Henry Gray (18251861). Anatomy of the Human Body. 1918.
by a dark dotted line. This line is termed Dobies line or Krauses membrane (Fig. 376,k), because it was believed by Krause to be an actual membrane, continuous with the sarcolemma, and dividing the light band into two compartments. In addition to the membrane of Krause, fine clear lines may be made out, with a sufficiently high power, crossing the center of the dark band; these are known as the lines of Hensen (Fig. 376,H).
Schäfer has worked out the minute anatomy of muscular fiber, particularly in the wing muscles of insects, which are peculiarly adapted for this purpose on account of the large amount of interstitial sarcoplasm which separates the sarco-styles. In the following description that given by Schäfer will be closely followed.
A sarcostyle may be said to be made up of successive portions, each of which is termed a sarcomere. The sarcomere is situated between two membranes of Krause and consists of (1) a central dark part, which forms a portion of the dark band of the whole fiber, and is named a sarcous element. This sarcous element really consists of two parts, superimposed one on the top of the other, and when the fiber is stretched these two parts become separated from each other at the line of Hensen (Fig. 376,A). (2) On either side of this central dark portion is a clear layer, most visible when the fiber is extended; this is situated between the dark center and the membrane of Krause, and when the sarcomeres are joined together to form the sarcostyle, constitutes the light band of the striated muscular fiber.
When the sarcostyle is extended, the clear intervals are well-marked and plainly to be seen; when, on the other hand, the sarcostyle is contracted, that is to say, when the muscle is in a state of contraction, these clear portions are very small or they may have disappeared altogether (Fig. 376,B). When the sarcostyle is stretched to its full extent, not only is the clear portion well-marked, but the dark portionthe sarcous elementis separated into its two constituents along the line of Hensen. The sarcous element does not lie free in the sarcomere, for when the sarcostyle is stretched, so as to render the clear portion visible, very fine lines, which are probably septa, may be seen running through it from the sarcous element to the membrane of Krause.
Schäfer explains these phenomena in the following way: He considers that each sarcous element is made up of a number of longitudinal channels, which open into the clear part toward the membrane of Krause but are closed at the line of Hensen. When the muscular fiber is contracted the clear part of the muscular substance is driven into these channels or tubes, and is therefore hidden from sight, but at the same time it swells up the sarcous element and widens and shortens the sarcomere. When, on the other hand, the fiber is extended, this clear substance is driven out of the tubes and collects between the sarcous element and the membrane of Krause, and gives the appearance of the light part between these two structures; by this means it elongates and narrows the sarcomere.
If this view be true, it is a matter of great interest, and, as Schäfer has shown, harmonizes the contraction of muscle with the ameboid action of protoplasm. In an ameboid cell, there is a framework of spongioplasm, which stains with hematoxylin and similar reagents, enclosing in its meshes a clear substance, hyaloplasm, which will not stain with these reagents. Under stimulation the hyaloplasm passes into the pores of the spongioplasm; without stimulation it tends to pass out as in the formation of pseudopodia. In muscle there is the same thing, viz., a framework of spongioplasm staining with hematoxylinthe substance of the sarcous elementand this encloses a clear hyaloplasm, the clear substance of the sarcomere, which resists staining with this reagent. During contraction of the musclei.e., stimulationthis clear substance passes into the pores of the spongioplasm; while during extension of the musclei.e., when there is no stimulationit tends to pass out of the spongioplasm.
In this way the contraction is brought about: under stimulation the protoplasmic