PHYSIOLOGY OF MUSCLE TISSUE. 55 



associated with the activity of the nervous system, it is nevertheless, 

 an independent endowment, and persists after all nervous connections 

 are destroyed. If the nerve terminals be destroyed, as they can be 

 by the introduction of curara into the system, the muscles become 

 completely relaxed and quiescent. The strongest stimuli applied to the 

 nerves fail to produce a contraction. Various external stimuli applied 

 directly to the muscle substance produce at once the characteristic 

 contraction. The excitability of muscle is therefore an inherent 

 property, dependent on its nutrition, and persisting as long as it is 

 supplied with proper nutritive materials and surrounded by those 

 external conditions which maintain its chemic or physical integrity. 



Muscle Contractions. All muscle contractions occurring in the 

 body under normal physiologic conditions are either voluntary, caused 

 by a volitional effort and the transmission of a nerve impulse from 

 the brain through the spinal cord and nerves to the. muscles, or reftex, 

 caused by a peripheral stimulation and the transmission of a nerve 

 impulse to the spinal cord, to be reflected outward through the same 

 nerves to the muscles. In either case the resulting contraction is 

 essentially the same. The normal or physiologic stimulus which 

 provokes the muscular contraction is a nerve impulse the nature of 

 which is unknown, but is perhaps allied to a molecular disturbance. 

 After removal from the body, muscles remain in a state of rest, 

 inasmuch as they possess no spontaneity of action. Though consist- 

 ing of a highly irritable tissue, they can not pass from the passive 

 to the active state except upon the application of some form ojE stimu- 

 lation. 



The stimuli which are capable of calling forth a contraction may 

 be divided into 



1. Mechanical. 



2. Chemic. 



3. Physical. 



4. Electric. 



Every mechanical stimulus of a muscle, e. g., pick, cut, or tap/ 

 provided it has sufficient intensity, and is repeated with sufficient 

 rapidity, will cause not only a single contraction, but a series of 

 contractions. 



All chemic agents which impair the chemic composition of the 

 muscle with sufficient rapidity -e. g., hydrochloric acid, acetic and 

 oxalic acids, distilled water injected into the vessels, etc. act as 



