242 INTRODUCTION TO NEUROLOGY 



no doubt play a very large part in shaping the organic back- 

 ground of the entire conscious life (see p. 259). 



Students of animal behavior are in the habit of investigating 

 the ability of animals to make simple associations by training 

 them to perform particular acts under conditions such that the 

 normal stimulus to the act is always accompanied by a second 

 stimulus of a different type. After many repetitions the re- 

 sponse may be obtained by presenting the second or collateral 

 stimulus without the first. For the nervous mechanism of 

 "associative memory" of this sort see p. 64. Pawlow has 

 found that variations in the amount of saliva secreted form an 

 especially good index of associations of this type, and he has used 

 this method extensively in analyzing complex reactions, or con- 

 ditional reflexes, as he calls them. See the summary of his 

 researches in the paper by Morgulis cited in the appended bib- 

 liography. 



Tactile sensibility is entirely absent throughout the entire 

 alimentary canal from the esophagus to the rectum, and the 

 same holds true for most of the other deep-seated viscera of the 

 body. Even the substance of the brain is insensitive to any 

 kind of mechanical irritation. Sensibility to changes in tem- 

 perature is feebly developed or absent in most of the viscera, the 

 esophagus and anal canal being very sensitive to heat and cold, 

 while the stomach and colon are feebly sensitive to these stimuli. 

 The entire alimentary canal is insensitive to hydrochloric and 

 organic acids in concentrations far in excess of what ordinarily QC- 

 curs in either normal or pathological conditions. The contact of 

 alcohol with all parts of the mucous membrane of the alimentary 

 canal gives rise to a sensation of warmth. This sensation is 

 different in character from that caused by hot fluids and is prob- 

 ably excited through the sympathetic nerves, while the sensa- 

 tion of warmth felt in consequence of the passage of hot fluid 

 through the esophagus is excited through the vagus. 



The demonstrated absence of tactile sensibility throughout 

 the mucous membrane of the stomach and intestine is considered 

 by Hertz to indicate that the sensations of fulness arising from 

 the distention of different parts of the alimentary canal are due 

 to the stretching of the muscular coat, and that, therefore, these 

 are to be regarded as varieties of the muscle sense. The same 



