58 TEXT-BOOK OF PHYSIOLOGY 



tion. All muscles which possess this capability are said to be irritable and 

 contractile; and all agents which call forth this response of the muscle are 

 termed stimuli. The rapid change of form which a highly irritable muscle 

 undergoes in response to the action of a stimulus of short duration is usually 

 termed a twitch or pulsation. With appropriate apparatus it can be shown 

 that the muscle at the time of the twitch becomes warmer and exhibits electric 

 phenomena. The muscle is therefore an apparatus for the conversion of 

 potential into kinetic energy: viz., heat, electricity, and mechanic motion. 



Though usually associated with the activity of the nerve system, and to 

 some extent dependent on it, irritability is nevertheless an independent 

 endowment of the muscle and persists for a longer or shorter period, as 

 shown by many experiments, after all nerve connections have been destroyed. 

 Among the proofs which may be presented in support of this view are the 

 following: The introduction of the drug, curara, into the body of an animal 

 produces in a short time complete paralysis. Experiment has shown that 

 curara suspends the conductivity of the receptor substance for a variable 

 time and thus separates the muscle entirely from the nerve. Though 

 the animal is incapable of executing a single movement, its muscles respond 

 promptly on the application of a stimulus. Moreover, portions of muscles 

 exhibit irritability although containing no trace of nerve structure. This is 

 the case with the ends of the sartorius muscle of the frog and the anterior 

 end of the retractor muscle of the eyeball of the cat. These and other 

 facts demonstrate the independence of muscle irritability. 



In the living body nutritive activity and irritability are maintained by a 

 due supply of oxygen, and of nutritive material, the removal of waste prod- 

 ucts, and a normal temperature. The muscles of the cold-blooded animals, 

 for example the frog, retain their irritability for a much longer period after 

 death than the muscles of the warm-blooded animals. This is the case also 

 with the individual muscles after removal from the body of the animal. 

 The reason for this is found in all probability in the difference in the rate 

 of their nutritive activities and in the quantity of nutritive material stored up 

 in their cells. The duration of the irritability of isolated muscles can be 

 considerably prolonged by keeping them in a moist atmosphere. 



Conductivity. All muscle protoplasm possesses conductivity. The 

 change excited in a muscle-fiber by the arrival of a nerve impulse is at once 

 conducted with great rapidity in opposite directions to the ends of the fiber; 

 the advance of the excitation process is immediately succeeded by the con- 

 traction process, the change of form which constitutes the contraction. 

 With the disappearance of the former, the latter also disappears and the 

 muscle resumes its previous passive condition. There is no evidence, how- 

 ever, that the excitation process travels transversely that is, into adjoining 

 fibers being prevented from doing so by the presence of the limiting 

 membranes, the sarcolemmata. The fact that each muscle-fiber receives 

 its own, or at least a branch of a nerve-fiber, and hence its own nerve impulse 

 or stimulus, would also indicate that the excitation process cannot be con- 

 ducted longitudinally into adjoining fibers, or at least with sufficient rapidity 

 for the purposes of ordinary muscle actions. Nevertheless if a long muscle, 

 such as the sartorius, from a curarized frog be stimulated at one end with 

 an induced electric current, the excitation and the contraction processes will 

 be conducted with extreme rapidity to the opposite end of the muscle. The 



