196 HUMAN PHYSIOLOGY 



directly. The muscle is directly irritable without the inter- 

 vention of the nerves, as is proven by the following facts : 



(a) In the sartorius muscle of a frog the nerve fibres dis- 

 tribute themselves only in the middle of the muscle, for the 

 two ends, as proven by microscopic examination, are free 

 of nerve fibres for about one-eighth of the total length of the 

 muscle. Yet stimulation of the ends, free of nerve fibres, 

 produces contraction of the muscle. 



(b) Ammonia stimulates the muscles directly, but does not 

 stimulate the nerve fibres. 



(c) Curare paralyzes the nerve endings in the muscle. 

 In an animal poisoned with curare, stimulation of the nerve 

 has no effect upon the muscle, yet the curarized muscle is 

 directly irritable. 



(</) " Idiomuscular" contraction is the local contraction 

 of the muscle fibres produced by the mechanical stimulation 

 of an abnormal muscle (fatigue, disease). This contraction 

 is produced only at the place of stimulation, the contraction 

 not being spread along the nerve fibres. 



The direct stimuli of the muscles are : 



1. Mechanical. Cutting and pinching of the muscle stimu- 

 late it. 



The external mechanical conditions also have an influence 

 on the irritability. With stimuli of the same strength, the 

 greater the resistance which the contraction of the muscle 

 encounters, the more energy is set free. This is useful, for 

 the muscle offers more force against greater resistance. 



2. Thermal. Heating above 40 produces a continuous 

 contraction which finally goes over into heat rigor identical 

 with rigor mortis (see page 198). Below the temperature 

 of 40, the irritability increases with the temperature. 



3. Chemical. Certain chemical agents stimulate the 

 muscle, e.g. ammonia, alkaline sodium salt solutions; but 

 these speedily injure it so that it becomes non-irritable. 

 Other substances, such as acids, merely injure the muscle 

 without previously stimulating it. Physiological salt solu- 

 tion (0.6^ NaCl) is indifferent. 



