COORDINATION. 619 



the cells of the cord employed in transmitting both sensory 

 and motor impulses. Not only is the movement a regular and 

 coordinated act, but in many cases, as has just been mentioned, 

 it is performed with a definite purpose, as if it were the result 

 of thought, but since there need be no consciousness it cannot 

 be mental. All these points may be easily studied on a frog 

 killed about an hour beforehand by decapitation. If a fragment 

 of blotting-paper, moistened in weak acid, be placed on the skin 

 of the body of such an animal, in a position not easily reached 

 by the foot, a most complex series of movements follows, first 

 with one leg, then the other, or with both. The muscular action 

 is both elaborately coordinated and purposeful ; indeed, the move- 

 ments of the headless animal might be called ingenious, and 

 usually result in the removal of the offending paper. 



If the degree of the stimulation be carefully regulated, it will 

 be found that the results obtained by peripheral stimulation 

 depend on (a) the strength of the stimulus, and the length of 

 time for which it is applied ; (6) the degree of excitability of the 

 cells of the cord; (c) the readiness with which the impulses pass 

 along the thin, conducting channels to the gray matter, and (d) 

 the functional activity of the muscles which act as the indicators 

 of the reflex effects. 



By graduating the strength of the solution of acid with which 

 a square millimetre of blotting-paper is saturated before it is 

 placed on a frog's foot, the following results are obtained: When 

 very weak acid is employed, only slight local and unilateral 

 movement is caused. If steeped in stronger acid, the same sized 

 paper produces a series of reflex movements, spreading to several 

 muscles on both sides of the body. If the stimulus be further 

 strengthened, the movements become violent and more extended 

 until the whole body is tossed about by coordinated motions. 

 The movements seem to spread from the local nerve cells to their 

 neighbors, and then to reach those governing the corresponding 

 muscles of the other side, in which, however, they are always less 

 marked than in those of the side stimulated. This spreading of 

 movement from one set of muscles to another, as the strength of 

 the stimulus is increased, of course, must be preceded by a spread- 



