STRUCTURE AND PHYSICAL PROPERTIES OF MUSCLE 533 



greater than it could lift, it would actually lengthen during its 

 stimulation, 



The above properties are of great importance in the body. If a 

 muscle were not readily extensible, the sudden contraction of one set 

 of muscles would tend to rupture those muscles (the antagonize^ 

 which perform the opposite action. Moreover, the contraction of a 

 muscle acting through an elastic medium is more efficient than through 

 a rigid medium. It is far less jerky in its effects. The smooth working 

 of the various body movements and of the circulation depends greatly 

 upon this elastic property. The muscles are kept in a slight state 

 of tension, so that no time is lost in " hauling in the slack." The 

 elastic property of the muscle insures its return to the normal state 

 after any contraction has been performed. Again, if the contracting 

 muscle were not more extensible, there would always be the risk, 

 when trying to lift a heavy weight, that the muscle would rupture 

 either itself, or its tendon, or the bones to which it is attached. Of 

 these three structures, the muscle is least often ruptured. 



Muscle is also excitable or irritable that is to say, it responds 

 with a contraction to different forms of stimulation. A muscle may 

 be stimulated directly or indirectly through its nerve. An excised 

 muscle may be directly stimulated by any sudden change in its physical 

 state by mechanical, chemical, thermal, or electrical stimuli. Striking 

 a muscle or pricking it causes its contraction; sudden heating or 

 cooling of a muscle may cause it to contract. Among chemical 

 excitants, we find that the application of such substances as ammonia, 

 dilute acids, strong saline solutions, induce muscular contraction. 

 Excitation follows any sudden alteration in the concentration of 

 electrolytes in tha fluid bathing the muscle fibre: . 



In experimental work, the electrical method is most generally 

 employed for purposes of stimulation, since it is convenient, easily 

 graduated and less injurious to the tissues.* The source of the electro- 

 motive force may be a Daniell or a Lee lane he cell. To make and 

 break the current, a mercury or a spring key is used, preferably the 

 latter. To protect the preparation from the current, a short- 

 circuiting key is used (the Du Bois-Reymond key). When shut, the 

 current is short-circuited through the metal blocks, which are 

 attached to a wooden or vulcanite base; when open, the current 

 flows to the preparation to be excited. 



Sometimes it is desired to reverse the direction of the current, or 

 to shunt it into another preparation. For this purpose, special 

 forms of keys are used. For making the actual stimulation of 

 the preparation, electrodes are used. These may be made of needles 

 insulated by a small piece of cork, and soldered to pieces of fine insu- 

 lated conducting wire. Such ordinary metal electrodes tend to 

 polarize, owing to the electrolysis which takes place in the tissue 

 fluids at the point of application. For accurate work, therefore, non- 

 polarizable electrodes are required. A form commonly employed 



* The student is advr-Kvl to consult a practical manral for details of apparatus. 



