group of muscles; at the same time, the antagonistic muscle or group 

 of muscles is relaxed. Thus, when the flexors contract and draw 

 up the leg, the extensors are relaxed (-Fig. 407); and when the ex- 

 tensors extend the leg, the flexors are relaxed. Each movement 

 is brought about by co-ordinate contraction of one group of muscles 

 .and relaxation of the antagonists. On this depends the perfect 

 balance of the movements of a technician or musician. 



In the body, it is difficult to get a reflex which does not either 

 .antagonize or reinforce other reflexes. This is well illustrated by 

 Fig. 409. Here the final motor neuron is that to the vasto-crureus of 

 the dog. It is excited by stimulation of the ear, fore-foot, tail, and 

 pressure on the pad of the foot of the same side, and by stimulation 

 of the shoulder and nocuous stimuli to the hind-foot of the opposite 



FIG. 405. 



To show that as the strength of acid is increased the reflex is more quickly performed. 

 The beakers contain respectively, from left to right, 0-1, 0-2, 0-3, 0-4, 0-5, and 

 1 per cent, sulphuric acid. 



side. It is inhibited by stimulation of the shoulder of the same side 

 (the scratch reflex), and by nocuous stimuli of the hind-foot of the 

 same side. 



Not only do sensory stimuli (the extero-ceptive mechanism) react 

 pon the reflexes of the body, but the impulses which arise from the 

 muscles themselves and from the joints and tendons (the proprio- 

 oeptive mechanism), and from the viscera (the entero-ceptive mechan- 

 ism), also play a part in determining the effector nature of reflexes. 



The chief points in connection with Spinal reflex action may be 

 summarized as follows : 



1. Reflexes are localized, definite, and purposive. 



2. Owing to the interposition of synapses in the course of the 

 reflex arcs, there is marked delay in the rate of conduction therein, 

 as compared with the rate of conduction in nerve. The synapses also 



