126 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



muscle, for this divides the character of the separate contractions, and, through 

 them, the effect of their combined action. 



The duration of the separate contractions, and the tendency of the muscle 

 to enter into contracture, are the predominant factors in determining the result. 

 ( !omplete tetanus can only be obtained in the case of a fresh muscle, when the 

 interval between succeeding stimuli is shorter 'than is required for the muscle 

 to reach its maximal contraction by a single stimulus. Thus the prolonged 

 contractions of smooth muscles permit of the development of a form of tetanus 

 by successive closures of the galvanic current at intervals of several seconds. 

 The non-striated muscle of the bladder of the cat can be tetanized by induc- 

 tion shocks given at a rate of a little less than one in two seconds. 1 The 

 contraction of some of the muscles of the turtle may last nearly a second, and 

 two or three excitations a second suffice to tetanize. The muscles of mar-* 

 mots during the winter sleep can be tetanized by 5 excitations per second 

 (Patrizi). Tetanus of the red (slowly contracting) striated muscles of the 

 rabbit can be obtained by 10 excitations per second, while 20-30 per second 

 are required to tetanize the pale (active) striated muscles (Kronecker and 

 Sterling) ; 100 stimuli per second are needed to tetanize the muscles of some 

 birds (Richet), and over 300 per second would be required to tetanize the 

 muscles of some insects (Marey). Any influence that will prolong the contrac- 

 tion process will lessen the rate of excitation required to tetanize. 



8. Effect of Exceedingly Rapid Excitations. — The question arises, Is there an 

 upper limit to the rate of excitation to which muscles will respond by tetanus? 

 There is no doubt that this is the case, but there is a difference of opinion as 

 to what the limit is, and how it shall be explained. 



Striated muscles and nerves can be excited by rates at which our most deli- 

 cate chronographs fail to act. The muscle ceases to be tetanized by direct exci- 

 tation at a rate by which it can still be indirectly excited through its nerve. 

 The highest rate for the nerve has been placed at from 3000 to 22,000 by differ- 

 ent observers, 2 and this wide difference is probably attributable to the methods 

 of excitation employed. That such different results should have been reached 

 is not strange, if we recall the many conditions upon which the exciting power 

 of the irritant depends. That tetanus should be obtained by such high rates 

 docs not show that the nerve responds to each of the separate shocks. As a 

 rule, when the rate of excitation is so high that tetanus fails a contraction is 

 observed when the current is thrown into the nerve, and often another when 

 it is withdrawn from the nerve — that is, the muscle behaves as if it were sub- 

 jected to a continuous battery current. A satisfactory explanation for this, as 

 well as for the failure of the tetanus, is at present lacking. 



9. Relative Intensity <>/ Tetanus and Single ( 'ontntctions. — The amount that 

 a muscle is capacle of shortening, when tetanized by maximal excitations, and 



1 C. C. Stewart: American Journal of Physiology, 191)0, iii. p. 25. 



1 Kronecker and Sterling: Archivfiir Anatomie und Physiologic, 1878, and Journal of Physi- 

 ology, 1880, vol. i. Von Frey nnd Wiedermann: Bcrichtc der sachsischen Gesellschaft der Wissen- 

 schuft, 1885. Roth: Pjluga's A rchiv, 1888. 



