112 



THE VERTEBRATE ANIMAL: IRRITABILITY 



is captured, its contact with the Uning of the mouth stimulates 

 the closure of the mouth. This, in turn, sets in operation the 

 swallowing reflexes, which occur in sequence. 



In the cases considered so far, the response to the stimulus has 

 been studied with respect to the usual external conditions that 



A.N. 



A.N. 



-^ 



C 



E.N. 



E.N. 



A.N. 



AD.N./; 



B 







E.N. 



^IG. 62. — Diagrams showing relations of neurones in different reflex arcs. A. 

 A simple reflex arc. B. A typical reflex arc showing possibihty of longitudinal 

 transmission. C. A chain of reflex arcs. D. Compound reflex arcs. If 

 the two effectors produce similar action the reflexes are alUed. If the action 

 of the effectors should )>e opposed one or the other is inhibited and the reflexes 

 are antagonistic. E. Com])Oimd reflex arcs. Here impulses from t.vo 

 receptors use as a "final common path" the efferent neurone to a single 



effector. 



A.N., afferent neurone; AD.N., adjuster neurone; C, central nervous system; E, effec- 

 tor; E.N., efferent neurone; R, receptor. (.Redrawn with modifications froia Herrick, 

 "Introduction to Neurology," copyright, 1915, by W. B. Saunders Co., printed by per- 

 mission.) 



produce the effect. Pavlov discovered that it was possible to 

 produce what he termed " conditioned " reflexes. For instance, 

 the flow of saliva is a reflex action stimulated normally by the sight 

 of food. Under experimental conditions, a bell may be rung 

 whenever food is given to an animal. After a number of such 

 experiments, the mere ringing of the bell, without the sight of food, 



