FACTOKS VARYING THE CHARACTER OF THE CONTRACTION 79 



at about 37 C. the muscle begins to lose its irritability and to pass, 

 at about 40^42 C., into the condition of heat rigor. Regarding the 

 cause of these variations little can be said; in fact, it has been stated 

 repeatedly that these fluctuations are not altogether constant. It 

 must be concluded, however, that muscle tissue requires a certain 

 optimum temperature which allows it to give reactions of maximal 

 amplitude. For the warm-blooded animals the most favorable tem- 

 perature is 37 C., and for the cold-blooded animals, the temperature 

 of the medium in which they are living. 



Heat rigor, or rigor caloris, is a permanent condition, correspond- 

 ing in a way to the coagulation of egg albumin. When entering this 

 state, the muscle gradually shortens and becomes firm to the touch 

 and opaque in its appearance. These characteristics it retains. 

 When placed under a greater tension than 15-20 grams per gram milli- 

 meter of substance, it ruptures abruptly. 



Effect of Drugs and Chemicals. Certain chemicals affect the 

 irritability and contractility of muscle in a very characteristic manner. 



'JfSec 

 FIG. 46. THE EFFECT OF VERATRIN ON MUSCULAR CONTRACTION. 



This is especially true of veratrin. A few drops of a 1.0 per cent, 

 solution of its acetate, injected into the dorsal lymph sac of a frog, 

 generally suffice to produce its characteristic effect. The muscle may 

 also be immersed in a solution containing 1 part of the alkaloid to 

 100,000 parts of a 0.7 per cent, solution of sodium chlorid. 1 By this 

 means a simple twitch of the gastrocnemius may be made to last 

 50-60 sec., instead of the normal 0.1 sec. Thus, the peculiarities 

 presented by a veratrinized muscle, consist in a surprisingly long period 

 of relaxation which usually presents two summits. It frequently hap- 

 pens, however, that a second stimulus sent into the muscle shortly after 

 it has completed one of these prolonged contractions, again results in 

 a very rapid twitch. If the muscle is then allowed to rest, the suc- 

 ceeding excitation may again produce a long drawn-out contraction. 

 Biedermann has stated that these peculiar effects are dependent upon 

 a dissociation of the red and pale fibers of the muscle. Carvalho and 

 Weiss, 2 however, have observed the same behavior in muscles which 

 are composed exclusively of either type of fibers ; hence, it is quite im- 



1 Bucannan, Jour, of Physiol., xxv, 1899, 137. 



2 Jour, de la Physiol. et de la path, gen., 1899. 



