THE SPINAL CORD AS A REFLEX CENTRE 



375 



insufficient to maintain the animal in such a position that a line 

 drawn vertically from its centre of gravity shall fall between its 

 points of support. On the other hand, swimming movements may 

 be carried out regularly. The frog deprived of its brain can swim 

 like a normal animal, but in consequence of the depression of its head 

 tends to swim ever deeper in the water. If a ' spinal ' dog be held 

 up by the fore limbs, the hind limbs nearly always enter into alternat- 

 ing movements of flexion and extension (' mark time ' movements), 

 the two limbs acting alternately as in normal progression. The 

 stimuli in this case seem to be started by the stretching of the skin 



THE SIMPLE REFLEX 



B 



FIG. 166. A. The receptive field, whence the scratch reflex of the left 

 hind limb can be evoked. 



B. Diagram of spinal arcs involved. L, afferent path from left foot ; 

 R, afferent path from right foot ; Ra, R/3, receptive paths from hairs on 

 ' scratch area ' ; FC, final common path (motor neuron) ; Pa, p/3, proprio- 

 spinal neurons. (SHERRINGTON.) 



and other 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. 166) 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. 



