GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 127 



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 nol show as great an 

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

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

 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 ten-ion, by isometric 

 contractions, that they give by single contractions ; and in this respect they 

 can be said to resemble red striated muscles. The following relations have 

 been found to exist between the strength of separate contractions and tetanus 

 in certain muscles : triceps and gastrocnemius of the frog, 1 : 2 or 3 ; the cor- 

 responding muscles of the turtle, 1:5; hyoglossus and rectus abdominalis of 

 the frog, 1 : 8 or 9. 1 It is evident that no just estimate of the part played 

 by different groups of muscles in the movement of the body can be reached 

 without a careful analysis of the nature of the contractions peculiar to each 

 of the muscles participating in the movement. 



Both the height and strength of the tetanus is controlled by the intensity 

 of the stimulus. A strong stimulus not only causes the separate contractions 

 of which the tetanus is composed to be higher, but is favorable to the develop- 

 ment of all the other factors which have been described as entering into the pro- 

 duction of tetanus. All normal physiological contractions are supposed to be 

 tetani, and everyone is conscious of the wonderful accuracy with which he can 

 grade the extent and strength of his voluntary movements. The remarkable 

 shading of the intensity of action observable in co-ordinated movements nin-t 

 find its explanation in the adjustment of protoplasmic activity in the nerve- 

 cells of the central nervous system. 



10. Continuous Contractions <ni<l Contractures. — Under ordinary circum- 

 stances a striated muscle, if excited by a single stimulus, gives a rapid con- 

 traction, followed almost immediately by a nearly equally rapid relaxation. 

 The duration and character of the period of relaxation are, however, subject to 

 great variation. In certain conditions the muscle may remain in a state of 

 continuous contraction for a considerable time, and then relax either slowly 

 or quite suddenly ; or it may begin to relax quickly and then suddenly stop > 

 as if the relaxation process had received a sudden check; or, after relaxing 

 quite rapidly for a short time, it may, without having received any visible 

 stimulus, contract again for a short distance and remain so contracted for a 

 considerable time. In any case when the relaxation period is unusually long, 

 the condition of prolonged contraction is termed "contracture." The form 

 of contracture which we are considering at present originates in the muscle 

 itself, and is to be sharply distinguished from a form of pathological contract- 

 ure, which originates in the central nervous system and in which the muscle 

 is kept continuously contracted by impulses coming from the spinal cord. 



There are a great variety of conditions under which muscles respond to 

 1 Biedermann : ElektrophysMogie, S. 109. 



