GENERAL PHYSIOLOGY OF MUSCULAR TISSUE. 49 



nutritive materials and surrounded by those external conditions which main- 

 tain 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 reflex, 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 musuclar 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 consisting of a highly irritable 

 tissue, they cannot pass from the passive to the active state except upon the 

 application of some form of stimulation. 



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 stimuli, and produce single and 

 multiple contractions. Physical agents, as heat and electricity, also act as 

 stimuli. A muscle heated rapidly to 30 C. contracts vigorously, and 

 reaches its maximum at 45 C. Of all forms of stimuli, the electric is the 

 most generally used. Two forms are used the induced current and the 

 make-and-break of a constant current. 



Changes in a Muscle During Contraction. When a muscle is stimu- 

 lated, either indirectly through the nerve or directly by any external agent, it 

 undergoes a series of changes, which relate to its form, volume, optic, physical 

 chemic, and electric properties. These changes, in their totality, constitute 

 the muscular contraction. 



i. Form. The most obvious change is that of form. The fibers become 

 shorter in their longitudinal and wider in their transverse diameters, and 

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