THE FROG 45 
are under the immediate control of the central nervous system; 
this class includes practically all the skeletal muscles. The in- 
voluntary muscle fibers are more or less 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 en- 
countered 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 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 possible the outlines 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 some- 
times 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’ and tease the fibers apart with 
dissecting needles. Cover with a cover glass and examine with 
the low and then with the high power. Observe the very long 
fibers or muscle cells, with their transverse striations consisting of 
alternate light and dark bands. There are also longitudinal 
striations, which divide the fiber into fibrilla. Observe the sar- 
colemma or cell-wall of the fiber; this is so delicate as to be 
visible only in crushed or twisted fibers. The nuclei are best 
seen after running a little dilute acetic acid (3 per cent.) under 
the cover glass; this is best done by placing a drop of the acetic 
acid on the slide in contact with one edge of the cover glass, 
and drawing out the liquid from the opposite edge by means of a 
piece of absorbent paper. Observe that there are many nuclei, 
and that they are elongated and highly refractive. Each nucleus 
1A physiologically normal salt solution is one whose osmotic pressure with 
respect to the tissue cells is equal to that of the plasma of the blood This 
solution is made by adding 7 grams of sodium chlorid to one liter of water. 
