SECTION VIII 



THE MECHANISM OF CO-ORDINATED 

 MOVEMENTS 



THE detailed study of the chief reflexes obtainable from a spinal 

 animal has, in Sherrington's hands, yielded much information as 

 to the linking of the various events which are concerned in the carrying 

 out of every co-ordinated movement, and as to the conditions 

 which determine the sequence and extent of the activities involved. 



We may take, as the type of such a reflex, the flexion of the leg 

 and thigh which ensues on the application of a painful stimulus 

 to the ball of the foot, such as pricking with a needle, or the applica- 

 tion of the faradic current. Of course, in the spinal animal no pain 

 can result from stimulation of any part below the level of section of 

 the cord, and it is better therefore under such circumstances to 

 speak of nocuous or pathic stimuli, since all stimuli which cause 

 pain are such that, if their operation continued, they would result 

 in damage to the material structure of the animal. This flexor 

 reflex is also easily obtainable in the frog as a result of stimulating 

 one of its toes by mechanical or chemical stimuli, but it is easier 

 to analyse the different events involved in the reaction in the case 

 of the larger animal. 



The effect varies with the strength of the stimulus. The minimal 

 effective stimulus causes simply movement of the foot. As its 

 strength is increased this movement is attended by flexion of the leg on 

 the thigh, and finally by flexion of the thigh on the body. With 

 still further increase there is a spread to the opposite hind limb, 

 which, however, performs the opposite movement of extension. 

 Increase in the strength of stimulus causes not only an increase in 

 the strength of contraction of the reacting muscles but also an 

 extension of the reaction to more and more muscles, or groups 

 of muscles. The spread occurs always in definite order. The 

 stimulus when represented by the prick of a needle can affect only 

 one or two nerve fibres. The impulse carried along these fibres 

 through a posterior root to the cord spreads in the cord, affects the 

 motor neurons of the anterior horns, and causes these to dis- 

 charge. The first discharge is as a rule limited to those in the 



immediate proximity of the entering impulses, but even when 



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