94 



VETEKINAEY PHYSIOLOGY 



kinds from the same area may induce one or other reflex. 

 Thus, while pressing the finger in the pad of the dog's foot 

 causes the extensor thrust, injurious stimuli cause a flexion-with- 

 drawal of the foot. When the two stimuli are applied at once, 

 one or other, but not a combination of the reflex actions, will 



manifest itself. In the per- 

 formance of each of these 

 reflexes the same outgoing 

 nerve paths are employed, 

 and in the contest between 

 the two stimuli one of the 

 stimuli overcomes and re- 

 places the other. In this 

 way ineo-ordinate m o v e- 

 ments by mixtures of re- 

 flexes are prevented. 



The ingoing fibres are pro- 

 bably five times as numerous 

 as the outgoing fibres, and 

 each one of them is con- 

 FIG. 42. To show the way in which the nected, or may become func- 



diirereut reflex arcs react on one another, tionally Connected, With 

 S, skin ; M, muscle ; V, yiscus ; C, spinal m differen t combinations 

 cord with synapses. (M'DouGALL.) 



of outgoing fibres. Hence 



it is difficult to get a reflex which does not interfere with 

 others, either by increasing or antagonising them. The out- 

 going paths are merely passive channels in the hands of certain 

 ingoing reflex paths. In illustration of this, Sherrington cites 

 the outgoing motor neuron to the vasto-crureus of the dog, and 

 in fig. 41 shows how this is the common path to different re- 

 flexes induced by different stimulation of different parts of the 

 body. Some stimuli excite its activity and may be grouped 

 together as " allied " ; others inhibit its activity and thus act as 

 " antagonistic " to the former group. 



Spread of Reflexes. While with gentle stimuli many re- 

 flexes tend to manifest themselves in the outgoing neurons of 

 the same region of the cord, other reflexes, such as the scratch 

 reflex of the dog, always involve neurons in widely different 

 regions. But even in reflexes which are localised when gentle 



