THE MUSCULAR TISSUE. 145 



This sheath is considered quite distinct from the cellular 

 tissue surrounding a fasciculus of fibres, and is perfectly 

 transparent 



The microscope reveals t^wo kinds of ultimate fibres, 

 one belonging to the muscular system of animal life, or 

 voluntary muscles, the other to that of organic life or the 

 involuntary class. The fibre of animal life is known by 

 being marked with transverse striae, by having the fibril- 

 lae beaded or knotted, and presenting a varicose appear- 

 ance. The fibre of organic life has no transverse striae, 

 and is much smaller than the fibre of animal life. It pre- 

 sents swellings at different points, and this is considered 

 as one of its most prominent characteristics. The form of 

 the ultimate fibre, according to Mr. Bowman, is polygonal. 



When the fibrils are separately examined, they are found 

 to present spaces of alternate dark and light color. The 

 size of these ultimate fibrils, according to Wagner, is nearly 

 the same in all the vertebrata, from the 1-8856 to 1-11076 

 of an inch in diameter. The diameter of the primitive fas- 

 ciculi is stated to be very variable in the different classes 

 and genera, and even in the same animal and same muscle. 



The size is greater in the male than in the female, the 

 average diameter, as given by Mr. Bowman, is about 1-400. 

 The microscopic observations of Mr. Bowman also show that 

 there exist in the substance of the ultimate fibre, small 

 discs, either circular or oval, frequently concave on one 

 or both surfaces, and having, near the centre, one, two, 

 or three minute granules or dots. These are found to be 

 connected with, and distributed in nearly equal numbers 

 between, the fibrils ; and these granules or corpuscles are 

 regarded as the nuclei, which being developed into the 

 nucleated cell, constitute the origin whence the muscular 

 fibre is formed. The corpuscles can be seen by treating 

 muscle with some of the milder acids, as the citric. Blood 

 vessels and nerves enter abundantly into the structure of 

 muscles. Muscles possess the vital property of contractility, 

 by which they can contract and shorten themselves, and 

 which, as already stated, they take part in a great variety 

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