822 



THE CENTRAL NERVOUS SYSTEM 



ously competing for the same final common path and those which occupy 

 it successively. 



INTEGRATION OF ALLIED REFLEXES 



Perhaps the simplest experiment to show this is performed by using 

 the scratch reflex. The skin area from which this reflex can be elicited 

 is very widespread (see Fig. 217), the type of reflex produced from any 

 given area being 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 then 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- 



Fig. 216. Diagram showing the reflex arcs involved in the scratch reflex. Ra and R/3 represent 

 the afferent neurons connected with hairs on the skin of the back and flank. The afferent im- 

 pulses are transmitted by these fibers, and on entering the corresponding segments of the spinal 

 cord terminate by synapses on cells of the internuncial neurons, whose arrows Pa and P0 travel 

 down in the lateral columns to terminate similarly around the cells of the motor neurons that 

 innervate the muscles of the hind limb. Since afferent impulses coming from elsewhere, par- 

 ticularly from the skin of the leg (R and L), also terminate on these neurons and may excite 

 them to a different type of action, the motor neuron is called the final common path (.F.C.). 

 (From Sherrington.) 



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 and produced, therefore, a 

 simultaneous integration of allied reflexes. 



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 (see page 788). For ex- 

 ample, if a stimulus inadequate to elicit a flexion reflex is applied to the 

 skin of the leg, and another stimulus, itself also inadequate, is ap- 

 plied to the central end of some deep afferent nerve in the same leg, 

 then the two subliminal stimuli will become effective in producing a 



