THE REFLEX CIRCUITS 57 



acquired automatisms must be clearly distinguished from the 

 reflexes, which belong to the innate nervous organization with 

 which we are endowed at birth (see pp. 31, 301). The lowly 

 organisms which lack a differentiated nervous system exhibit 

 many kinds of behavior which closely resemble reflexes and, in 

 fact, are physiologically of the same type; but these non-nervous 

 responses are usually termed tropisms or taxes, though some 

 physiologists call them reflexes, and some reflexes, as above de- 

 fined, are often called tropisms. 



The structure of the simple reflex circuit is diagrammatically 

 illustrated in Fig. 18, A. The receptor (R) may be a simple 

 terminal expansion of the sensory nerve-fiber or a very complex 

 sense organ. The effector (E) may be a muscle or a gland. The 

 cell body of the afferent neuron (1) may lie within the center (C) 

 or outside, as in the diagram. The latter condition is more usual, 

 as seen in the spinal and cranial ganglia (Fig. 1, p. 25). The 

 synapse and the cell body of the efferent neuron (2) lie in the 

 center. 



A simple reflex act involving the use of so elementary a mech- 

 anism as has just been described is probably never performed 

 by any adult vertebrate. The nervous impulse somewhere in its 

 course always comes into relation with other reflex paths, and in 

 this way complications in the response are introduced. Some 

 illustrations of the simpler types of such complex reflex circuits 

 will next be considered. 



Separate reflex circuits may be so compounded as to give the 

 so-called chain reflex (Fig. 18, B). Here the response of the 

 first reflex serves as the stimulus for the second, and so on in 

 series. The units of these chain reflexes are usually not simple 

 reflexes as diagrammed, but complex elements of the types next 

 to be described. 



Figure 18, C illustrates another method of compounding re- 

 flexes so that the stimulation of a single sense organ may excite 

 either or both of two responses. If the two effectors, El and 

 E2, can cooperate in the performance of an adaptive response, 

 the case is similar to that of Fig. 18, A, with the possibility of a 

 more complex type of reaction. This is an allied reflex. If, 

 however, the two effectors produce antagonistic movements, so 

 that both cannot act at the same time, the result is a physiological 



