VI 



HISTOLOGY OF THE FROG 



129 



lungs, urinary and gall bladders, around many of the glands 

 of the skin, and in the iris and ciliary muscle of the eye. It 

 is concerned in the production of slow movements, like the 

 contractions of the intestine, the expansion and contraction 

 of blood vessels, the change in shape of the pupil of the eye. 

 The fibers of striated muscle are more complicated in 

 structure. They possess several spindle-shaped nuclei, 

 scattered about 

 through the cell, 

 each of which is 

 surrounded by a 

 small amount of 

 unmodified cyto- 

 plasm. There is a 

 thin, but well-de- 

 fined, cell wall, or 

 sarcokmma, which 

 is best seen in 

 places where the 

 contents of the 

 fiber are crushed 

 or broken apart. 

 Each fiber of vol- 

 untary muscle is to 

 be regarded as a 

 single cell, with nu- 

 merous nuclei scattered about through its cytoplasm. In 

 its early stages of development a voluntary muscle cell pos- 

 sesses but one nucleus. As the fiber grows, the nucleus 

 divides repeatedly, but as the cytoplasm does not divide at 

 the same time, there come finally to be numerous nuclei 

 within the limits of a single cell wall. The cytoplasm shows 

 both a longitudinal striation, and a cross striation consist- 



K 



Fig. 36. — A, part of a fresh muscle fiber of a frog ; 

 B, the same after treatment with distilled water 

 followed by methyl green, b, light bands; d t 

 dark bands ; n, nuclei ; s, sarcolem ma showing 

 more clearly where the fiber is broken. (After 

 Parker and Parker.) 



