LABORATORY MANUAL OF PHYSIOLOGY. 



In which of these areas are the two points of the test needles 

 distinguished as separate, when brought nearest together ? Why ? 



In which axis of a limb are the two points of the test object most 

 readily distinguished? Explain. 



Is there any after-sensation in any of the skin areas stimulated ? 



(i) Stimulation of the touch-spots, through pressure, occurs as 

 a consequence of deformation of the skin. If a constant pressure 

 is applied to all parts of a skin surface equally, there is no sensa- 

 tion of touch. This may be accomplished by immersing the hand 

 in a vessel of water at the same temperature as that of the skin. 

 So long as the hand remains quiet and the water is still, there is no 

 sensation of touch, except at the junction of air and water at the 

 surface of the liquid. 



2. Temperature Sense. Temperature sensation is of two 

 kinds, sensation of cold and sensation of heat. There are, ap- 

 parently, separate sets of nerve fibres and endings for these two 

 sensations, since certain skin areas are irritable to cold objects but 

 not to warm, and others are irritable to objects warmer than the 

 skin but not to cold stimuli. 



(a) These areas may be mapped out as so-called warm and cold 

 spots by a method similar to that used in mapping out the touch- 

 spots. Take a metal cannula, drawn to a fine point a small ar- 

 tery cannula may be used and run hot water through it for a time. 

 Choose an area of skin, about 4 sq. cm. on the back of the hand, 

 for testing. Bring the tip of the cannula sufficiently near the skin 

 to obtain the sensation of heat without mixing this with the touch 

 sense. Mark those points where the heat sensation occurs, with 

 a fine-pointed colored pencil. 



(b) Go over the same area of skin as in (#), running cold water 

 through the cannula instead of hot as before. Mark the cold spots 

 with a fine pencil of another color than that employed in marking 

 the warm spots. 



(c) A rough estimate of the temperature sense, in various skin 

 areas of the body, may be made by filling two test tubes of small 

 calibre with hot and cold water, respectively. These are applied, 



