MUSCLE 219 



checks excitability, produces the phenomena of fatigue, and 

 may finally kill. A balance between the proportion of these 

 ions, such as exists in the lymph bathing the tissues, is 

 necessary for normal action. Ringer's solution is an attempt 

 to reproduce the balance required for the tissues of the frog. 



2nd. A sudden mechanical change such as may be pro- 

 duced by pinching, tearing, or striking the muscle will cause 

 it to contract. This may be seen in fractures, and in opera- 

 tive interference with muscles (Practical Physiology). 



8rcZ. Any sudden change of temperature, either heating 

 or cooling, stimulates. A slow change of temperature has 

 little or no effect. Muscle, however, passes into a state of 

 contraction, heat rigor, when a temperature sufficiently high 

 to coagulate its protein constituents is reached — ^in mammals 

 about 46° C. This, however, is not a true living contraction. 



Mh. Muscle like nerve is stimulated by any sudden 

 change in the strength of an electric current passed through 

 it, and what has been said of the stimulation of nerve (pp. 62 

 to 67) applies also to the stimulation of muscle. 



The slower make and break of the galvanic current is a more 

 effective stimulus to muscle than the more rapid combined make 

 and break of the faradic current, which is more effective upon 

 nerve (p. 67). 



3. The Changes in Muscle when Stimulated. 



1 . Change in Shape. 



The result of stimulation of a muscle is the sudden 

 development of tension. It is as if a piece of string passing 

 between two points were suddenly changed into a stretched 

 rubber band. If the two ends are fixed, tension develops, but 

 the muscle cannot shorten. If the ends are not fixed the most 

 manifest change is shortening and thickening of the muscle. 

 This any one can see in the contracting biceps muscle. 



In skeletal muscle the development of tension and the 

 shortening and thickening of the muscle as a whole is due 

 to the development of tension in and the shortening and thicken- 

 ing of the individual fibres and their fibrils. 



In these fibrils the shortening and thickening is most 



