CHAP, v.] THE CATS MUSCLES. 127 



conditions and excitations), not due to any mere physical or chemical 

 change, but^is a vital action. The capability of being acted on by 

 such excitations is called irritability, and the special form of irrita- 

 bility possessed by muscle is called CONTRACTILITY. 



This contractility may be seen to act in a single fibre, and it is by 

 the simultaneous action of the fibres composing it that each muscle 

 performs its proper function, and contracts as a whole. 



The change of shape referred to, is a temporary shortening of the 

 fibres in length, with a consequent transverse enlargement. It is a 

 familiar fact that when, in ourselves, the fore-arm is bent upwards a 

 temporary swelling takes place on the front of the upper arm. This 

 is due to the thickening which accompanies the shortening of the 

 muscle mainly employed in effecting such movement. 



The contracted state of any muscle can only endure for a limited 

 period, and cannot be repeated without an interval of rest, which 

 must be greater according to the exhaustion induced by frequently 

 repeated contractions. There is one muscle, however, t which acts 

 throughout the whole of life, the contractions being continually 

 reiterated after short regular intervals of rest. This muscle is the 

 heart, which takes its needful interval of repose after each contraction. 



Unstriped muscle contracts slowly and but slowly relaxes, while 

 striped muscle can contract suddenly and be suddenly relaxed. In 

 certain pathological states, as, e.g., in lockjaw, muscular contraction 

 may be greatly prolonged. 



The amount of force with which a muscle contracts depends on 

 the number of its fibres, but the length of the muscle determines the 

 degree of shortening which can be effected. 



The irritability of muscular tissue persists for a certain period 

 after death, which varies somewhat according to the cause of dissolu- 

 tion speedily disappearing after death from poisoning by noxious 

 vapours, or from lightning, while occasionally it has been found in 

 man to persist for twenty-four hours after death. Sixteen hours, 

 however, is the ordinary limit, even of that part of the heart (the 

 right auricle) which was called by Galen the " ultimum moriens," 

 on account of its long-enduring irritability. This property persists 

 very much longer in cold-blooded animals, e.g., the frog. 



The agent which induces muscular contraction is called a stimulus, 

 and there are various kinds of STIMULI. 



Thus, there may be a direct stimulus, such as the application to 

 the muscular fibres of a sharp-pointed body, or of an acid or some 

 acrid substance, or by sudden heat or cold, or by a shock of 

 electricity.* There may also be an indirect stimulus, i.e. when the 

 excitation is applied not directly to the muscular tissue, but to the 

 nerves distributed to it, or there may be a mental stimulus due to 

 emotional excitement. Stimuli, physically equal, have a more 

 powerful effect when acting on a muscle through a nerve, than 

 when acting directly on the muscle itself. 



* The resistance of a muscle to elec- , transversely as it is in the direction of its 

 trical conduction is seven times as great length. 



