MUSCLE. 



81 



the protoplasm. They are associated with the power of con- 

 tractility possessed by the cells. 



1. Smooth Muscle. 



This tissue consists of spindle-shaped cells, usually 50-200 p 

 long and 4-7 [t thick (Fig. 45). In the pregnant uterus they 

 may he as much as 500 ft in length. They do not possess a 

 true cell membrane. In the middle of the cell, at its thickest 



FIG. 4-x 



Four smooth muscle cells from the stomach of a frog, isolated in 33 per cent. KOH. 

 In the centre of each cell lies an oval nucleus, at either end of which there is a collection 

 of granular protoplasm. X 400. 



part, there is an oval rod-shaped nucleus, rounded at the 

 ends and containing one or more nucleoli. The nucleus is 

 surrounded at both ends by granular protoplasm. In the 

 protoplasm there can be made out a number of fibrils run- 

 ning longitudinally. These are seen more distinctly in the 



FIG. 46. 



Nucleus- 

 Intercellular 

 bridges' 



Longitudinal section of the muscle layer 



>g's large intestine. X 530. 



lower animals. The differentiated fibrils are doubly refractive, 

 and lie in the undifferentiated sarcoplasm. 



The smooth muscle cells usually lie close together in groups, 

 which may be combined to form definite layers, as is seen on 

 the muscular coats of the intestine. In cross-section the cells 

 appear as polygonal or round areas of unequal size, on account 



