SENSITIVITY IN PLANTS AND ANIMALS 75 



of sensitivity in animals by studying what is termed 

 in physiology a muscle-nerve preparation (Fig. 31). 

 It is well known that the tissues of the lower animals 

 retain their vitality for some considerable time after 

 death, and thus permit of the performance on them 

 of certain simple physiological experiments which 

 cannot conveniently be carried out on the living 

 animal. One of the muscles of the hind leg of a frog 

 is dissected off a recently killed animal and the nerve 

 supplying it is also carefully exposed. If one end 

 of the sinew attached to a muscle be now fixed in 

 a rigid clamp, and the other free end be attached to 

 a weight, we are able, by applying a stimulus to the 

 nerve, to cause contraction in 

 the muscle, thereby raising the 

 weight. Moreover, if we attach 

 to the weight a pointer, placed 

 in such a way as to write on 

 smoked paper covering a re- 

 volving drum, we are able to Fl : 32 - Tra <>ing of a 

 i , . c j f ,-, simple muscular con- 



obtam a record of the amount traction, 

 of the muscular contraction 



in relation to the nature or intensity of the stimulus 

 applied to the nerve. Let us suppose the nerve to 

 be stimulated by an electric shock, we obtain a pro- 

 nounced contraction on the part of the muscle, but 

 the beginning of the contraction and the moment of 

 application of the stimulus are not synchronous ; 

 a longer or shorter period elapses between the applica- 

 tion of the stimulus and the response (Fig. 32). This 

 period is known as the latent period, and during it. 

 various chemical and molecular rearrangements are 

 no doubt taking place, both in the nerve, in carrying 

 the message along, and in the muscle fibres, pre- 

 liminary to their contraction. If the stimulus be 



