LABORATORY WORK 211 



Taste-Buds 



These will be found in sections across the " papilla foliata " of 

 the rabbit's tongue. These are two small oval areas, one on each 

 side of the back of the tongue. 



Olfactory Cells 



Take a small piece of the mucous membrane of the upper part 

 of the nasal cavity of the frog, beneath the olfactory lobes pro- 

 jecting from the front of the brain. Place in i per cent, osmic acid 

 for a few hours. Soak in water for two or three days. Tease in 

 dilute glycerine, or break up by tapping on the cover-glass. Look 

 for narrow cells with brush-like outer ends. These are the smell 

 receptors. 



Heat and Cold Spots 



Explore the skin of the back of your hand for these, using a 

 simple instrument made thus : Draw out a piece of half-inch glass 

 tubing to about --th in. Cut it at the narrow part. Cement with 

 sealing wax into the end a short piece of thick copper wire filed to 

 a small rounded point. Wrap flannel around the tube so that the 

 temperature may remain fairly constant for some minutes. For heat 

 spots, put water at 40 C. into the tube and allow it to rest gently 

 on various places of the skin. When one is found which is sensitive 

 to heat, mark it with red ink for future identification. For cold 

 spots, put finely broken ice into the tube and proceed in a similar 

 way, marking the spots black. They will be found to have different 

 situations from the heat spots. 



Stimulate both kinds with induction shocks by placing on them 

 finely pointed electrodes. The sensation from each will be its own 

 , appropriate one, so that, if stimulated at all, the sensation is always 

 the same. A temperature which feels distinctly warm to the heat 

 spots does not affect the cold spots, and vice versa. But a tempera- 

 ture sufficiently high to stimulate the nerve fibres themselves will 

 produce a sensation of cold from a cold spot. Thus, while 15 pro- 

 duces a sensation of cold from a cold spot and none from a heat 

 spot, one of 45 produces a sensation of heat from a heat spot and 

 cold from a cold spot. This paradox is due, of course, to the 

 operation of the law of specific sensation, as applied to the fact 

 that the receptors for temperatures above that of the skin and below 

 it are separate organs. 



