332 



PHYSIOLOGY 



THE SIMPLE REFLEX 



165) causes rhythmic movements of flexion and extension of the hind limb of 

 the same side, the effect of which would be to scratch away the irritant 

 object. These movements are repeated at the rate of about four per second. 

 (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 op- 

 posite hind limb. 



(3) Extensor or ' step- 

 ping ' reflex. Gentle pres- 

 sure applied to the plan- 

 tar surface of the hind 

 foot, especially if the 

 limb is somewhat flexed, 

 causes a movement of 

 extension of the limb, 

 accompanied sometimes 

 by a flexion of the oppo- 

 site hind limb. 



In such an animal the 

 carrying out of the vis- 

 ceral reflexes may be very 

 efficient. The blood pres- 

 sure has attained its 

 normal height and may 

 be altered reflexly in very 

 much the same way as in 



B 



FIG. 166. A. The receptive field, whence the scratch a normal animal, although 

 reflex of the left hind limb can be evoked. ^ medul) vaso -motor 



B. Diagram of spinal arcs involved. L, afferent path T , 



from left foot ; B, afferent path from right foot ; Ra, R/3, centre can no longer 

 receptive paths from hairs on * scratch area ' ; FC, final 

 common path (motor neuron) ; Pa, p/3, proprio-spinal neu- 

 rons. (SHERRINGTON.) 



centre can no longer 

 concerned. Thus in the 

 diagram (Fig. 166) is re- 

 presented 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 con- 

 nections 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 accumulated, con- 

 tracts 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 may be effected in or by such a maimed 



