This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
Striped Muscular Tissue consists of very long fibres which, in their best developed varieties, approach of an inch in diameter. In most instances each fibre exhibits a delicate sheath or sarcolemma, which, in its resistance of reagents, resembles elastic tissue. The sarcolemma is filled with substance which is closely striped or striated transversely, the striation depending on a regular alternation of parts of different refractive properties. By careful manipulation this striated substance may be separated up into a bundle of fine threads called fibrillę, each of which exhibits the same alternation of parts which causes the striation of the fibre, and may be looked on as a series of dark or highly refractive parts, the sarcous elements of Bowman, connected by means of a less dense material. By other modes of manipulation the striped fibres may be partially cleft across, so as to present the appearance of a series of discs; and there is no sufficient reason for supposing that the division into fibrillę is more natural than this division into discs. To the interior of the sarcolemma are adherent a number of elongated nuclei; and it is important to observe that in the frog, and also in early foetal development of mammals, the nuclei are imbedded in the middle of the striated contractile substance. The fibres of the heart have no sarcolemma.