I30 THE BIOLOGY OF THE FROG chap. 



ing of alternate light and dark bands. The longitudinal 

 striation is due to the existence of minute strands, the sar- 

 costyles ox fibrillce, which extend the length of the cell. The 

 fibrilte, which are supposed to represent the contractile ele- 

 ments of the fiber, are separated by a semi-fluid substance, 

 the sarcoplasm. There is an arrangement of the fibrillae 

 into bundles, the muscle columns, which are separated from 

 each other by a thicker layer of sarcoplasm than that be- 

 tween the fibrillae. 



The appearance of cross striation is brought about by the 

 division of the fibrillae into segments, or sarcomeres. The 

 sarcomeres are separated from each other by a very fine 

 dark line known as Krause's membrane, which extends not 

 only across the individual fibrillae, but across the sarcoplasm 

 between the fibrillae of the fiber. Krause's membrane lies 

 in the center of a comparatively clear and lightly staining 

 band formed by the opposed ends of the two contiguous 

 segments. The middle portion of each sarcomere forms 

 the so-called dark band. Across the center of this band 

 there extends a second very delicate membrane, known as 

 the line of Hensen. When the fiber is relaxed, this line may 

 be seen to lie in the center of a comparatively light band, 

 which is usually not evident when the muscle is in a con- 

 tracted state. The dark bands of the muscle fiber are 

 composed of material which is anisotropic, or doubly refract- 

 ing, while the lighter areas on either side of Krause's mem- 

 brane are isotropic, or singly refracting, like the sarcoplasm. 

 When viewed with polarized light the differences between 

 these two substances are clearly brought out. 



A transverse section of a muscle fiber presents the ap- 

 pearance of a number of polygonal areas called Cohnheim's 

 fields^ which, represent the cut ends. of the muscle columns, 

 the spaces between the fields being filled with sarcoplasm. 



