STEUCTURE OF THE FIBRES. HI 



thicker and apparently contracted parts of the fibre, but it is by no 

 means confined to such parts. This cross-striped appearance, which is 

 most beautifal and characteristic, is found in all the voluntary muscles : 

 ])ut it is not altogether confined to them, for it is seen in the fibres of 

 the heart, which is a strictly involuntary organ : striped fibres are also 

 found in the pharynx and upper part of the gullet, in the muscles of 

 the internal ear, and those of the urethra, parts which are not under 

 the direct control of tlic will. 



Structure of the fibres. — A muscular fibre may be said to consist 

 of a soft contractile substance inclosed in a tubular sheath. 



This, the proper sheath of the fibre, is named sarcoJemma or myo- 

 lemma. It consists of transparent and apparently homogeneous mem- 

 brane agreeing in chemical characters with elastic tissue, and, beiug 

 comparatively tough, will sometimes remain entire when the included 

 fibrils are ruptured by stretching the fibre, as represented in fig. 67. 

 In this way its existence may be 

 demonstrated; and it is especially ^ig. 67. 



well seen in fish and other animals 

 which have large fibres, for in these 

 it is thicker and strouger. It may 

 also be well shown in fresh muscular 

 filjres from the frog, by exposing 

 them to water under the micro- 

 scope. The fluid is imbibed, and 

 then collects between the substance ^% 67. -Muscular Fibre of Fish. 



^ ,, £., T •. 1 .1 X teUBSTAXCE OP J^IBRE RUPTURED SO 



of the fibre and its sheath so as to ^^ ^^ ^^^,^^^ Sarcolemma. (After 



separate the membrane and make Bowman.) 



it apparent. At the same time, as 



regards mammalian muscles, it must be admitted that it is not always 



easy to bring the sarcolemma distinctly into view. 



The proper substance of the fibre presents, besides the cross-striped 

 appearance already mentioned, also an appearance of longitudinal 

 striation, which is the better marked where the transverse striation is 

 less distinct. On separating the fibre with needles, especially after 

 hardening in alcohol or solution of osm.ic acid, it may be broken up 

 longitudinally into the so-called ^^//vVs, which, when of a certain fineness, 

 appear to consist of a row of dark quadrangular particles (fig. 6G B, i), 

 with bright intervals, the latter being commonly traversed by a dark 

 dotted line, c. These rows of quadrangular particles may, however, be 

 further separated, and the finest of the filaments so obtainable present 

 the appearance of lines regularly broken at short distances with a dot in 

 each of the breaks, d. Each such thread may be looked upon as an ulti- 

 mate fibril. It must, however, be borne in mind that the fibre is not 

 actually co)nposed of these, but that there is in addition a not inconsider- 

 able amount of connecting substance. Under certain circumstances the 

 fibres show a tendency to cleave across in a direction parallel to the 

 stripes, and even to break up into transverse plates or disks, which are 

 formed by the lateral cohesion of the ])articles of adjacent fibrils. To 

 make up such a disk, therefore, every fibril contributes a particle, which 

 separates from those of its own fibril, but coheres with its neighbour 

 on each side, and this with perfect regularity. Indeed, Bowman con- 

 ceives that the subdivision of a fibre into fibrillar is merely a pheno- 

 menon of the same kind, only of more common occurrence, the cleavage 



