45o 



A MANUAL OF VETERINARY PHYSIOLOGY 



sets up an intraspinal excitation of the motor neurones of the 

 flexor muscles and an intraspinal inhibition of the motor neurones 

 innervating the extensor muscles. 



When the flexion phase of the act of stepping has been passed 

 through, the leg extends again by return of activity in the motor 

 neurones of the extensor muscles which had been inhibited. In 

 due course the foot reaches the ground. When it does so the 

 weight of the body gradually comes upon it, and soon presses 

 the sole of the foot with its full force against the ground. A 

 stimulus is thus given to nerve-endings in the sole. This 

 stimulus can be imitated — for instance, by pressing against the 

 toe-pads with a finger. This, in the spinal dog, even when the 



Fig. 133. — The Scratch Reflex (Sherrington). 



The, 'receptive field,' as revealed. after low cervical transection, a saddle-shaped 

 area of dorsal skin, whence the scratch reflex of the left hind-limb can be 

 evoked. Ir marks the position of the last rib. 



animal is lying on its side, excites a strong reflex extension of 

 the limb, the ' extensor thrust.' Just such an extension occurs 

 when the foot is pressed against the ground by the weight of 

 the body in the act of stepping. This extensor thrust gives the 

 propulsive movement of the body forward, which is the contri- 

 bution made by the limb in its reflex step toward the progres- 

 sion of the animal. The extensor thrust is particularly marked 

 in the gallop. 



The Scratch Reflex. — Good opportunity for study of this cor- 

 relation between reflexes is given in the ' scratch reflex.' This 

 reflex can be easily elicited in many normal dogs ; when the 

 spinal cord has been transected in the neck, it becomes 

 abnormally prominent. Stimuli applied within a large saddle- 

 shaped field of skin (Fig. 133) excite a scratching move- 



