128 



THE BIOLOGY OF THE FROG 



CHAP. 



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 voluntary, 

 and the nnstriated, or involuntaiy. 

 In the latter the cell structure is 

 relatively simple ; the fibers are 

 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, 

 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 fibrillcB, which are con- 

 sidered by most investigators to 

 be the contractile elements of the 

 cell. The cell wall is very thin 

 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. 

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

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



7ZU 



Fig. 35, — Unstriated muscle 

 fibers from the intestine of 

 the frog, nil, nucleus. (After 

 Howes.) 



