122 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Complete 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 contraction of some of the muscles of the turtle may last nearly a second; 

 and two or three excitations a second suffice to tetauize. Tetanus of the red 

 (slowly contracting) striated muscles of the rabbit can be obtained by 10 exci- 

 tations 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). 

 Strange to say, the heart- muscle cannot be tetanized; if it replies at all to 

 frequent excitations, it gives the simple contractions characteristic of the heart- 

 beat. Any influence which will prolong the contraction 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, 1 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. 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 nerve, and often another when it is withdrawn from the nerve. A satisfactory 

 explanation for this, as well as for the failure of the tetanus, is at present lack- 

 ing. 



9. Relative Intensity of Tetanus and Single Contractions. The amount that 

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

 the strength of the tetanic contraction, depends very largely on the kind of 

 muscle. For example, pale striated muscles, although capable of higher and 

 more rapid single contractions than the red striated, do not show as great an 

 increase in the height and strength of contractions when tetanized as do the 

 red ; the latter, which are very rich in sarcoplasma, have likewise the greater 

 endurance. Gruetzner has called them " tetanus muscles," since they seem to 

 be particularly adapted to this form of contraction. Fick found that human 

 muscles when tetanized develop ten times the amount of tension, by isometric 



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

 ology, 1880, vol. i. Von Frey und Wiedermann : Berichte der sdchsischen OeseUschaft der Wissenr 

 sckafi, 1885. Roth : Pftiiger's Archiv, 1888. 



