FUNCTIONS Of THE SPINAL CORD 909 



anatomical relations of afferent and efferent paths, partly by the 

 varying resistance of the synapses or other structures whose proper- 

 ties fix the threshold value of the excitation by which an arc can be 

 forced. In general it is not enough that the channel of the final 

 common paths (p. 899) to the muscles whose contraction produces 

 the reflex movement should be thus open to the afferent arcs that 

 elicit the movement; they must be closed to other afferent arcs 

 which might disturb the reflex. Not only so: there is evidence 

 that very frequently the final common paths are, so to say, more 

 widely opened to the afferent arcs in question by the ' reinforcing ' 

 or ' facilitating ' influence of allied, though it may be distant, 

 afferent arcs, which are simultaneously excited (p. 911). Further, 

 the final common paths to antagonistic muscles must also be 

 temporarily closed. The closing of these central connections, or 

 rather the raising of their threshold sufficiently to bar the impulses 

 from passing through the door, is an inhibitory phenomenon. Ex- 

 citation of the desired movements and inhibition of antagonistic 

 movements go hand-in-hand in the simultaneous combination of 

 reflexes. It is obvious that if a movement is to be efficiently exe- 

 cuted, it cannot be the result of a compromise between competing 

 reflexes. A segment of a limb can be either flexed or extended, but 

 cannot at the same time undergo both flexion and extension. In 

 the interests of an effective movement, one or the other must give 

 way utterly. The reflex which eventuaUy prevails makes a clear 

 field for itself by inhibiting all other reflexes which do not co-operate 

 with it. For example, if the receptive area of skin from which the 

 scratch reflex is elicited be stimulated and a painful stimulation be 

 at the same time applied to the foot, we do not obtain a mixed 

 scratch and flexion reflex, which would result in a confused and 

 ineffective combination of movements, but a pure scratch or flexion 

 reflex, as a rule the latter. 



The successive combination of reflexes is well illustrated by the 

 contraction of the oesophagus in deglutition. First one portion of 

 the tube and then the next below are involved in the reflex action. 

 The combination consists in the orderly sequence. The manner in 

 which this is secured in this class of reflex action has been lumin- 

 ously discussed by Sherrington,* but details cannot be given here. 

 While only allied reflexes i.e., such as mutually reinforce and 

 therefore harmonize with each other can be simultaneously com- 

 bined, and antagonistic reflexes cannot, both allied and antagonistic 

 reflexes can be successively combined. An example of the succes- 

 sive combination of allied reflexes is the series of scratch reflexes 

 caused by a parasite travelling across the receptive field of the 

 reflex. An example of the successive combination of antagonistic 



* ' Integrative Action of the Nervous System, ' to which work the advanced 

 student is referred. 



