); pain, however, is felt in the terminal organs of the ulnar nerve, namely, in the finger points ; this pain, at the same time, deadens the local sensation of cold." If the sensation of cold were due to the irritation of a specific-nerve fibre, the sensation of cold would be referred to the tips of the fingers. When any part of the skin is above its normal mean temperature, warmth is felt ; in the opposite case, cold. The normal mean temperature of a given area varies according to the distribution of hot blood in it and to the activity of nutritive changes occurring in it. When the skin is brought into contact with a good conductor of heat there is a sensation of cold. A sensation of heat is experienced when heat is carried to the skin in any way. The following are the chief facts that have been ascertained regarding the temperature sense. (1) E. H. Weber found that, with a skin temperature of from 15'5 C. to 35° C., the tips of the fingers can distinguish a difference of '25° C. to '2° C. Temperatures just below that of the blood (33° C.27° C.) are distinguished by the most sensitive parts, even to '05° C. (2) The thermal sense varies in different regions as follows :tip of tongue, eyelids, cheeks, lips, neck, belly. The "perceptible minimum" was found to be, in degrees C. : - breast, '4°; back, '9° ; back of hand, '3° ; palm, '4° ; arm, '2° ; back of foot, '4° ; thigh, '5° ; leg, '6° to '2° ; cheek, -4° ; temple, '3°. (3) If two different temperatures are applied side by side and simultaneously, the impressions often fuse, especially if the areas are close together. (4) Practice is said to improve the thermal sense. (5) Sensations of heat and cold may curiously alternate ; thus " when the skin is dipped first into water at 10° C. we feel cold, and if it be then dipped into water at 16° C. we have at first a feeling of warmth, but soon again of cold " (Landois). (6) The same temperature applied to a large area is not appreciated in the same way as when applied to a small one ; thus " the whole hand when placed in water at 29°'5 C. feels warmer than when a finger is dipped into water at 32° C.
There is every reason to hold that there are different nerve fibres and different central organs for the tactile and thermal sensations, but nothing definite is known. The one sensation undoubtedly affects the other. Thus the minimum distance at which two compass points are felt is diminished when one point is warmer than the other. Again, a colder weight is felt as heavier, " so that the apparent difference of pressure becomes greater when the heavier weight is at the same time colder, and less when the lighter weight is colder, and difference of pressure is felt with equal weights of unequal temperature" (E. H. Weber). Great sensibility to differences of temperature is noticed after removal, alteration by vesicants, or destruction of the epidermis, and in..the skin affection called herpes zoster. The same occurs in some cases of locomotor ataxy. Removal of the epidermis, as a rule, increases tactile sensibility and the sense of locality. Increased tactile sensibility is termed hyperpselaphesia, and is a rare phenomenon in nervous diseases. Paralysis of the tactile sense is called hypopselaphesia, whilst its entire loss is apselaphesia. Brown-SCquard mentions a case in which contact of two points gave rise to a sense of a third point of contact. Certain conditions of the nerve centres affect the senses both of touch and temperature. Under the influence of morphia the person may feel abnormally enlarged or diminished in size. As a rule the senses are affected simultaneously, but cases occur where one may be affected more than the other. Herzen states that "limbs which are sleeping" feel heat and not cold (Landois).