130 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 Q^ fibril Ice, which extend the length of the cell. The 

 fibrillae, 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. 



