THE RECEPTORS 861 



light pressures will excite them. Consequently the threshold for touch 

 is lowered on the hairy parts of the body. 



The examination of the sense of touch in the skin is important in the 

 study of nervous disease. The presence or absence of the sensation is 

 tested by touching the skin with a pledget of cotton or a soft paint 

 brush. If the sensation is impaired but not destroyed, more exact in- 

 formation can be obtained by measuring the threshold, i. e., the smallest 

 pressure which will stimulate the touch spots by repeating the test with 

 a series of hairs or brushes of graded stiffness. The ability to localize 

 the spot touched is known as spot finding or one dimensional localiza- 

 tion. Two dimensional localization is the process of recognizing that 

 stimulation is being applied to two spots at the same time. If the points 

 of a pair of compasses are applied to the skin at once they will be recog- 

 nized as distinct points only if separated by a certain distance. This 

 distance becomes greater as the number of touch spots becomes fewer, 

 it apparently being necessary that a certain number of unstimulated 

 spots should separate them before the points of stimulation are recog- 

 nized as distinct. This process of recognizing the distinctness of two 

 spots is called two dimensional localization. The compass test is use- 

 ful in detecting deficiencies in the process which may result from de- 

 struction of the afferent paths for the sense of touch. 



Very closely associated with the sense of touch are the sense organs, 

 which give us knowledge of the position and movements of the parts of 

 our body. These are located in the muscles, tendons and joints, and are 

 doubtless stimulated by deforming pressures arising from the contrac- 

 tion of muscle and the movement of the bones at the joints. The senses 

 are tested by the ability of a patient to touch one part of the body with 

 another, the eyes being shut and the affected member being moved into 

 various positions. Quantitative measurements can also be obtained by 

 determining through how many degrees a joint may be bent before the 

 patient recognizes that the limb is being moved. The ability to recog- 

 nize the position and movement of the parts of the body is called three 

 dimensional localization. 



Heat and Cold. The heat and cold spots are stimulated by abnormal 

 temperatures, heat spots being excited by temperatures above and cold 

 by temperatures below that previously existing in the skin. Whether 

 one or the other sensation will be felt depends not so much on the ab- 

 solute temperature to which the heat and cold spots are brought as to 

 the relation of this temperature to that of the skin. If the skin is 

 chilled luke warm water will feel warm to it, whereas the same water 

 will feel cool if the skin has previously been brought to a high tempera- 

 ture. Thus if one finger is placed in cold water and the other in hot 

 water for a few minutes and then both are thrust into water at an 



