632 A MANUAL OF PHYSIOLOGY 



little difference in the muscles of a living man, although fatigue 

 increases the extensibility. 



The great extensibility and elasticity of muscle must play a 

 considerable part in determining the calibre of the vessels, and in 

 lessening the shocks and strains which the heart and the vascular 

 system in general are called upon to bear, and must contribute much 

 to the smoothness with which the movements of the skeleton are 

 carried out, and immensely reduce the risk of injury to the bones 

 as well as to the muscles themselves, the tendons and the other 

 soft tissues. And not only is smoothness gained, but economy also ; 

 for a portion of the energy of a sudden contraction, which, if the 

 muscles were less extensible and elastic, might be wasted as heat 

 in the jarring of bone against bone at the joints, is stored up in the 

 stretched muscle and again given out in its elastic recoil. The 

 skeletal muscles, too, are even at rest kept slightly on the stretch, 

 braced up, as it were, and ready to act at a moment's notice without 

 taking in slack. This is shown by the fact that a transverse wound 

 in a muscle 'gapes,' the fibres being retracted, in virtue of their 

 elasticity, towards the fixed points of origin and insertion. Smooth 

 muscle, as we meet it in the hollow viscera, is highly distensible and 



FIG. 220. EXTENSIBILITY OF SMOOTH MUSCLE (GRUTZNER). 



The upper group of four cells (i to 4) is from a hollow organ, whose walls arc 

 contracted, and its lumen almost abolished ; the under group represents the same 

 fibres when the organ is full. The fibres are longer and somewhat darker. They 

 are also displaced somewhat along each other. 



elastic, as is suited to organs whose capacity is continually varying 

 within wide limits (Fig. 220). 



In the further study of muscle it is necessary first of all to consider 

 the means we have of calling forth a contraction in other words, 

 the various kinds of stimuli. 



Stimulation of Muscle. A muscle may be excited or stimu- 

 lated either directly or through its motor nerve. It is usual to 

 classify stimuli as electrical, mechanical, chemical, or thermal. 

 Electrical stimuli are by far the most commonly employed, and 

 will be discussed in detail. A prick, a cut, or a blow are examples 

 of mechanical stimuli. The action of a fairly strong solution of 

 common salt or of a dilute solution of a mineral acid is usually 

 described as chemical stimulation. But in considering the 

 excitation of nerve (p. 680) we shall see that physical changes are 

 often mixed up with so-called chemical stimulation. The con- 

 traction caused is not a single brief twitch, as is the case with a 

 not too severe mechanical excitation, but a sustained contraction 

 or a tetanus, Sudden cooling or heating acts as a stimulus for 



