CHAPTER X. 



THE PROPERTIES OF MUSCULAR TISSUE. 



Contractility. The characteristic physiological property 

 of muscular tissue, and that for which it is employed in the 

 Body, is the faculty possessed by its fibres of shortening 

 forcibly under certain circumstances. The direction in 

 which this shortening occurs is always that of the long axis 

 of the fibre, in both plain and striped muscles, and it is 

 accompanied by an almost equivalent thickening in other 

 diameters, so that when a muscle contracts it does not 

 shrivel up or diminish its bulk in any appreciable way; it 

 simply changes its form. When a muscle contracts it also 

 becomes harder and more rigid, especially if it has to over- 

 come any resistance. This and the change of form can be 

 well felt by placing the fingers of one hand over the biceps 

 muscle lying in front of the humerus of the other arm. 

 When the muscle is contracted so as to bend the elbow it 

 can be felt to swell out and harden as it shortens. Every 

 schoolboy knows that when he appeals to another to " feel 

 his muscle" he contracts the latter so as to make it thicker 

 and apparently more massive as well as harder. In statues 

 the prominences on the surface, indicating the muscles be- 

 neath the skin, are made very conspicuous when violent 

 effort is represented, so as to indicate that they are in vigor- 

 ous action. In a muscular fibre we find no longer the slow, 

 irregular, and indefinite changes of form seen in the undif- 

 ferentiated cells of early development; this is replaced 

 by a precise, rapid, and definite change of form in one di- 

 rection only. Muscular tissue represents a group of cells 

 in the bodily community, which have taken up the one spe- 

 cial duty of executing changes of form, and in proportion 



