474 THERMAL AND MECHANICAL STIMULI. 



Method. In making experiments upon the chemical stimulation of muscle, it is inadvisable 

 to dip the transverse section of the muscle into the solution of the chemical reagent (Hering). 

 The chemical stimulus ought to be applied in solution to a limited portion of the uninjured 

 surface of the muscle ; after a few secouds, we obtain a contraction or fibrillar twitchings of the 

 superficial muscular layers (Hering). 



[Rhythmical Contraction. While rhythmical contractions are very marked in smooth muscle, 

 (especially if it is stretched or subjected to considerable internal pressure, as in the hollow 

 viscera), e.g., the intestine, uterus, ureter, blood-vessels, and also in the striped but involuntary 

 cardiac musculature ( 58), they are not, as a rule, very common in striped voluntary muscle. 

 Chemical stimuli are particularly effective in producing them.] If the sartorius of a curarised 

 frog be dipped into a solution composed of 5 grms. NaCl, 2 grms. alkaline sodium phosphate, 

 and 0*5 grm. sodium carbonate in 1 litre of water, at 10 C, the muscle contracts rhythmically, 

 and may do so for several days, especially with a low temperature (Biedermann). This recalls 

 the rhythmical contraction of the heart. [Kuhne found a similar result. The rhythm is 

 arrested by lactic acid and restored by an alkaline solution of NaCl.] Rhythmical movements 

 may also be induced in the sartorius (frog), by the combined action of a dilute solution of sodic 

 carbonate and an ascending constant electrical current. Compare also the action of a constant 

 current on the heart ( 58). 



3. Thermal Stimuli. If an excised frog's muscle be rapidly heated to 28 C, a 

 gradually increasing contraction occurs, which, at 30 C, is more pronounced, 

 reaching its maximum at 45 C. If the temperature be raised, " heat-stiffening . 

 rapidly ensues. The smooth muscles of warm-blooded animals also contract when 

 they are warmed, but those of cold-blooded animals are elongated by heat (Grtin- 

 hagen). If a frog's muscle be cooled to 0, it is very excitable to mechanical 

 stimuli (Grilnltagen) ; it is even excited by a temperature under (Eckhard). 



CI. Bernard observed that the muscles of animals, artificially cooled, remained excitable many 

 hours after death ( 225). Heat causes the excitability to disappear rapidly, but increases it 

 temporarily. 



4. Mechanical Stimuli. Every kind of sudden mechanical stimulus, provided 

 it be applied with sufficient rapidity to a muscle (and also to a nerve), causes a 

 contraction. If stimuli of sufficient intensity be repeated with sufficient rapidity, 

 tetanus is produced. Strong local stimulation causes a weal-like, long-continued 

 contraction at the part stimulated ( 297, 3, a). Moderate tension of a muscle 

 increases its excitability. 



5. Electrical Stimuli will be referred to when treating of the stimulation of 

 nerve ( 324). 



Other Actions of Curara. When it is injected into a frog, either into the blood or sub- 

 cutaneously. it causes at first paralysis of the intra-muscular ends of the motor nerves (p. 471), 

 while the muscles themselves remain excitable. The sensory nerves, the central nervous system, 

 viscera, heart, intestine, and the blood-vessels are not affected at first (CI. Bernard, Kblliker). 

 [If the skin be stimulated, the frog pulls up the ligatured leg reflexly, although the other leg 

 remains quiescent ; this shows that the sensory nerve and nerve-centres are still intact ; but 

 when the action of the drug is fully developed, no amount of stimulation of the skin or the 

 posterior roots of the nerves will give rise to a reflex act, although the motor nerve of the 

 ligatured limb is known to be excitable ; hence, it is probable that the nerve-centres in the cord 

 themselves are ultimately affected. If the dose be very large, the heart and blood-vessels are 

 affected.] In warm-blooded animals, death takes place by asphyxia, owing to paralysis of the 

 diaphragm, but of course there are no spasms. In frogs, where the skin is the most important 

 respiratory organ, if a suitable dose be injected subcutaneously, the animal may remain motion- 

 less for days and yet recover, the poison being eliminated by the urine (Kiihnc). If the dose 

 be large, the inhibitory fibres of the vagus may be paralysed. In electrical fishes, the sensory 

 nerves, and in frogs, the lymph-hearts are paralysed. A dose sufficient to kill a frog, when in- 

 jected under its skin, will not do so if administered by the mouth, because the poison seems to 

 be eliminated as rapidly by the kidneys as it is absorbed from the gastric mucous membrane. 

 For the same reason the flesh of an animal killed by curara is not poisonous when eaten. If, 

 however, the ureters be tied, the poison collects in the blood, and poisoning takes place 

 (L.Hermann). [In this case the mammal may exhibit convulsions. Why? Curara paralyses 

 the respiratory nerves, so that asphyxia is produced from the venosity of the blood. It affects 

 the respiratory nerve-endings before those in the muscles generally, so that when the venous 

 blood stimulates the nerve-centres, the partially affected muscles respond by convulsions. Other 

 narcotics may excite convulsions indirectly by inducing a venous condition of the blood, while 

 the motor centres, nerves, and muscles are still unaffected.] Large doses, however, poison un- 



