24 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



in water, and Bernard showed that when such an extract is injected 

 into the blood or hypodermically it paralyzes the motor nerves 

 at their peripheral end, so that direct stimulation of these nerves 

 is ineffective. Direct stimulation of the muscle substance, on the 

 contrary, causes a contraction.* We are justified, therefore, in 

 saying that skeletal muscle possesses the properties of independ- 

 ent contractility (Haller) and independent irritability (Ber- 

 nard). By the former term we mean that the shortening of the 

 muscle is due to active processes developed in its own tissue, 

 by the latter we mean that the muscular tissue may be made 

 to enter into contraction by artificial stimuli applied directly 

 to its own substance. This latter property cannot be said to 

 hold for all the tissues. Whether a nerve cell or a gland cell may be 

 made to enter into its specific form of activity by the direct appli- 

 cation of an artificial stimulus is still a debatable question. 



Artificial Stimuli. If we designate the stimulus that the 

 muscle receives normally from its nerve as its normal stimulus, 

 all other forms of energy which may be used to start its contraction 

 may be grouped under the designation artificial stimuli. Experi- 

 ments have shown that a contraction may be aroused by mechani- 



Fig. 6. The induction coil as used for physiological purposes (du Bois-Reymond 

 pattern): A, The primary coil; B, the secondary coil; P', binding posts to which are at- 

 tached the wires from the battery, they connect with the ends of coil A; P", binding posts 

 connecting with ends of coil B, through which the induction current is led off; S, the slide, 

 with scale, in which coil B is moved to alter its distance from A. 



caf stimuli, for instance, by a sharp blow applied to the muscle; by 

 thermal stimuli, that is, by a sudden change in temperature; by 

 chemical stimuli, for example, by the action of concentrated solu- 

 tions of salts, and finally by electrical stimulL> In practice, how- 

 ever, only the last form of stimulus is found to fee convenient. The 

 mechanical and thermal stimuli cannot be well applied without at 

 the same time injuring the muscle substance, and the same is prob- 

 ably true of chemical stimuli, which possess the disadvantage, more- 

 over, of not exciting simultaneously the different fibers of which 

 the muscle is composed. Electrical stimuli, on the contrary, are 



* "Lemons sur les effets des substances toxiques et m^dicamenteuses," 

 1857, pp. 238 et seq. 



