16 HAND-BOOK OF PHYSIOLOGY. 



tary nor involuntary, which form part of the walls of the pharynx, and 

 exist in many other parts of the body, as the internal ear, urethra, etc. 



Structure. All these muscles are composed of larger or smaller 

 bundles of muscular fibres called, fasciculi, enclosed in coverings of fibro- 

 cellular tissue (perimysium), by which each is at once connected with 

 and isolated from those adjacent to it (Fig. 265). Supporting the fibres 

 contained in each fasciculus is a scanty amount of fine connective tissue 

 endomysium. 



Each muscular fibre is thus constructed: Externally is a fine, trans- 

 parent, structureless membrane, called the sarcolemma (Eig. 266, A), 

 which in the form of a tubular investing sheath forms the outer wall of 



FIG. 265. FIG. 266. 



FIG. 265. A small portion of muscle natural size, consisting of larger and smaller fasciculi, ^en 

 in a transverse section, and the same magnified 5 diameters. (Sharpey.) 



FIG. 266. Part of a striped muscle-fibre of a water-beetle (hydrophilus) prepared with absolute 

 alcohol. A, sarcolemma; B, Krause's membrane. Owing to contraction during hardening, the sar- 

 colemma shows regular bulgings. Above and below Krause's membrane are seen the transparent 

 "lateral discs." The chief mass of a muscular compartment is occupied by the contractile disc com- 

 posed of sarcous elements. The substance of the individual sarcous elements has collected more at 

 the extremity than in the centre: hence this latter is more transparent. The optical effect of this is 

 that the contractile disc appears to possess a ''median disc" (Disc of Hensen). Several nuclei of 

 muscle corpuscles, C and D, are shown, and in them a minute netAvork. X 300. (Klein and Noble 

 Smith.) 



the fibre, and is filled up by the contractile material of which the fibre is 

 chiefly composed. Sometimes, from its comparative toughness, the sarco- 

 lemma will remain untorn, when by extension the contained part can be 

 broken (Eig. 269), and its presence is in this way best demonstrated. 

 The fibres, which are cylindriform or prismatic, with an average diameter 

 of about -g fa of an inch, are of a pale yellow color, and apparently marked 

 b} r fine striae, which pass transversely round them, in slightly curved or 

 wholly parallel lines. Each fibre is found to consist of broad dim bands 

 of highly refractive substance representing the contractile portion of the 

 muscle fibre the contractile discs (Fig. 267, A, c) alternating with nar- 

 row bright bands of a less refractive substance the interstitial discs 

 (Fig. 267, A, i). After hardening, each contractile disc becomes longi- 

 tudinally striated, the thin oblong rods thus formed being the sarcous 

 elements of Bowman. The sarcous elements are not the optical units, 

 since each consists of minute doubly-refracting elements the disdiaclasts 



