332 



PHYSIOLOGY 



THE SIMPLE REFLEX 



structures at the front of the thighs. In such animals three reflexes, 



amongst others, can be excited almost invariably, viz. : 



(1) Scratch reflex. Gentle stimulation, mechanical or electrical, of any 



point over a saddle-shaped area on the dorsum behind the shoulders (Fig. 



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 re- 

 peated 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 accom- 

 panied by extension of 

 the opposite hind limb. 



(3) Extensor or ' step- 

 ping' reflex. Gentle pres- 

 sure applied to the 

 plantar surface of the 

 hind foot, especially if the 

 limb is somewhat flexed, 

 causes a movement of ex- 



B 



L, afferent path 



FlG ' fl 165 V .A ? h u- ^P* fi t w w ! ience the scratch tension of the limb accom- 



renex of the left hind limb can be evoked. 



pamed sometimes by a 

 of the opposite 



receptive paths from hairs on 'scratch area' ; re, final i/ rl V -u 

 common path (motor neuron) ; *>- ~ a -~ i ~" 

 rons. (SHERRINGTON.) 



B. Diagram of spinal arcs involved. 



from left foot : B, afferent path from right foot ; Ka, B/3, flexion 



' scratch area ' ; re, final 

 Pa, P/3, proprio- spinal neu- 



In such an animal the 

 carrying out of the vis- 

 ceral reflexes may be very efficient. The blood pressure has attained its 

 normal height and may be altered reflexly in very much the same way as 

 in a normal animal, although the medullary vaso-motor centre can no 

 longer be concerned. Thus in the diagram (Fig. 166) 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 accumulated, con- 



