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 application 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 effec- 

 tive stimulus causes simply movement of the foot. As its strength is in- 

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

 the immediate proximity of the entering impulses, but even when minimal, 

 involves the simultaneous action of more than one anterior root. We may 

 say that the motor response is determined to a certain extent by the spatial 

 proximity of the afferent to the efferent tracts, but that it is always pluri- 



338 



