556 IRRITABILITY AND STIMULATION OF THE MUSCLE. 



Frogs' muscles that have been cooled, or those in which desiccation has 

 begun, exhibit an excessively increased irritability, especially to mechanical 

 stimuli. This fact may explain the remarkable muscular movements that often 

 take place in cholera-cadavers. Cooled muscles from the frog or the tortoise 

 may preserve their irritability for as long as ten days, but the muscles of warm- 

 blooded animals often degenerate in from one and one-half to two and one-half 

 hours. The irritability of the heart-muscle is considered on p. 118. Curarized, 

 isolated frogs' muscles exhibit the least amplitude of contraction at o, the greatest 

 at 30; if heated beyond the latter temperature, the contraction gradually dimin- 

 ishes, until the point is reached where rigor sets in. The duration of contraction 

 and the latent period are also shortest at 30. 



Since the time of Alb. v. Haller (1743) it has been thought necessary to 

 attribute to muscle a peculiar irritability (even without the intermediation of the 

 motor nerve) . In more recent times attempts have been made to adduce further 

 support in favor of this specific muscular irritability: (i) There are chemical 

 irritants that induce no movement when applied to the motor nerves, but cause 

 contraction when applied directly to the muscle; for example ammonia, lime- 

 water, carbolic acid. (2) The extremities of the sartorius muscle of the frog, in 

 which no nerve-endings can be demonstrated by means of the microscope, never- 

 theless react to direct stimulation by contractions. (3) Curare paralyzes the 

 motor nerves, while the muscle itself remains irritable. The action of cold, or 

 the arrest of the circulation in the muscle of an animal, will likewise abolish the 

 irritability of the nerve, but not of the muscle at the same time. In general, 

 the directly stimulated muscle will still contract for some time after its motor 

 nerve has degenerated. (4) After section of the nerves, the muscles still remain 

 irritable, even though the nerves have undergone total fatty degeneration. (5) At 

 times electrical stimuli act only upon the nerves, and not upon the muscles them- 

 selves. 



In lower animals (hydra, medusa) unicellular structures, neuro-muscular cells, 

 have been found in which nervous and muscular tissue are represented in one 

 and the same cellular structure. 



With regard to the stimuli that act upon the muscles, the following are to 

 be noted: 



1. The normal stimulus under ordinary circumstances acts upon the muscle 

 by way of its nerve, as in voluntary movement, the automatic motor impulse, 

 reflex excitation. Its nature is unknown. The irritation of a muscle through the 

 intermediation of its nerve is designated indirect stimulation. Pseudomotor 

 effects are considered on p. 559. 



2. Chemical Stimuli. All chemical agents that alter the chemical constitution 

 of muscular tissue with sufficient rapidity act as muscle-stimuli. According to 

 Kuhne, the mineral acids (o.i per cent hydrochloric acid), acetic and oxalic acids, 

 the salts of iron, zinc, copper, silver, and lead, bile, all act as stimuli to the muscle 

 in dilute solution, and only on the nerves in much stronger solutions. Lactic 

 acid and glycerin, when concentrated, excite only the nerve (?) ; when dilute, only 

 the muscle. The neutral alkaline salts act equally on muscle and nerve. Alcohol 

 and ether both act feebly. Water, especially if injected into the muscular vessels, 

 causes fibrillary contractions. Solutions of sodium chlorid, from 0.6 to 0.9 per 

 cent., or normal solutions of other sodium-salts, act indifferently toward the 

 muscular substance, even after the latter is exposed to their influence for days; 

 this is especially true after the addition of a trace of calcium chlorid or calcium 

 phosphate. A 6 per cent, solution of sodium chlorid causes the sartorius, when 

 deprived of its nerve, to contract much more strongly than when its nerve is 

 preserved, and especially in its active, thick fibers. Acids, potassium-salts, and 

 meat-extract diminish the irritability of the muscle, while other muscle-stimuli, 

 such as alcohol, sodium-salts, some metallic salts, in small doses increase the 

 irritability. Gases and vapors also have a stimulating influence on the muscles, 

 either exciting simple contractions or immediately causing contracture. Pro- 

 tracted exposure to the gases causes rigidity. Only the vapor of carbon disulphid 

 has an irritating effect on the nerves, while most vapors (for example, of hydro- 

 chloric acid) destroy without causing excitation. 



In comparative observations on the influence of chemically related substances, 

 only chemically equal quantities, for example normal solutions, should be employed. 

 Thus, among the halogens, sodium iodid, with its high molecular weight, has the 

 strongest effect; while sodium chlorid, with its low molecular weight, has the 

 feeblest effect. The combinations of the metals act in like manner; also the salts 

 of the alkaline earths. Those with the highest molecular weight cause the 



