128 



THE BIOLOGY OF THE FROG 



CHAP. 



10- 



layers may also be added from within by a layer of cells lin- 

 ing the inner surface of the walls of the marrow cavity. 



Muscle is composed of elongated cells or muscle fibers - 

 united by connective tissue. Two varieties of muscle are 

 commonly distinguished, the striated, or so-called volimtary, 1 



and the unstriated, or involimtary. ] 



In the latter the cell structure is 

 relatively simple ; the fibers are y 

 commonly spindle-shaped, with a 

 single nucleus near the center, 

 which is usually elongated in the 

 direction of the fiber. The ends of 

 the fibers are sometimes branched, 

 but they are more commonly en- 

 tire. /The length of the unstriated 

 muscle fibers varies greatly ; they 

 may be very narrow and attenu- 

 ated, as in the walls of the bladder, 

 '^u-M '^^W'^^'^F^^^^^l^ or short and comparatively thick, 



as in the walls of the smaller blood 

 vessels.^ While the fibers usually 

 show no cross striation, the cyto- 

 plasm shows delicate longitudinal 



,, . , , strands, or fibrilla;, which are con- 



FlG. 35. — Unstriated muscle ■' 



fibers from the intestine of sidered by most investigators to 

 the frog. ««, nucleus. (After ^g ji^g contractile elements of the 



Howes.) ,1 ,1 • , • 



. cell, ihe cell wall is very thm 



and 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 the 

 body where there is little occasion for sudden movement.) Q^ 

 It occurs in the muscular coats of the alimentary canal, in 

 the walls of the blood vessels and of many ducts, in the 



