142 SPECIAL PHYSIOLOGY. 



it be mechanical, chemical, or electrical, influence the result. The 

 electrical current used to excite the motor nerve, may be derived from 

 the contraction of other muscles, as will hereafter be explained in de- 

 scribing the so-called rheoscopic frog's limb. 



Cause of Muscular Contractility. 



Various theories have been advanced as to the nature and cause of 

 muscular contraction, but all may be dismissed as being unsatisfactory, 

 and at present our real knowledge may be said to be limited to the 

 phenomenon itself, and to certain of its conditions and accompani- 

 ments. Two very opposite general views have been, and are still, en- 

 tertained by physiologists on this subject. According to one view, the 

 muscular tissue owes its contractility to the nerves distributed to it, a 

 certain force generated in the nerves being transferred, as it were, to 

 the muscles, and so imparting to them their special irritability or con- 

 tractile property. We have seen, indeed, that the muscular contrac- 

 tility may be excited through a nerve. In the living body, this is 

 the ordinary mode of stimulation; and when the motor nerve of a 

 muscle is divided, neither the will nor the so-called reflex nervous 

 stimulus, both of which require that the nerves should be in connection 

 with nerve centres, can any longer excite contractions in it. By irri- 

 tating the portion of nerve connected with the muscle, movements 

 can be excited, however, for a short time ; but after a period, varying 

 from four to eight days, this can no longer be accomplished, although 

 the muscle may still be excited to contract by the direct stimulation 

 of its fibres a condition which may continue, though in a much less 

 degree, for more than two months, but which at last is completely 

 lost. This fact has been held to show that the contractility of muscle 

 is not inherent, but is dependent upon, or derived from, the nerves 

 still contained in it ; for the separated muscle, though continuously 

 nourished, does not retain its contractility for a lengthened period. 

 But a muscle, so separated from the nervous centres, is not perma- 

 nently well nourished ; it becomes atrophied or wasted, its fibres lose 

 their transverse striae, and undergo a fatty degeneration, so that both 

 its structure and properties are destroyed. Moreover, its power of 

 being directly stimulated, lasts longer than that of being indirectly 

 excited through the divided nerve, and the frequent continuous appli- 

 cation of galvanism to such detached muscles, will prevent their atro- 

 phy, and at the same time preserve their contractility. 



These, and many other, considerations serve, therefore, rather to 

 support the second and opposite view, celebrated from having been 

 that adopted by Haller, and now very generally entertained, viz., that 

 the muscular irritability is a special property of the muscular tissue 

 itself, and inherent in it, a vis musculosa, or vis insita. The following 

 numerous facts and considerations are usually quoted, as supporting 

 this important doctrine in physiology. Nerves possess no contractil- 

 ity, but muscular tissue always does. Very small and isolated por- 

 tions of single muscular fibres are seen to contract under the micro- 

 scope. A contractile tissue is found in minute unicellular animal 



