18 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



voluntary movements are effected. What we call a muscle is an 

 organ composed of many thousands of muscle fibers, bound to- 

 gether by connective tissue, and surrounded by a sheath of the 

 same tissue. Each fiber is a minute cyhndrical or prismatic 

 thread, whose diameter varies perhaps between 0.1 and 0.01 mm., 

 and whose length does not exceed 36 mm. It constitutes the unit 

 of structure of the muscle, and the shortening of the muscle, as a 

 whole, is the expression of the combined effect of the contractions 

 of the constituent fibers. In considering the physiology of the 

 muscle as a contracting mechanism it is convenient to discuss the 

 phenomenon as it occurs in a single fiber. As a matter of fact, 

 each fiber is a complex structure, consisting of several distinct 

 parts. It is developed from a single cell, and since it contains a 

 number of nuclei, we may regard it histologically as a sort of a 

 multinuclear giant cell of elongated form. Each fiber is enclosed 

 within a thin, structureless, elastic membrane, the sarcolemma. 

 The material of the fiber inside the sarcolemmal bag shows the 

 characteristic cross-striation, and is supposed to be of a semi- 

 liquid or viscous consistency when living. This material, as a 

 whole, is designated as the muscle plasma. 



There is on record an interesting observation by Kiihne* which seems 

 to demonstrate the fluid nature of the living muscle substance. He hap- 

 pened, on one occasion, to find a frog's muscle fiber containing a nematode 

 worm within the sarcolemma. The animal swam readily from one end of 

 the fiber to the other, pushing aside the cross bands, which fell into place 



Fig. 1. — A cross-section of muscle 

 fiber of rabbit. The bundles of fibrils are 

 dark; the intervening small amount of 

 sarcoplasm is represented by the clear 

 Bpa,ces.—(.K6lliker.) 



Fig. 2. — Cross-section of two muscle 

 fibers of the fly: Ms, The columns of 

 fibrils; Sp, the sarcoplasm. — (Schieffer- 

 decker.) 



again after the animal had passed. At one end, where the fiber had been 

 injured, the worm was unable to force its way. The muscle substance at 

 tlus point was dead and apparently had passed into a solid condition. The 

 fact that the cross bands were displaced only temporarily by the movement 

 and fell back into their normal position would indicate that they may have 

 a more soUd structure. 



* Kiihne, "Archiv fur pathologische Anatomic," 26, 222, 1863. 



