TACTILE SENSATIONS. 815 



ature of bodies in contact with it. Bodies of exactly the same temperature 

 as the region of the skin to which they are applied produce no such thermal 

 sensations, though we can, from the very absence of sensation, form a judg- 

 ment as to their temperature ; and good conductors of heat appear respec- 

 tively hotter and colder than bad conductors raised to the same temperature. 



742. We may consider the skin as having at any given time and in 

 any given spot a normal temperature at which the sensation of temperature 

 is at zero ; for under ordinary circumstances we are not directly conscious 

 of the temperature of our skin ; it is only when the normal temperature at 

 the spot is raised or lowered that we have a sensation of heat or cold re- 

 spectively. This normal temperature may be at the same time different in 

 different parts of the body ; thus, at a time when neither the forehead nor 

 the hand are giving rise to any sensation of temperature, we may, by putting 

 the hand to the forehead, frequently feel the former hot or cold because the 

 normal temperatures of the two parts differ. The normal temperature in 

 any spot may also vary from time to time. Thus, when the hand is placed 

 in a warm medium for some time, the sensation of warmth ceases; a new 

 normal temperature is established with the zero of sensation at a higher level, 

 a depression or elevation of this new temperature giving rise, however, as 

 before, to sensations of heat and cold respectively. That it is the changed 

 condition, and not the change itself, of which we are conscious, is shown by 

 the fact that when a portion of the skin is cooled, by brief contact with a 

 cold metal, for instance, we are still conscious of the spot being cold after the 

 cooling agent has been removed that is, at a time when a cooled spot is in 

 reality being heated by the surrounding warmer tissues. 



743. The change in temperature of the skin necessary to produce a sen- 

 sation must have a certain rapidity ; and the more gradual the change the 

 less intense the sensation. The repeated dipping of the hand into hot water 

 produces a greater sensation than when the hand is allowed to remain all 

 the time in the water, though in the latter case the temperature of the skin 

 is most affected. The effects of contrast are also seen in these sensations as 

 in those of pressure. 



We can with some accuracy distinguish variations of temperature, espe- 

 cially those lying near the normal temperature of the skin. These sensations, 

 in fact, follow Weber's law, though apparently sensations of slight cold are 

 more vivid than those of slight heat, the range of most accurate sensation 

 seeming to lie between 27 and 33. 



The regions of the skin most sensitive to variations in temperature are not 

 identical with those most sensitive to variations in pressure. Thus the 

 cheeks, eyelids, temples, and lips are more sensitive than the hands. The 

 least sensitive parts are the legs, and front and back of the trunk. 



744. The simplest view which can be taken with regard to the dis- 

 tinction between pressure sensations and temperature sensations, and which is 

 suggested by the facts just mentioned, is to suppose that two distinct kinds 

 of terminal organs exist in the skin, one of which is affected only by pressure, 

 and the other only by variations in temperature ; and that the two kinds of 

 peripheral organs are connected with different parts of the central sensory 

 organs by separate nerve-fibres. Certain pathological cases have been 

 quoted as showing not only that this is the case, but that the two sets of 

 fibres pursue different courses in the spinal cord. Thus in certain diseases 

 or injuries to the brain or spinal cord, hypera3sthesia as regards temperature 

 has been observed unaccompanied by an augmentation of sensitiveness to 

 pressure ; and, conversely, instances have been seen where the patient could 

 tell when he was touched, but could not distinguish between hot and cold. 

 On the other hand, there are facts which show a close dependence between 



