

MUSCULAR STIMULI. 473 



responds to a feebler stimulus because its motor nerve terminations are not paralysed, while the 

 poisoned leg does not do so, because the motor terminations are paralysed. A feebler induced 

 shock suffices to cause a muscle to contract when it is applied to the nerve, than when it is 

 applied to the muscle itself directly. In large doses, curara also affects the spinal cord (p. 474).] 



[On what structures does curara act? These experiments prove that curara does not 

 paralyse the motor nerve-trunks, nor the muscular fibre's, and that it acts on the motor termin- 

 ations within the muscles, but they do not enable us to state the precise part of the nerve-ending 

 so affected. It may act on (1) the nerve just before it pierces the sarcolemma, (2) the sub-lemmar 

 axis-cylinder, (3) the end-plates, (4) the terminal branches or spray. Kiihne and Pollitzer 

 have made it probable that, even when a muscle is thoroughly impregnated with curara, some 

 of the nervous apparatus is unaffected. The sartorius is most excitable where there are most 

 nerves (fig. 318), and even in a muscle profoundly poisoned with curara, the distribution of 

 excitability varies with the number of nerves in the several parts of the muscle (tig. 317) just 

 as in a normal muscle, with this difference, that the excitability of all the parts of the muscle 

 containing nerves is less than normal. That this variation in excitability is due to nervous 

 structures, is shown by using a polarising anelectrotonic current ( 335), which depresses the 

 excitability of nerve-fibres, and then this difference of excitability disappears, the curve of ex- 

 citability running parallel with the abscissa, so that the difference does not seem to be due 

 to purely muscular causes.] 



[Pollitzer, speculating as to which part of the terminal nerve is affected, supposes that all 

 parts beyond the last node of Ranvier retain their functions, and he supposes that it is not the 

 axis-cylinders themselves, but the cement at the nodes, on which the drug exerts its specific 

 action. ] 



Neuro-Muscular Cells. Even in the lower animals, e.g., Hydra and Medusa?, there are uni- 

 cellular structures called " neuro-muscular cells," in which the nervous and muscular sub- 

 stances are represented in the same cell (Kleinenbcrg and Eimer). [The outer part of these 

 cells is adapted for the action of stimuli, and corresponds to the nervous receptive organ, while 

 the inner deeper part is contractile, and is the representative of the muscular part.] 



Muscular Stimuli. Various stimuli cause a muscle to contract, either by 

 acting upon its motor nerve, or upon the muscular substance itself ( 324). [The 

 former is called indirect stimulation, the latter direct stimulation.] 



1. Under ordinary circumstances, the normal stimulus exciting a muscle to 

 contract is the nerve impulse which passes along a nerve, but its exact nature is 

 unknown. 



2. Chemical Stimuli. All chemical substances which alter the chemical com- 

 position of a muscle with sufficient rapidity, act as muscular stimuli. Mineral 

 acids (HC1 0*1 per cent.), acetic and oxalic acids, the salts of iron, zinc, copper, 

 silver, and lead, bile, all act in weak solutions as muscular stimuli ; they act upon 

 the motor nerve only when they are more concentrated. Lactic acid and glycerin, 

 when concentrated, excite only the nerve; when dilute, only the muscle. [The 

 lower end of the sartorius, which contains no nerves, may be dipped into glycerin, 

 and it will not contract, but if it be dipped deeper to where there are nerve- 

 endings, it will contract at once.] Neutral alkaline salts act equally upon nerve 

 and muscle \ alcohol and ether act on both very feebly. When water is injected 

 into the blood-vessels, it causes fibrillar muscular contractions (v. Wittich), while a 

 0*6 per cent, solution of NaCl may be passed through a muscle for days without 

 causing contraction (Kolliker, 0. Nasse). [Carslaw, under Ludwig's direction, 

 however, found that solutions containing 0*5 to 0*2 per cent. NaCl, when perfused 

 through the muscles of a frog, excite many short, powerful attacks of tetanus, 

 separated from each other by periods of rest. Solutions containing 0'5 to 0*7 per 

 cent. NaCl, i.e., so-called " indifferent fluids " or " normal saline," are not without 

 influence, but of all known saline solutions, they injure a nerve-muscle preparation 

 least. Solutions of 1 to 2 per cent, rapidly kill the muscle.] Acids, alkalies, and 

 extract of flesh diminish the muscular excitability, while the muscular stimuli, 

 in small doses, increase it (Ranke). Gases and vapours stimulate muscle; they 

 cause either a simple contraction (e.g., HC1), or at once permanent contraction or 

 contracture (e.g., CI). Long exposure to the gas causes rigidity. The vapour of 

 bisulphide of carbon stimulates only the nerves, while most vapours (e.g., HC1) kill 

 without exciting them (Kiihne and Jani). 



