76 



repeated at very short intervals the muscle becomes 

 at length rigid in the contracted condition ; it is 

 said to be in a state of tetanus (Fig. 33). Gradually, 

 however, the muscle becomes less and less contracted 

 as fatigue sets in, until, finally, it is unable to raise 

 the weight at all, and in this condition it remains for 

 some time. If the stimulus be reapplied after a 

 short period of rest, the muscle is again able to raise 

 the weight, but not so far as it did at first. 



The nature of the stimulus applied may be of the 

 most varied character ; it may be a chemical reagent, 

 an electric shock, or merely a tap from a pencil. 



It is not difficult to see 

 "~"*\ that the chief character- 

 V_ istic of the animal as con- 



r-r iiniiiiinimiiiuiir trasted with the plant in 



****** relation to sensitivity is 



FIG. 33. Tracing of imperfect wnat ma y be termed the 

 tetanus in muscle. centralisation of adminis- 



tration. The plant has, as 



we have seen, diffused sensitiveness to certain stimuli, 

 but in the animal, not only is the perception of 

 many of these stimuli localised, but one or more 

 centres are developed to which these stimuli are 

 transmitted ; there they are analysed before a 

 reaction takes place, which reaction is caused in 

 turn by a stimulus generated in the centre and 

 transmitted to the region of response. In the 

 simplest condition the same cell that receives the 

 stimulus also brings about the response, but in most 

 multicellular organisms the element that receives 

 the stimulus and ths element that reacts are distinct, 

 but put in communication with each other by means 

 of a central element, so that the motive impulse 

 to contract or secrete as it may be, is transmitted 



