18 THE PHYSIOLOGY OF MUCLES AND NERVE. 



THE PHYSIOLOGY OF SKELETAL MUSCLE TISSUE. 



This tissue makes up the essential part of the skeletal muscles by 

 means of which our voluntary movements are effected. Each muscle 

 fiber arises from a single cell and in its fully developed condition 

 may be regarded as a multinuclear giant cell. It is inclosed entirely 

 in a thin, structureless, elastic membrane, the sarcolemma. The 

 material of the fiber is supposed to be semifluid or viscous when 

 in the living condition; it is designated in general 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 



Ms. 



Fig. 1. A cross-section of muscle Fig. 2. Cross-section of two muscle 



fiber of rabbit. The bundles of fibrils are fibers of the fly : Ma, The columns of 



dark; the intervening small amount of fibrils; Sp, the sarcoplasm. (Schieffer- 



sarcoplasm is represented by the clear decker.) 

 spaces. (Kolliker.) 



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 

 this 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 solid structure. 



Disregarding the nuclei, the muscle plasma consists of two 

 different structures: the fibrils, which are long and thread-like and 

 run the length of the fiber, and the intervening sarcoplasm. The 

 fibrils consist of alternating dim and light discs or segments, which, 

 coming together in the different fibrils, give the cross-striation 

 that is characteristic. In mammalian muscles the fibrils are grouped 

 more or less distinctly into bundles or columns, between which lies 

 the scanty sarcoplasm. The relative amount of sarcoplasm to 

 fibrillar substance varies greatly in the striped muscles of different 

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



