. The biology of the frog . fibers arecommonly spindle-shaped, with asingle nucleus near the center,which is usually elongated in thedirection of the fiber. The ends ofthe fibers are sometimes branched,but they are more commonly en-tire. The length of the unstriatedmuscle fibers varies greatly; theymay be very narrow and attenu-ated, as in the walls of the bladder,or short and comparatively thick,as in the walls of the smaller bloodvessels. While the fibers usuallyshow no cross striation, the cyto-plasm shows delicate longitudinalstrands, or fibrillce, which are con-sidered by most investigato
. The biology of the frog . fibers arecommonly spindle-shaped, with asingle nucleus near the center,which is usually elongated in thedirection of the fiber. The ends ofthe fibers are sometimes branched,but they are more commonly en-tire. The length of the unstriatedmuscle fibers varies greatly; theymay be very narrow and attenu-ated, as in the walls of the bladder,or short and comparatively thick,as in the walls of the smaller bloodvessels. While the fibers usuallyshow no cross striation, the cyto-plasm shows delicate longitudinalstrands, or fibrillce, which are con-sidered by most investigators tobe the contractile elements of thecell. The cell wall is very thinand transparent. In its action unstriated muscle is slow;a considerable time elapses before it responds to a stimulus,and it is also slow to relax. It is found in those parts of thebody where there is little occasion for sudden occurs in the muscular coats of the alimentary canal, inthe walls of the blood vessels and of many ducts, in the nu. Fig. 35. — Unstriated musclefibers from the intestine ofthe frog, nu, nucleus. (AfterHowes.) VI HISTOLOGY OF THE FROG 129 lungs, urinary and gall bladders, around many of the glandsof the skin, and in the iris and ciliary muscle of the eye. Itis concerned in the production of slow movements, like thecontractions of the intestine, the expansion and contractionof blood vessels, the change in shape of the pupil of the fibers of striated muscle are more complicated instructure. They possess several spindle-shaped nuclei,scattered aboutthrough the cell,each of which issurrounded by asmall amount ofunmodified cyto-plasm. There is athin, but well-de-fined, cell wall, orsarcokmma, whichis best seen inplaces where thecontents of thefiber are crushedor broken fiber of vol-untary muscle is tobe regarded as asingle cell, with nu-merous nuclei scattered about through its cytoplasm. Inits early stages of development a voluntary muscle cell pos-sesses
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