THE SPECIAL NEKVOUS MECHANISMS 



119 



in pairs, some of them being the cells of axons that go to the 

 flexor muscles, while others are the cells of axons that go to the 

 extensors. Now the same impulse breaks upon both these series 

 of axons, but it causes, at the same time, the flexor to contract 

 and the (antagonistic) extensor to relax, thus bending the limb. 



But the muscle fibres contain receptors which are stimulated 

 by the acts of <^hl 



contraction and [^^eAfor ^ ^^fj ^^ A 

 relaxation, and 

 thus set up 

 impulses that 

 are propagated 

 along the affer- 

 ent nerve fibres 

 coming from the 

 muscle. These 

 afferent fibres 

 pass into the 

 grey matter of 

 the cord, and 

 are received by 

 nerve cells there. 

 From the latter 

 cells axons pass 

 off and enter in- 

 to synapses with 

 the motor cells 

 which control 

 the movements 

 of the muscles. 



Thus the mus- 

 cular apparatus 

 responds to a 

 stimulus which 

 enters the central nervous system at a place removed some 

 distance from that place from which the motor nerves emerge, 

 so that there are internal paths of communication in the cord. 

 Also the contracting and relaxing muscles " advise " the motor 

 centres {via their own receptors and afferent nerves) what they 

 are doing, so to speak, so that the entire series of actions may 

 be adjusted, or controlled, or accelerated, or inhibited, while 

 they are still in progress. 



tensormuscle 

 xor muscle 



Fig. 34. — The Main Mechanisms involved in the 

 Scratching Reflex of the Dog. 



An afferent fibre enters the cord, and is connected by 

 various cells, their synapses and axons with two 

 motor cells in a lower segment. The latter send 

 out axons to the two antagonistic muscles, and 

 the muscles send up afferent fibres which come 

 into connection, via synapses, with the motor cells. 



