Page:Life in Motion.djvu/113

Rh striated muscle, because it is apparently formed of fibres, the surfaces of which show markings or lines running transversely across the fibre, as you see in this diagram. Each fibre is really an enormous cell, about one and a half inch long by about one- three-hundredth of an inch broad. The wall of the cell is a fine membrane called the sarcolemma, and in it we find the muscle -substance showing alternately dark and narrower and clear and broader bands. The distance between two of the dark bands is on an average about the one-ten-thousandth of an inch. These dark and light bands are really the edges of discs, so that we have a disc of light substance alternating with a disc of dark substance. Each clear disc, however, when the muscle-fibre is looked at with a high power, shows a thin dark line passing through it, as shown in this diagram, and there is a fainter line, not so well marked, in the centre of the dark disc. Sometimes the fibre splits crossways into discs, and at other times into fine fibrils, each of which shows the same transverse markings as are seen in the fully formed fibre. The dark portions, usually called the sarcous