8 PRACTICAL PHYSIOLOGY 



there for a few minutes place both in 3 and record the sensation in 

 each hand. What conclusion do you draw ? 



2. What Factors modify the Sensation? 



METHOD. Bring a piece of metal and a piece of flannel, which 

 have been kept at the room temperature, upon the skin and notice 

 the difference in the sensation produced. What is the explanation ? 



3. Is the Power of Determining Temperature equally distributed over 

 the Skin ? 



METHOD. With a cold metal point gently touch the back of 

 the hand at different points between the fourth and fifth metacarpal 

 bones, and notice if the sensation of cold is produced by contact 

 everywhere or only at certain spots. 



Repeat the experiment with the metal at a higher temperature 

 than the body. 



The experiment may be repeated on the back of the forearm. 

 What conclusions do you draw ? 



4. What is the smallest Difference of Temperature which can be 

 appreciated ? 



METHOD. Take two large test-tubes and half fill them with 

 water at between 35 and 40 C., making one slightly colder than 

 the other. Now find the smallest difference of temperature which 

 can be appreciated (a) on the side of the face, (6) on the back of the 

 forearm. Start at differences of temperature distinctly perceptible, 

 and add cold water to the warm tube till the difference of 

 temperature can just be detected. Then with a thermometer take 

 the temperature of the water in each tube. 



The Possibility of Erroneous Judgments involving Tactile and 

 Temperature Senses. Apply first a cold penny, then a warm penny to 

 the forehead, with the head thrown back, and note whether or not 

 the weight feels the same in both cases. 



Conclusions from the Examination of Cutaneous Sensibility. 

 The result of the study of the sensations connected with the skin 

 is to show that there are three kinds of receptors each specially 

 responding to one kind of external change, to one kind of stimulation. 

 1. Receptors responding specially to contact with gross matter ; 

 2. receptors responding to injurious stimuli, e.g. the prick of a pin, 

 and 3. receptors responding to the withdrawal or addition of heat. 



It appears that these are distributed in a spotted manner over the 

 surface, being more abundant at some parts, less abundant at others. 



It is to changes in the external condition that they respond, and 

 the suddenness of the change in time or place is the most potent 

 factor. 



While each kind of receptor is specially tuned to one kind of 

 change, there is evidence that they may be acted upon by others if 

 these are sufficiently powerful. 



(Read Cutaneous Senses in Text Book.) 



