EFFECT OF RATE OF STIMULATION 489 



gating them with defibrinated and aerated blood (von Frey). Mammalian 

 muscles will remain irritable for 30 minutes, or longer if cooled, after being 

 shut off from their blood supply and isolated from the body, but both irrita- 

 bility and contractility soon disappear entirely. 



Effect of Nerve Supply. The voluntary or skeletal muscle normally 

 contracts in the body only when stimulated through its motor nerve. If the 

 motor nerve is severed, the muscle is cut off from its normal source of activity, 

 hence will undergo the changes resulting from disuse, which will be presently 

 discussed. Aside from this, it is held by most observers that there are dis- 

 tinct nutritive or trophic nerves which exercise a controlling influence over 

 the growth, development, and general nutritive processes going on in muscle. 



When a motor nerve is cut, the muscle at first exhibits heightened irrita- 

 bility to all forms of stimuli. In a couple of weeks it decreases in its power 

 to respond to rapidly changing stimuli like induced currents. It responds 

 more readily to mechanical shocks and to galvanic currents for six or seven 

 weeks, then gradually loses the power of contracting through as many months. 

 The changes are due to protoplasmic degeneration. It is not clear in what 

 degree these changes are due to loss of trophic nerve influence, to inactivity, 

 and to changes in nutritive conditions. Since degeneration occurs when 

 the vascular supply is maintained, it would seem that the nutritive condition 

 must be chargeable to one or the other of the first two factors, probably to 

 both. 



Use of muscle increases its power and also its irritability. A properly 

 regulated exercise is well known to contribute to the health and development 

 of muscles. In cases of paralysis, mechanical or electrical stimulation is 

 applied directly to the muscle in an effort to supply artificial exercise until the 

 nerves are regenerated and motor connections re-established. If such stim- 

 ulation is not applied, the muscle degenerates from disuse and loses its 

 irritability often before the nerves regenerate. 



The Effect of Drugs. Drugs affect the irritability of muscle, some 

 augmenting, others depressing it. Voluntary muscle, which does not ordina- 

 rily contract except when stimulated, can be made so irritable by certain 

 salts that it contracts automatically like heart muscle, and the converse. 

 Ether, chloroform, etc., anesthetize muscle just as they do nerve, suppressing 

 both irritability and contractility. Suprarenal extract increases the ampli- 

 tude of contraction, as do also caffeine, digitalis, nicotine, and others. Ver- 

 atrine is well known greatly to prolong the relaxation phase of the simple 

 contraction without materially affecting the contraction phase or the latent 

 period. 



TETANIC AND VOLUNTARY MUSCULAR CONTRACTIONS. 



Effect of Rate of Stimulation. If we stimulate the muscle-nerve prep- 

 aration \vith two induction shocks, one immediately after the other, when the 



