UNSTRIPED MUSCLE 



plasm, which appears almost clear or more or less granular, 

 containing in its interior a rounded body, the nucleus, 

 specially distinguished by the affinity of parts of its substance 

 for colouring matters. To a body having this essential 

 structure, whatever its form, the name cell is applied. 



Unstriped Muscle. Examination of a teased preparation 

 of the muscular coat of the intestine, stomach, or urinary 

 bladder will show that it is com- 

 posed of delicate fibres (Fig. 31) 

 tapering at both ends, and with 

 a nucleus in the middle. These 

 are called smooth or unstriped 

 muscular fibres : they are ob- 

 viously cells which have under- 

 gone a great elongation in length. 



During the peristaltic move- 

 ments of the intestine (p. 7 5) each 

 fibre alternately contracts and 

 relaxes, becoming shorter and 

 thicker during the former pro- 

 cess, like the large muscles of 

 the body (p. 60). The move- 

 ments, in this case, however, are 

 not under the control of the will, 

 and unstriped muscular tissue is 

 therefore often spoken of as 

 involuntary muscle. 



Contractility of Protoplasm. 

 We have now studied three 

 different kinds of movement in 



cells : muscular movement in the unstriped muscle-fibres, 

 ciliary movement in the ciliated epithelial cells, and amoeboid 

 movement in the colourless blood-corpuscles. Muscular 



FIG. 31. Unstriped muscular 

 fibres from the frog's intestine. 

 To the right are shown fibres from 

 the longitudinal and circular 

 layers (see Chap. VIII) cross- 

 ing one another ; to the left 

 isolated fibres. (After Howes.) 



