STIMULATION OF MUSCLE 



711 



energy of a sudden contraction, which, if the muscles were less ex- 

 tensible 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 elastic, as is suited to organs 

 whose capacity is continually varying within wide limits (Fig. 237). 



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 stimulated 

 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 dis- 

 cussed in detail. A prick, a cut, or a blow are examples of mechani- 



Fig. 237. Extensibility of Smooth Muscle (Griitzner). The upper group of four 

 cells (i to 4) is from a hollow organ, whose walls are 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. 



cal 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. 757) we shall see that physical changes are often mixed up with 

 so-called chemical stimulation. The contraction caused is not a 

 single brief :> \itch, as is the case with a not too severe mechanical 

 excitation, b;i(; a sustained contraction or a tetanus. Sudden cooling 

 or heating acts as a stimulus for muscle, but thermal stimulation is 

 somewhat uncertain. It is not quite settled whether the contrac- 

 tion which can be obtained from a muscle when it is subjected to 

 brief local heating to a ' thermic shock,' as some writers prefer 

 to say (e.g., by the momentary glow of a platinum wire below but 

 not touching it) is an ordinary muscular contraction, or a physical, 

 although transient, contracture analogous to that caused by certain 

 drugs (Waller). Smooth, like striped, muscle is susceptible to 

 electrical, mechanical, thermal, and chemical stimulation. In 

 addition, in certain situations it can be excited by light (photic 

 stimulation), as in the case of the excised iris of fish and amphibia. 



