MUSCULAR FIBRES. cxix 



mediate pellucid substance in very sparing quantity ; they may be partially 

 separated and spread out by breaking across a fibre, and gently bruising 

 the broken end, as at c (fig. LX.), or by splitting up its substance with 

 fine needles ; and the separation is facilitated by previous immersion of the 

 muscle for some time in alcohol or in 

 a weak solution of chronic acid, p- 



which either strengthens the fibrils, 

 or, by acting on the uniting substance, 

 weakens their lateral cohesion. But 

 whilst in this way the fibrillar struc- 

 ture is made apparent, and the fibre 

 may be split up into fine bundles or 

 skeins of fibrils (fig. LXI. B), and 

 threads apparently single may be 



detached, yet it is by no means easy to say when we thus arrive at an 

 insulated ultimate fibril. A thread so separated (fig. LXI. B, c) when viewed 

 iu proper focus with a magnifying power of 400 or 600, appears to consist 

 of a row of dark quadrangular particles, named sarcous elements by Mr. Bow- 

 man, with bright intervals between them, as if they were connected by some 

 pellucid substance of less refractive power. For the most part also a dark 

 line may be discovered passing across the middle of each bright space. I 

 am disposed, however, to think that the filaments thus described consist of 

 more than one ultimate fibril ; for I have now and then seen in specimens of 

 human muscle treated with chronic acid, a finer filament (as at d, fig. LXT.) 

 lying alongside one or more of those above described, and, whether itself an 

 ultimate fibril or not, showing at least that those with the quadrangular 

 particles are composite. In such a fine fibril the dark sarcous elements, 

 whilst agreeing in length with those alongside, are slender, rod-shaped, or 

 linear in figure ; and in the middle of the bright intervals between them 

 there is a dark point. In short, the fibril looks like a line regularly broken 

 at short distances, with a clot in each of the breaks. From this it may be 

 inferred that the greater breadth of the quadrangular particles is caused by 

 the lateral apposition of several rod-shaped particles ; and it is plain that 

 the appearance of a dark line in the bright interval is produced by a 

 transverse range of the intervening dots. 



This account corresponds very much with what is seen on a larger scale in the mus- 

 cular fibres of insects, by which I do not mean the fine, naturally separated, fibres of 

 the thoracic muscles, sometimes taken for fibrils, but the larger fibres, in which fibrils 

 answering to the above description are readily separable. In these, the rather long 

 rod-shaped sarcous elements, of which the fibrils consist, give a fluted character to 

 the broad cross stripes or bands which, by mutual apposition, they produce in the 

 fibre (fig. LXIII). 



The intermediate dotted line was long since noticed by Busk and Huxley, and was 

 considered by them to be produced, most probably, by the interposition of a row of 

 minute sarcous elements ; but, as they justly observe, it is not invariably present. 



Cause of the stripes, and cleavage into disks. When the fibrillse lie undis- 

 turbed in the fibre, the elementary particles of collateral fibrils are situated 

 in the same transverse plane, and it is to this lateral coaptation of the 

 particles that the transverse striping of the fibre is due. (See fig. LXTV.) 

 Accordingly, the cross stripes are not confined to the surface of the fibre, 

 but may be seen throughout its entire thickness on successively deepening 

 the focus of the microsdope. The fibres, moreover, when treated with 

 certain reagents (such as very dilute hydrochloric acid), show a tendency to 



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