THE PHENOMENON OF CONTRACTION. 25 



DURATION OF A SIMPLE MUSCULAR CONTRACTION. 



Insect 0.003 sec. 



Man 0.050 " 



Frog 0.100 " 



Terrapin 1.000 " 



The series may be continued by the figures obtained from the 

 plain muscle, thus: 



The involuntary muscle (mammal) 10.00 



Foot muscle of slug* (Ariolimax) 20.00 



There is reason to believe that the rapidity of contraction is re- 

 lated to the distinctness of the cross-striation. This is indicated 

 by the properties of the so-called red and pale muscles that occur 

 in some animals the rabbit, for instance. The pale muscles con- 

 tract much more rapidly than the red ones, and corresponding with 



Fig. 8. Curve of simple muscular contraction. 



this fact it is found that the cross-striation is more distinct in the 

 former. As was explained above, the active agent in contraction 

 is contained in the dim bands of the fibers, and the more highly 

 differentiated this structure becomes the more perfect apparently 

 is its work as a mechanism for shortening. According to Cash, the 

 duration of contraction of the soleus muscle (red) in rabbits is one 

 second, while that of the gastrocnemius medialis (white) is only 

 0.25 second. 



The Curve of Contraction. When a contracting muscle is 

 attached to a lever this lever may be made to write upon a smoked 

 surface and thus record the movement, more or less magnified 

 according to the leverage chosen. If the recording surface is sta- 

 tionary the record obtained is a straight line and indicates only the 

 extent of the shortening. If, however, the recording surface is in 

 movement during the contraction the record will be in the form of a 

 curve, which, making use of the system of right-angled co-ordinates, 

 * Carlson, "American Journal of Physiology," 10, 418, 1904. 



