710 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



presence of nerve-filaments, as in the extremes of the sartorius muscle of 

 the frog, and yet in them stimulation applied directly to a muscle pro- 

 duces contraction. The most conclusive evidence, however, of the inde- 

 pendent irritability of muscles is found through the use of the poison 

 curare. 



This substance, a South American arrow-poison, possesses the prop- 

 erty of entirely paralyzing the terminal filaments of the motor nerves. 

 If an animal, such as the frog, be poisoned with this drug, stimuli ap- 

 plied, to the motor nerves will be entirely incapable of producing muscular 

 contraction. The same stimulus, however, applied directly to the muscle 

 still produces a characteristic normal contraction. This poison acts, not 

 on the nerve-trunks, but on the intra-muscular terminations of the nerves. 

 This fact may be demonstrated by ligating the sciatic artery in one hind 

 leg of the frog and injecting curare into the dorsal lymph-sac. The 

 poisoned blood will then, of course, circulate in every part of the body 

 with the exception of the limb in which the circulation has been arrested. 

 If a stimulus be then applied to the sciatic nerve of the non-poisoned 

 limb it will still succeed in calling forth a contraction, even although the 

 sciatic trunk has been exposed to the action of curare. Stimulation of 

 the sciatic, on the other hand, in the limb in which the circulation has 

 been maintained, and in which, of course, the poison has had access to 

 the nerve-filanients produces no contraction, while local stimulation of 

 the muscle does. 



Muscular contraction may be produced by various stimuli, acting 

 either indirectly upon the muscle through its motor nerve, or directly 

 by being immediately applied to the muscle substance. 



Muscular stimuli may be either chemical, thermal, mechanical, or 

 electrical. All chemical substances, such as acids and various metallic 

 salts, which alter the composition of muscle are muscular stimuli. 

 Variations of temperature also produce muscular- contraction. Thus, if 

 an excised frog's muscle be heated rapidly to about 28° C, contraction 

 commences and reaches its maximum at 45° C. If the temperature be 

 raised above this point the muscle passes into a condition of heat-rigor, 

 due to .the coagulation of the proteids of muscle. Sudden mechanical 

 stimuli, whether applied directly to the muscle or indirectly to the nerve, 

 if repeated with sufficient rapidity, also produce contraction of muscle. 

 Strong local irritation, as by a blow, produces a long-continued, weal- 

 like contraction of the part stimulated. 



(c) The Phenomena of Muscular Gorttraction. — Muscular contraction 

 consists in the shortening of muscle-fibres in the direction of their long 

 axes, with a proportionate and simultaneous increase in their transverse 

 diameter. Such a contraction is accompanied by a number of phenomena, 

 of which the most evident is the change in form. 



