STRUCTURE OF A MUSCULAR FIBRILLA. 



455 



Sarcolemma. Each muscular fibre is completely enclosed by a thin colourless, structureless, 

 transparent elastic sheath (fig. 302, 1, S), which, chemically, is midway between connective 

 and elastic tissue, and within it is the contractile substance of the muscle. [When a muscular 

 fibre is being digested by trypsin, Chittenden observed, at the beginning, the sarcolemma raised 

 from its sarcous contents as a folded tube, but it is ultimately digested by trypsin. It is thus 

 distinguished from the collagen substance of connective-tissue, which is not digested by trypsin. 

 It is not dissolved by boiling, and it resists the action of acids and dilute alkalies, while it is 

 dissolved by concentrated alkalies. Thus, it differs from elastic fibres, and on the whole, 

 chemically, it seems to be most closely related to the membrana propria of glands. It has 

 much more cohesion than the sarcous substance which it encloses, so that sometimes, when 

 teasing fresh muscular tissue under the microscope, one may observe the sarcous substance torn 

 across, with the unruptured sarcolemma stretching between the ends of the ruptured sarcous 

 substance. If muscular fibres be teased in distilled water, sometimes fine clear blebs are seen 

 along the course of the fibre, due to the sarcolemma being raised by the fluid diffusing under 

 it. The sarcous substance, but not the sarcolemma, may be torn across by plunging a muscle 

 in water at 55 C, and keeping it there for some time {Ranvier).] 



Sarcous Substance. The sarcous substance is marked transversely by alternate light and 

 dim layers, bands, stripes or discs (fig. 302, 1, Q), so that each fibre is said to be " transversely 

 striped." [The stripes do not occur in the sarcolemma, but are confined to the sarcous sub- 

 stance, and they involve its whole thickness.] 



[The animals most suited for studying the structure of the sarcous substance are some of the 

 insects. The muscles of the water-beetle, Dytiscus marginalis, and the Hydrophilus piceus are 

 well suited for this purpose. So is the crab's muscle. In examining a living muscle micro- 

 scopically, no fluid except the muscle-juice should be added to the preparation, and very high 

 powers of the microscope are required to make out the finer details.] 



Bowman's Discs. If a muscular fibre be subjected to the action of hydrochloric acid (1 per 

 1000), or if it be digested by gastric juice, or if it be frozen, it tends to cleave transversely 

 into discs {Bowman), which are artificial products, and resemble a pile of coins which has been 

 knocked over (fig. 302, 5). 



Fibrillae. Under certain circumstances, a fibre may exhibit longitudinal striation. This 

 is due to the fact that it may be split up longitudinally into an immense number of (1 to 1 '7 /* 

 in diameter) fine, contractile threads, the primitive fibrillse 

 (fig. 302, 1, F), placed side by side, each of which is also 

 transversely striped, and they are so united to each other by 

 semi-fluid cement-substance, that the transverse markings of 

 all the fibrillse lie at the same level. Several of these fibrils 

 are united together owing to the mutual pressure, and prismatic 

 in form, so that when a transverse section of a perfectly fresh 

 muscular fibre is observed after it is frozen, the end of each 

 fibre is mapped out into a number of small polygonal areas 

 called Cohnheim's areas (fig. 302, 2). [Each bundle of fibrils 

 or polygonal area represents what Kolliker called a "Muscle- 

 Column."] 



Fibrillse are easily obtained from insects' muscles, while 

 those from a mammal's muscle are readily isolated by the 

 action of dilute alcohol, Muller's fluid [or, best of all, per 

 cent, solution of chromic acid] (fig. 302, 3). 



[In studying the structure of muscle, it is well to re- 

 member that there are considerable differences between the 

 muscles of Vertebrates and those of Arthropoda.] 



[When a living unaltered vertebrate muscular fibre is 

 examined microscopically, in its own juice, we observe the 

 alternate dim and light transverse discs. Amici, Krause, and 

 Dobie showed that a fine dark line runs across the light 

 disc, and divides it into two (fig. 303). Amici resolved it 

 into a row of granules, and by others {e.g., Krause) it is -p. g 



regarded as due to the existence of a membrane, hence TT \ '' OA/ . 



it is called Krause's membrane, -which runs transversely Human muscular fibre, x 300. 

 across the fibre, being attached all round to the sarcolemma, thus dividing each fibre into a 

 series of compartments placed end to end. Hensen described a disc or stripe in the centre of 

 the dim disc] 



[On Krause's theory, each muscular compartment contains (1) a broad dim disc, which is the 

 contractile part of the sarcous substance. It is doubly refractive (anisotropous), and is com- 

 posed of Bowman's sarcous elements. (2) On each end of this disc, and between it and Krause's 

 membranes, is a narrower, clear, homogeneous, and but singly refractile (isotropous), soft or 

 fluid substance, which forms the lateral disc of Engelmann. In some insects it contains a row 

 of refractive granules, constituting the granular layer of Flbgel. If a muscular fibre be 



