STRUCTURE OF MUSCLE. 57 



STRIATED MUSCLE. 



The striped muscular tissue forms the conspicuous masses known as the 

 "muscles" or "flesh" attached to the bony framework of the body. These 

 organs are the active agents in moving the passive levers, the bones, and in 

 producing the movements of the animal. The structural unit of voluntary 

 muscle is the transversely striated muscle-fibre, which represents a highly 

 specialized cell. The fibres are the contractile elements by whose shortening 

 the length of the entire muscle is decreased and its force exerted. 



The muscle-fibres are cylindrical, or prismatic with rounded angles, 

 in form and vary from .01- i mm. in diameter. No constant relation exists 

 between the thickness of the fibres and the size of the muscle of which they 

 are components, and, indeed, their diameter varies in the same muscle. The 

 length of the muscle-fibres is likewise subject to great variation. As a rule, 

 the fibres are of limited length, not exceeding from 4-5 cm. ; in exceptional 

 cases, as in the sartorius muscle, they may attain a length of over 1 2 cm. 

 The fibres are usually slightly larger in the middle than at the ends, which 

 are more or less pointed, but sometimes blunted or club-shaped or, rarely, 

 branched. Branched and anastomosing fibres occur in the lingual, facial 

 and ocular muscles. 



Each muscle-fibre corresponds to an enormously elongated multinu- 

 cleated cell and consists of a sheath, or sarcolemma, and the contained sar- 

 cous substance. The sarcolemma forms 

 a complete investment of the fibre and 

 alone comes in contact with the sur- 

 rounding connective tissue by which the 

 muscle-fibres are attached to one another 

 or to the fibrous structures upon which 

 they directly exert their pull. The 



sarcolemma is a transparent, homo- Sarcoiemma" 



geneous, elastic membrane and envelops F io. 7 i.-Portion of muscle-fibre, showing 



the Contained SarCOUS Substance SO sarcolemma bridging break in sarcous sub- 

 11, i ,. ,. . stance. X 370. 



closely that, under ordinary conditions, 



it is almost or entirely invisible. Being tougher than the muscle-substance, 

 it often withstands teasing with needles while the muscle is broken ; where 

 such breaks occur, the muscle-substance sometimes contracts within the 

 sarcolemma, which then- becomes visible at the fractures as a delicate tubular 

 sheath (Fig. 71). Sometime^ the sarcolemma may be seen projecting 

 beyond the sarcous substance, as a coat sleeve covers the stump of an arm. 

 The sarcous substance, everywhere enclosed within the sarcolemma, 

 also consists of two parts, the less differentiated and passive sarcoplasm and 

 the highly specialized contractile fibrillce in which take place the active 

 changes resulting in the contraction of the muscle-fibre. The characteristic 

 cross-striation, resolvable into alternating light and dark bands, that dis- 

 tinguishes the fibres of voluntary muscle depends upon the constitution and 

 arrangement of the contractile fibrillae. These are threads of great tenuity, 

 which extend the entire length of the muscle-fibre and present series of alter- 

 nating light and dark segments that probably correspond to differences of 

 density. The dark denser areas are doubly refracting (amsotropic)\ the light 

 less dense ones are singly refracting (isotropic). The cross-striation of the 

 muscle-fibre as a whole results from the definite and orderly arrangement of 

 the fibrillae. Close lateral approximation of the denser and deeply staining 



