THE SIMPLE MUSCLE CONTRACTION 513 



will produce a contraction, repeated on the repetition of the stimulus. If 

 applied to the same point for a number of times such stimuli will soon destroy 

 the irritability of the preparation. 



2. Thermal Stimuli. If a needle or glass rod be heated and applied to a 

 muscle or its nerve, the muscle will contract. A temperature of over 45 C. 

 will cause the muscles of a frog to pass into a condition known as heat rigor. 

 The sudden change of temperature acts as a stimulus. 



3. Chemical Stimuli. A great variety of chemical substances will excite 

 the contraction of muscles, some substances being more potent in irritating 

 the muscle itself and other substances having more effect upon the nerve. 

 Of the former may be mentioned dilute acids, salts of certain metals, e.g., 

 zinc, copper, and iron; to the latter belong strong glycerin, strong acids, 

 ammonia, bile salts in strong solution, etc. 



4. Electrical Stimuli. Any form of electrical current may be employed 

 to stimulate a muscle to contract, but either galvanism or the induced current 

 is usually chosen. For experimental purposes electrical stimuli are most 

 frequently used, as the strength of the stimulus maybe conveniently regulated. 

 In order that a stimulus shall be effective, it must have a certain amount of 

 energy and the application to the muscle must have a certain abruptness. 

 For example, a comparatively weak galvanic current suffices to stimulate a 

 muscle to action when suddenly applied in full force. But if the electric 

 current be applied very gradually, a current many times stronger will fail to 

 arouse contraction of a muscle. 



Conductivity in Muscle. In an ameba or other simple undifferentiated 

 contractile protoplasmic unit a stimulus applied at any point is quickly 

 transmitted throughout the entire mass. Just so is it with differentiated 

 muscle. A stimulus applied at any point of a muscle will quickly be propa- 

 gated through the mass as far as there is protoplasmic continuity. In cardiac 

 muscle and in smooth muscle there is uninterrupted conduction from cell 

 to cell. But in voluntary muscle each fiber is physiologically isolated from 

 its neighbors. When a voluntary muscle fiber is stimulated either at the ex- 

 tremities or at its middle, the effect of the stimulus quickly passes through 

 the entire fiber, whether it arouses a distinct act of contraction or not. 



The rate at which conduction takes place when a contraction accom- 

 panies it has been carefully measured by numerous observers. It varies 

 greatly in the different kinds of muscle, from two-tenths of a meter per second 

 in the rabbits' ureter (Engelmann) to ten meters per second in the voluntary 

 muscles of man. 



SINGLE MUSCLE CONTRACTIONS. 



Characteristics of a Single Contraction. The Myogram. The con- 

 traction of a muscle in response to a single effective stimulus of short dura- 

 tion is called a simple muscle contraction. A record of such a contraction 



33 



