CHAP. IX.] THE CONTRACTILE TISSUES. 343 



is not such as to admit of the assumption that gravity alone deter- 

 mines it; and it becomes more than probable when we learn that 

 the heat-developing processes 1 and the chemical processes 2 are also 

 carried over into the relaxing period. The duration of the period of 

 relaxation is about ^ of a second. Muscle appears to have no power 

 of active re-extension 3 . 



Tetanus When stimuli are thrown into muscle with sufficient 



rapidity, contractions overtake one another, sum their 

 effects, and maintain the muscle against extending forces, in a 

 position more or less of maximum contraction. Such continued con- 

 traction is called a tetanus, the laws of which belong to the physics 

 of muscle. 



The course of contraction is not similar in all kinds of muscle; 

 nor is the course the same in any one muscle under all circumstances. 

 Thus the rate of contraction is much quicker in the muscles of 

 insects than in those of frogs ; and quicker in the latter again than 

 in those of the tortoise ; and in these than in the heart-muscle ; and 

 in this than in the smooth muscles of the intestines or ureter. Such 

 differences are of the greatest interest to the physiologist as indicating 

 either differences in the machinery for the conversion of energies, 

 or different capacities for the chemical changes upon which contraction 

 depends. 



Red and In respect of such internal machinery, or capacity, the 



Pale striated differences of red and pale striated muscles are remark- 

 Muscles, able. The contraction of the red variety is slow and 

 enduring, 10 stimuli a second being enough to cause almost unbroken 

 tetanus; while of the pale variety, the contractions are short and sharp, 

 20 30 stimuli a second being needed for a perfect fusion of them 4 . In 

 the former the latent period is so long as T X H sec. ; while in the latter 

 it has the value of ^ sec. 3 . 



These physiological differences are all associated with varieties of 

 structure; but differences of a similar nature may be exhibited by the 

 same muscle when it contracts under varied conditions. Thus cold, 

 many poisons, and incipient exhaustion prolong contraction and 

 diminish its amplitude. Indeed the stimulus of a sharp blow to dying 



idio-mus- muscle often produces a local contraction which may be 

 cuiar Con- likened to a wheat, and which may persist for a long 

 traction. time : such a contraction is described as 'idio-muscular'. 



Absolute The force of muscular contraction is measured by 



force. the weight which is just sufficient to prevent the 



1 Steiner, "Ueber die Warmeentwicklung bei der Wiederausdehnung des Muskels. " 

 Pfliiger's Arch., Vol. xi. p. 196. 



2 Heidenhain with Landau and Faculty, Loc. cit. Pfliiger's Arch., Vol. n. p. 429. 



3 Kuhne, Loc. cit. Arch. f. Anat. Physiol. u. wiss. Med. (Reichert u. du Bois- 

 Reymond), 1859, p. 815. 



4 Kronecker and Stirling, "The Genesis of Tetanus." Journal of Physiol. (Foster)^ 

 Vol. i. p. 395. 



5 Ranvier, Loc. cit. Arch, de physiol. norm, et path., 2 s6r. Vol. i. p. 5. 



