720 STRUCTURE OF MUSCLE. 



of this tissue as consisting of ' fibres,' yet the ultimate structure of 

 the 'Muscular Fibre' is very different from that of the Simple 

 Fibrous tissues already described. When we examine an ordinary 

 Muscle (or piece of 'flesh') with the naked eye, we observe that 

 it is made-up of a number of fasciculi or bundles of fibres, which 

 are arranged side-by-side with great regularity in the direction in 

 which the muscle is to act, and are united by Areolar tissue. 

 These Fasciculi may be separated into smaller parts, which appear 

 like simple fibres ; but when these are examined by the Micro- 

 scope, they are found to be themselves fasciculi, composed of 

 minuter fibres bound together by delicate filaments of Connective 

 tissue. By carefully separating these, we may obtain the ultimate 

 Muscular Fibre. This fibre exists under two forms, the Striated 

 and the N on- striated. The former is chiefly distinguished by the 

 transversely-striated appearance which it presents (Fig. 387), and 

 which is due to an alternation of light and 

 Fig. 387. dark spaces along its whole extent ; the 



breadth and distance of these stria? vary, 

 however, in different fibres, and even in 

 different parts of the same fibre, according 

 to its state of contraction or relaxation. 

 Longitudinal stria? are also frequently visible, 

 which are due to a partial separation be- 

 tween the component fibrillye into which the 

 fibre may be broken up. — When a fibre of 

 this kind is more closely examined, it is 

 seen to consist of a delicate tubular sheath, 

 quite distinct on the one hand from the 

 Connective tissue which binds the fibres into 

 fasciculi, and equally distinct from the in- 

 ternal substance of the fibre. This mem- 

 branous tube, which has been termed the 

 Sarcolemma, is not perforated either by 

 Fasciculus of Striated Capillary vessels or by Nerves; and forms, 



striae, and at 6 its junc- elements of Muscular structure and the 

 tion with the Tendon, surrounding parts. The diameter of the 

 Fibres varies greatly in different kinds 

 of Vertebrated animals. Its average is greater in Reptiles and 

 Fishes than in Birds and Mammals, and its extremes also are 

 wider; thus its dimensions vary in the Frog from l-100th to 

 l-1000th of an inch, and in the Skate from l-65th to 1 -300th; 

 whilst in the Human subject the average is about 1 -400th of an 

 inch, and the extremes about l-200th and l-600th. 



568. The elements of Muscular Fibre appear to be very minute 

 cylindrical particles with flattened faces of nearly uniform size, 

 adherent to each other both longitudinally and laterally. The 

 former adhesion is usually the most powerful ; and causes the 



