CXV111 



MUSCULAR TISSUE. 



Fig. LXI. 



of the fibre, but it is by no means confined to such parts. This cross- 

 striped appearance, which is most beautiful and characteristic, is found in all 

 the voluntary muscles ; but 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 the will. 



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

 number of extremely fine filaments or fibrils inclosed in a tubular sheath. 

 This, the proper sheath of the fibre, is named sarcolemma or myolemma. 

 It consists of transparent and apparently homogeneous membrane agreeing 



in chemical characters 

 with elastic tissue, and, 

 being comparatively 

 tough, will sometimes 

 remain entire when 

 the included fibrils are 

 ruptured by stretching 

 the fibre, as represented 

 in fig. LXII. In this 

 way its existence may 

 be demonstrated ; and 

 it is especially well 

 seen in fish and other 

 animals which have 

 large fibres, for in 

 ^ese it is thicker and 

 stronger. It may also 

 be well shown in fresh 



5? 



Fig. LXI. A. PORTION OP A MEDIUM-SIZED HUMAN MUS- 

 CULAR FIBRE, MAGNIFIED NEARLY 800 DIAMETERS. 



B, Separated bundles of Fibrils, equally magnified, a, a, 

 larger, and b, b, smaller collections ; c, still smaller ; d, d, 

 the smallest which could be detached, possibly representing 

 a single series of sarcous elements. 



muscular fibres from 

 the frog, by exposing 

 them to water under 

 the microscope. The 

 fluid is imbibed, and 

 then collects between 

 the substance of the 

 fibre and its sheath so 

 as to separate the mem- 

 brane and make it 

 apparent. At the same 

 time, as regards mam- 

 malian muscles, it must 



be admitted that it is not always easy to bring the sarcolemma distinctly 

 into view. 



Fibrils. Lines and fissures are sometimes seen running lengthwise in the 

 substance of the fibres, and indicating their fibrillar structure, as in somo 

 of those represented in fig. LX. ; and when those longitudinal lines are well 

 marked, the transverse striae are comparatively indistinct. In a thin trans- 

 verse section the ends of the fibrils may be seen, when highly magnified, as 

 small dots or points, which occupy the whole sectional area of the fibre, 

 showing plainly that the latter is not hollow, as has sometimes been main- 

 tained, but possesses the same fibrillar structure throughout its whole thick- 

 ness. The fibrils are closely connected together in the fibre by an inter- 



