84 CONTRACTILE ELEMENTS. 



Analysis of Contraction. The muscle, after obeying 

 these irritants, and passing from the lirst into the second 

 form, returns to the first; this succession of changes is what 

 is called the contraction of the muscle. It is made up of 

 several periods : that during which the muscle passes into 

 the second form ; the time during which it remains in it ; 

 and, finally, that occupied by its return to the first. It has 



Fig. 20. Kymo-graphic tracings of muscular contraction.* 



been also discovered that, when an excitant acts upon a 

 muscle, a short space of time elapses before the latter 

 responds to the excitation (Helmholtz). This forms a period 

 preceding the three others, and is called latent excitation. 



* lj Analysis of diagram representing muscular contraction. AB, Latent 

 excitation. fiC, Line of ascent. CD, Line drawn during the continuance of 

 form No. 2. D E, Line of descent and return to form No. 1 (E F). 



2, Ordinary form of a jerk (secousse). A B, Latent excitation. From B to 

 CD, ascent, or passage from form No. 1 to form No. 2; the latter lasts for a 

 moment only in C D, and is immediately followed by the line of descent, D E, 

 or return to form No. 1 (E F). 



3, Physiological tetanus. A B, Latent excitation. B C, Ascent. C E, De- 

 scent interrupted by a fresh ascent ; the jerks (secousses) thus produced in suc- 

 cession (c, c', c", c"')) afterwards succeed each other so rapidly as to become 

 blended in one, causing the muscle to retain the form No. 2, and to describe the 

 line F. The dotted lines indicate the descents or return to the form No. 2, which 

 would take place if fresh excitations did not force the muscle to describe a new 

 line of ascent before it has had time to complete the line of descent produced by 

 the preceding jerk (secousse). 



