CHAPTER IV. 

 MUSCLES. 



29. THE active element in which the force resides by which 

 not only the bones and joints are set in motion, but likewise 

 all the movements of the organs are accomplished, is called 

 muscular fibre. 



Muscular fibre presents two great varieties, the striped 

 and the unstriped. The striped is the more complex, and, as 

 it is the variety of which all the muscles consist, and over which 

 the will has control, it is likewise termed voluntary muscular 

 fibre ; while the unstriped or smooth variety, being employed 

 in the viscera, blood-vessels, and other structures subserving 

 the purposes of organic life, and beyond the control of the 

 will, is termed involuntary or organic muscular fibre. There 

 are, however, various instances in which striped muscular 

 fibre is not under the direction of the will, the principal being 

 the heart ; and there is a circumstance other than the rela- 

 tion to the will, on which more probably the kind of fibre 

 used in each structure depends, namely, that striped muscle con- 

 tracts suddenly when irritated, and becomes suddenly relaxed, 

 whereas the unstriped contracts slowly and is relaxed slowly. 

 The heart is required to contract rapidly, and striped muscu- 

 lar fibre is used in its construction. The mode of contrac- 

 tion of striped muscle is exemplified by the immediate 

 response of the muscles of the skeleton to the impulses of 

 the will ; while the contraction of unstriped fibre may be 

 seen by watching the enlargement and diminution of the 

 pupil, occasioned by altering the focus of the eye or the 

 amount of light admitted to it (p. 242). But birds, which 

 probably require more rapid action of the iris in connection 

 with their extraordinary keenness of vision, have the muscu- 

 lar tissue of the iris composed of striped fibres; and in niam- 



