LXXVT.] TOUCH, SMELL, TASTE, HEARING. 369 



(d.) Hold one hand for a time in water at 10" C, and afterwards place it 

 in water at 20° C, at first the latter causes a sensation of heat, which soon 

 gives place to that of cold. 



(^.) Test with the finger the acuteness of the sense of temperature, i.e., in 

 two given fluids of ditierent temperatures, what fraction of a degree C. can be 

 distinguished. One can usually distinguish |°, although the acuteness is 

 greater when the fluids are about 30" C. 



if.) Use two brass tubes (5 cm. long and i cm. in diam.), terminating in a 

 point. Cover both, all except the point, with india-rubber tubing. Fill one 

 with warm water and the other with cold. Test the j)osition of the warm and 

 cold points on another person on various parts of the skin. 



(7.) Warm and Cold Spots. 



With a blunt metallic point touch different parts of the skin. 

 Certain points excite the sensation of warmth, others of cold, 

 although the temperatures of the skin and the instrument remain 

 constant. Map the position of the cold and hot spots by means of 

 different colours. 



3. Sense of Pressure. 



(rt.) Rest the back of the hand on a table, cover a small area of 

 the palm with a non-conducting material, e.g., a wooden disc. On 

 the latter place different weights. Estimate the smallest difference 

 of weight which can be appreciated. 



(&.) Dip the hand or a finger into mercury. The greatest sensation is felt 

 at the plane of the fluid in the form of a ring, but even this is best felt on 

 moving the hand up and down. 



4. Peripheral Projection. 



(a.) Press the ulnar nerve at the elbow, the prickling feeling is 

 referred to the skin on the ulnar side of the hand. . 



{b.) Dip the elbow in ice-cold water ; at first one feels the sensation of cold 

 owing to the effect on the cutaneous nerve-endings. Afterwards, when the 

 trunk of the ulnar nerve is affected, the pain is felt in the skin of the ulnar 

 side of the hand where the nerve terminates. 



5. Reference of Tactile Impressions to the Exterior. — Gene- 

 rally speaking, the sensation of touch is referred to our cutaneous 

 surfaces. In certain cases, however, it is referred even beyond 

 this. 



(a.) Holding firmly in one hand a cane or a pencil, touch an 

 object therewith ; the sensation is referred to the extremity of the 

 cane or pencil. 



(b.) If, however, the cane or pencil be held loosely in one's hand, 

 one experiences two sensations, one corresponding to the object 

 touched, and the other due to the contact of the rod with the skin. 

 The process of mastication affords a good example of the reference 

 of sensations to and beyond the periphery of the body. 



2 A 



