158 COMPARATIVE PHYSIOLOGY 



myenteric plexus of the gut ; which is probably in essentials 

 similar to a nerve-net. 



It is possible that many segmental reflexes in Crustacea 

 may employ a simple afferent- efferent reflex arc. In Verte- 

 brates this is rarely the case ; the same internuncial neurones 

 are involved in a multipHcity of reflex arcs. In the adjust- 

 ment of response an important result of Sherrington's 

 analysis of the properties of reflex action is the recognition 

 of the principle of the final common path. If two receptive 

 areas when stimulated evoke response of one kind or another 

 in the same set of muscles, the effect of simultaneous stimula- 

 tion is in general either one of reinforcement, or the complete 

 exclusion of one reflex response in favour of the other. When 

 it is remembered that in normal life the organism is subject 

 to a large variety of stimuli simultaneously impinging upon 

 reflex systems the majority of which may make use of a common 

 path in some part of the C.N.S., the general importance of 

 this fact in defining the behaviour of the animal at any moment 

 is obvious. 



For the scientific analysis of behaviour in animals possessing 

 a nervous system the term *' tropism " is a convenient label 

 for grouping reflexes concerned with bodily orientation in 

 response to a particular type of stimulus. According to 

 established usage the term is extended also to modes of 

 behaviour in metazoa which do not possess a central nervous 

 system, as well as to protista and plants. Thus we speak 

 of photo tropisms, geotropisms, thigmotropisms, chemotropisms 

 to classify reflexes concerned with orientation with reference 

 to light, gravity, contact, or chemical stimuH. The analysis 

 of behaviour has progressed chiefly by studying : (i) the isolation 

 of tropisms or other reflex systems by experimental procedure 

 involving the exclusion of particular receptors or parts of the 

 C.N.S. itself ; (ii) tropistic reactions or other reflexes which 

 normally predominate over other modes of behaviour in the 

 intact animal ; (iii) the modification of normal modes of 

 response by physico-chemical means. 



The study of tropisms in animals has been advanced 

 especially through the labours of Loeb, whose most valuable 



