82 



HISTOLOGY. 



becomes nearly full of these fibrils, so arranged that their light and dark 

 bands appear to form continuous stripes across the muscle fiber (Fig. 94). 

 That the transverse striations are optical effects is shown by the readiness 

 with which they may be broken up by the separation of the longitudinal 

 fibrils (Fig. 96). The dark bands stain more deeply than the light ones, 

 which perhaps is not due to chemical differences but is because they are 

 denser, containing less water. In polarized light the dark bands are 

 "doubly refractive" or anisotropic and the light ones are "singly refrac- 

 tive" or isotropic. 



The finer structure of the fibrils such as occur both in cardiac and in 

 the skeletal muscles, is shown in the diagram, Fig. 97. The light band 

 is bisected by a slender dark one said to be continuous from one side of 

 the fiber to the other, thus connecting the fibrils with one another. Since 

 such a transverse membrane is not present from the first it has been 

 suggested that it forms by lateral outgrowths of the fibrils. It is named 

 the ground membrane of Krause, and is always designated by the letter 

 Z. The light band is J. The large dark band seen with ordinary lenses 



FIG. 97. DIAGRAM OF MUSCLE STRIATIONS. 



The fibrils consist of alternating dark bands, q, and light bands, j. j is traversed by the ground mem- 

 brane z, and q by the median membrane m. In the right of the three muscle segments shown 

 in the figure the bands, n, have been drawn. (The portion of j between n and z is designated e.) 



is called Q. It grows lighter toward its middle part where it is sometimes 

 crossed by the median membrane of Heidenhain, M. This is thought to 

 be similar to the ground membrane Z, but more delicate. The light 

 portion of Q through which it passes is designated H. In some highly 

 developed muscles of insects a dark band N is found in /. It is of un- 

 certain nature. The fiber as a whole is divided by the ground membranes 

 which cross it, into a series of similar compartments called muscle segments 

 (sarcomeres) . Additional sarcomeres may be formed at the ends of 

 muscle fibers; it has not been found that the median membrane can become 

 a ground membrane, thus producing two segments from one. 



The contraction of muscles corresponds in its rate with the complexity 

 of the striae. Thus smooth muscles which are non-striated contract 

 slowly. The more rapidly acting muscles of some invertebrates have 

 banded fibrils but lack the orderly arrangement which produces transverse 

 striations. The highest development of striated structure is perhaps in 

 the wing muscles of insects which contract with great rapidity. As the 



