CAUSES AND PHENOMENA OF MOTION. 25 



tissue of the heart, that the cessation of the action of this organ in 

 asphyxia is in some measure due. 



Sensibility. Besides the property of contractility, the muscles, 

 especially the striated, possess sensibility by means of the sensory nerve- 

 fibres distributed to them. The amount of common sensibility in muscles 

 is not great; for they may be cut or pricked without giving rise to severe 

 pain, at least in their healthy condition. But they have a peculiar sensi- 

 bility, or at least a peculiar modification of common sensibility, which is 

 shown in that their nerves can communicate to the mind an accurate 

 knowledge of their states and positions when in action. By this sensibil- 

 ity, we are not only made conscious of the morbid sensations of fatigue 

 and cramp in muscles, but acquire, through muscular action, a knowledge 

 of the distance of bodies and their relation to each other, and are enabled 

 to estimate and compare their weight and resistance by the eifort of which 

 we are conscious in measuring, moving, or raising them. Except with 

 such knowledge of the position and state of each muscle, we could not 

 tell how or when to move it for any required action; nor without such a 

 sensation of eifort could we maintain the muscles in contraction for any 

 prolonged exertion. 



MUSCULAE CONTRACTION. 



The power which muscles possess of contraction may be called forth 

 by stimuli of various kinds, viz., by Mechanical, Thermal, Chemical, and 

 Electrical means, and these stimuli may also be applied directly to the 

 muscle or indirectly to the nerve supplying it. There are distinct advan- 

 tages, however, in applying the stimulus through the nerves, as it is more 

 convenient, as well as more potent. 



Mechanical stimuli, as by a blow, pinch, prick of the muscle or its 

 nerve, will produce a contraction, repeated on the repetition of the stim- 

 ulus; but if applied to the same point for a limited number of times 

 only, as such stimuli will soon destroy the irritability of the preparation. 



Thermal stimuli. If a needle be heated and applied to a muscle or 

 its nerve, the muscle will contract. A temperature of over 100 F. 

 (37 '8 C.) will cause the muscles of a frog to pass into a condition known 

 as heat rigor. 



Chemical stimuli. A great variety of chemical substances will excite 

 the contraction of muscles, some substances being more potent in irrita- 

 ting 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 and bile salts in strong solution. 



Electrical stimuli. These are most frequently used as muscle stimuli, 

 as the strength of the stimulus may be more conveniently regulated. 



