Two portions of muscle; one of which, a, is covered with
membrane; the other, b, is uncovered; c, the muscular
fibres terminating in tendon.
40. The ultimate thread, or the minutest division of which the muscular fibre is susceptible, is called a filament; the smallest thread which can be distinguished by the naked eye is termed a fibre (fig. XXVI.); and the bundle which is formed by the union of fibres is denominated a fasciculus. The proper muscular substance is thus arranged into three distinct forms progressively increasing in size,—the filament, the fibre, and the fasciculus. The filament, the fibre, the fasciculus, as well as the muscle itself, formed by the aggregation of fasciculi, is each inclosed in its own distinct sheath of cellular membrane (fig. XXVI. a).
Portion of a muscle enclosed in a sheath of fascia
or aponeurosis.
41. The composition of the ultimate filament has been very carefully examined by many distinguished physiologists with microscopes of high magnifying power. Under some of these microscopes the filament appears to consist of a series of rounded particles or globules of the same size as the particles of the blood when deprived of their colouring matter, so that it looks like a string of pearls (fig. XXVIII.), each globule being commonly stated to be about the 2000th part of an inch in diameter. But it is now pretty generally agreed that this globular appearance of the ultimate muscular fibre vanishes under the more improved microscopes of the present day, and, as viewed by the latter, appears as a peculiar pulpy substance arranged into threads of extreme minuteness, placed close and parallel to each other, intersected by a great number of delicate lines passing transversely across the muscular threads (fig. XXIX.),
Ultimate fibres of muscle, very greatly magnified; showing
the strings of globules of which they are supposed by
some to consist.
42. With the exception of the organs of sense, the muscular tissue is more abundantly supplied with arteries, veins, and nerves, than any other substance of the body. Every ultimate thread or filament appears to be provided with the ultimate branch of an artery, vein, and nerve. These vessels are seen ramifying on the surface of the delicate web of membrane that incloses the pulp, but cannot be traced into it.
The appearance of the ultimate muscular fibres and of
their transverse lines, as seen under the microscope of Mr. Lister,
when the object is magnified 500 diameters.