THE MECHANISM OF CO-ORDINATED MOVEMENTS 339 



segmental, the most important determining factor being the adaptation of 

 the movement to the stimulus which is applied. 



The gradual spread of the response with increasing strength of stimulus 

 is spoken of as ' irradiation' The nature of the response is determined by 

 the locus or place of application of the stimulus and by the quality of the 

 latter. While a painful stimulus causes flexion of the leg, deep pressure on 

 the plantar surface of the paw causes extension the ' stepping ' reflex. 

 However extensive the irradiation, the muscles which are set into action are 

 always such that their actions co-operate towards a given end. Thus, 

 when the impulse spreads to the opposite limb and produces an extension, 

 the reaction is such as would ensue when the dog steps on a sharp point and 

 immediately retracts the irritated limb away from the injurious agent while 

 it extends the other limb in the first act of progression or movement away 

 from the dangerous spot. 



A superficial study of this reflex would therefore lead us to the conclu- 

 sion that, by the varying resistance in the different synapses on the course 

 of the connections of the stimulated afferent nerves, the impulses are directed 

 so as to affect solely and exclusively the muscles whose activity will co- 

 operate and aid the primary reflex. Such a description would however re- 

 present only one half of the process. Every muscle in the body is in a state of 

 tone varying with its extension. If this tone is not to interfere with the 

 carrying out of a reflex movement, there must be some means by which it 

 can be inhibited. Such an inhibition we have seen occur as the result of con- 

 traction of antagonistic muscles ; but the remarkable fact has been brought 

 out by Sherrington that the impulses, which start on the surface of the 

 body and set loose a chain of motor impulses resulting in the co-ordinated 

 contraction of certain muscles, spread at the same tune to the motor 

 mechanisms governing the muscles antagonistic to the movement, and exer- 

 cise on these an inhibitory effect. 



This inhibition can be easily shown in a spinal animal in the following 

 way. The anterior thigh muscles are cut away from their attachments to 

 the tibia and the patellar tendon is connected by a thread with a recording 

 lever. On then exciting the flexor reflex by nocuous stimulation of the foot, 

 the lever attached to the patellar tendon falls (Fig. 169B), showing that the 

 extensor muscles have undergone actual elongation. The same effect is 

 observed even when the hamstrings, the flexors of the knee, have been divided. 

 The inhibition of the extensor tone is thus not only determined by the in- 

 creased tension of the flexors, but is a direct result of the primary cutaneous 

 stimulus. 



From a broad standpoint the function of the nervous system is the 

 co-ordination of all the activities of the body so that these may be com- 

 bined to one common end, viz. the preservation of the organism. For 

 this purpose there must be no clashing between opposing activities of 

 different parts. If one part is engaged in any action this action must be 

 the policy of the body as a whole. Yet the surface of the body is being 

 continually played upon by ever-changing stimuli, tending to excite first 



