946 CENTRAL NERVOUS SYSTEM 



The Integration of Allied Reflexes 



Simultaneous Combination. The scratch reflex is well adapted for 

 the study of this subject since the skin area from which this reflex can be 

 elicited is very widespread (see Fig. 238). The type of reflex produced 

 from any given area is in general the same, although "the local sign" 

 that is, the point at which the animal scratches will vary according 

 to the point stimulated. If we take point A in the reflex scratch area 

 and apply to it a stimulus which is just inadequate to produce any reflex 

 at all, and then, while this stimulus is still in progress, apply a similar 

 subliminal stimulus to point B a little removed from it, the two sub- 

 liminal stimuli will become effective and produce a typical scratching 

 movement. In other words, the subliminal stimulus of point A be- 

 comes added on the final common path with the subliminal stimulus of 

 point B; the one has reinforced the other. 



In a similar way two stimuli each of which are adequate to excite 

 a weak reflex response from a common motor mechanism, will reinforce 

 one another and produce a strong response if they are applied at the 

 same time. 



The receptors from which these mutually reinforcing impulses are re- 

 ceived need not, as in the above example, be of the same kind, similar 

 results being obtained by stimulation of receptors of widely different 

 kinds, such as exteroceptors and proprioceptors. For example, if a stimu- 

 lus inadequate to elicit a flexion reflex is applied to the skin of the leg, 

 and another stimulus, itself also inadequate, is applied to the central end 

 of some deep afferent nerve in the same leg, then the two subliminal 

 stimuli will become effective in producing a flexion movement. 



We have seen that in the development and maintenance of posture 

 the closest alliance takes place between the exteroceptive reflexes which 

 initiate movement and the proprioceptive reflexes which adapt the tone 

 of the muscles to the new positions of the limbs (page 917). Allied re- 

 flexes may reinforce one another even though the exciting stimuli are 

 applied far apart. The flexion of the fore limb which results from 

 stimulation of the fore paw is reinforced, for example, by a simultaneous 

 excitation of the contralateral hind foot a stimulation that gives rise 

 to a reflex figure involving flexion of the fore limb as we have seen in the 

 last chapter. Reflexes which give rise to inhibition may also reinforce 

 one another in their action on the final common path. 



Successive Combination. The reinforcement which one reflex lends to 

 a second allied reflex lasts for a short time after the stimulus for the 

 first response has been removed. Consequently successive allied reflexes 

 may reinforce one another. This reinforcement also can be illustrated 

 in the case of the scratch reflex. If a stimulus, inadequate to excite when 



