376 PHYSIOLOGY 



(2) Flexor reflex. Nocuous stimuli, such as the prick of a needle 

 applied to any part of the foot, causes flexion of the leg and thigh, 

 often accompanied by extension of the opposite hind limb. 



(3) Extensor or ' stepping ' reflex. Gentle pressure applied to 

 the plantar surface of the hind foot, especially if the limb is some- 

 what flexed, causes a movement of extension of the limb accom- 

 panied sometimes by a flexion of the opposite hind limb. 



In such an animal the carrying out of the visceral reflexes may 

 be very efficient. The blood pressure has attained its normal height 

 and maybe altered reflexly in very much the same way as in a normal 



aoo mm Hg. 



150 mm. Hg. 



100 mm. Hg- 

 b. p. 



Signal 

 Time in 2' 



FIG. 167. Blood-pressure tracing from a spinal dog. The signal indicates 

 the time during which the afferent nerve was stimulated. (SHERRINGTON.) 



animal, although the medullary vaso-motor centre can no longer 

 be concerned. Thus in the diagram (Fig. 167) is represented the 

 effect on the blood pressure of exciting the central end of the digital 

 nerve in a spinal dog. The pressure rises from 90 to 208 mm. Hg. 

 a pressor effect as great as any which can be obtained in an animal 

 still possessing all the connections of the vascular system with the 

 vaso-motor centre. The height of the rise shows that as regards the 

 influence on the blood pressure the spinal cord must be acting as a 

 whole. No effect on the blood-vessels confined to the segment, or 

 segments, adjacent to that of the nerve stimulated would suffice to 

 cause a rise of more than a few mm. Hg. 



The reflex apparatus for other visceral functions seems to be 

 equally perfect. The urinary bladder, when sufficient urine is accumu- 

 lated, contracts forcibly, the contraction being accompanied by 

 relaxation of the sphincter and followed by rhythmic contractions 

 of the urethral muscles ; accumulation of faeces in the rectum leads 

 to their normal evacuation. With a little assistance impregnation 



