VTT 



TXSTRIPED MUSCLE 



111 



of a mass of living protoplasm, 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 pre- 

 paration of the muscular coat of the intestine, stomach, 

 or urinary bladder will show 

 that it is composed of delicate 

 fibres (Fig. 31) tapering at both 

 ends, and each with a nucleus in 

 the middle. These are called 

 smooth or unstriped muscular 

 fibres : they are obviously cells 

 which have undergone a great 

 elongation in length. 



During the peristaltic move- 

 ments of the intestine (p. 75) 

 each fibre alternately contracts 

 and relaxes, becoming shorter 

 and thicker during the former 

 process, like the large muscles 

 of the body (p. 60). The 

 movements in this case, how- 

 ever, 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 move- 

 ment in the unstriped muscle-fibres, ciliary movement in 

 the ciliated epithelial cells, and amoeboid movement in the 



FIG. 31. Unstriped muscular 

 fibres from the Frog's intes- 

 tine, (x 500.) 



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.) 



