THE FROG 65 



under the control of the sympathetic nervous system; this 

 class includes both the non-striated muscle fibers and the 

 cardiac muscular tissue. 



1. Non-striated muscle fibers. These have already been 

 encountered in the study of cross-sections of the stomach 

 and intestine. They may be studied to better advantage in 

 stained and mounted microscopical preparations of the 

 mesentery, or the wall of the bladder. In such a preparation, 

 observe that the fibers are usually grouped in bundles; the 

 more isolated fibers are better for study. Make out if pos- 

 sible the outline of a single fiber; observe that it has the 

 form of a very slender spindle, usually unbranched but in 

 rare cases forked at one end or even at both ends. In its 

 center lies a single elongated nucleus, and at each end of this 

 nucleus is some granular cytoplasm. The fibers sometimes 

 exhibit a faint longitudinal striation. Under high power, 

 draw a non-striated muscle fiber. 



2. Striated muscle fibers. Only the fibers of the skeletal 

 muscles will be considered here. Cut out a small piece of 

 such a muscle from a freshly-killed frog; mount it on a slide 

 in a drop of normal salt solution 1 and tease the fibers apart 

 with dissecting needles. Cover with a cover glass and ex- 

 amine with the low and then with the high power. Observe 

 the very long fibers or muscle cells, with their delicate trans- 

 verse striations consisting of alternate light and dark bands. 

 There are also longitudinal striations, which divide the fiber 

 into fibrillae. Observe the sarcolemma or cell-wall of the 

 fiber; this is so delicate as to be visible only in crushed or 

 twisted fibers. The nuclei may be seen after running a little 



1 For the success of this experiment it is important that the solution 

 be of the proper strength. See Appendix, p. 99. 



