THE MUSCULAR SYSTEM 73 



along the afferent sensory nerve through the posterior nerve 

 root to the cells of the spinal cord, and an impulse is at 

 once sent forth along efferent motor fibers to the muscles 

 of the hand and arm, which promptly jerk the hand away 

 (Fig. 48). 



A great multitude of reflex actions are possible to the 

 muscles, and one of the chief functions of the spinal cord 

 is to act as a center of reflex action. 



94. Automatic Movement. Action of the muscles often 

 occurs without any obvious stimulation of the nerves from 

 without. That is, the nervous impulse may apparently 

 arise in the nerve center itself, and efferent, or outgoing 

 influences are not preceded by afferent, or incoming influ- 

 ences. Such changes often, occur rhythmically activity 

 and rest, or diminished activity, following each other in 

 regular alternation as in the movement of the muscles 

 of respiration ; and this characteristic is believed to be 

 due to certain rhythmic changes which take place in some 

 of the nervous material of the medulla oblongata. 



95. The Muscular Sense. It is not motor nerve fibers 

 only that are distributed to the muscles ; the muscles 

 receive sensory fibers also which pass to the posterior 

 roots of the spinal nerves and convey impressions from 

 the muscles to the spinal cord and thence to the brain. 

 These impressions are called the muscular sense. They 

 assist our judgments of weight, and inform the brain of 

 the general condition of the muscles. 



96. Hygiene of the Muscles. Muscles increase in size 

 and in strength by appropriating suitable material from 

 the food, and by use. If ill fed and inactive, they 

 become small and weak. If one limb is made useless from 

 disease or injury, as when a bone is broken, its 

 muscles shrink and grow soft, so that the unused limb 



