2^2 



BIOLOGY AND HUMAN LIFE 



system or of stimuli coming through the sense organs. The 

 involuntary muscle system works constantly, as long as there 

 is life, even while we are asleep. Even if the skeletal muscles 

 are paralyzed, life may go on for an indefinite time. If the 



smooth muscles are paralyzed, then 

 the end comes quickly. 



207. Our double nervous system. 

 Corresponding to the two sets of 

 muscles we have two sets of nerves : 

 (i) The spinal cord and the brain, 

 with their connections to the recep- 

 tors and effectors, regulate the ad- 

 justment of the organism to its 

 surroundings. (2) The autonomic, or 

 self-regulating, system connects the 

 internal organs with one another (see 

 Fig. 120). It has no central organ: 

 it consists of a double series of gan- 

 glia, or nerve-cell clusters, located 

 in front of the spinal column. Be- 

 cause of its many nerve connections 

 the various activities of the organism 

 become very closely tied together 

 and the organism acts as a whole. 



We have already seen that as the 

 activities of the muscles and of the 

 brain vary there is an automatic reg- 

 ulation of the heart, of breathing, of 

 the blood vessels, and of various 

 glands. This regulation is brought about by reflexes of the 

 autonomic system. The autonomic system includes in its 

 control, however, much more than muscles. Certain glands 

 are of special importance in connection with the work of the 

 autonomic nervous system. 



208. Our emotions. When stimuli from the skin or some spe- 

 cial sense organ start a nervous discharge into neurons that 



Fig. 119. Fibers of muscle 



The flesh, or muscle, of animals 

 is made up of fine threads, or 

 fibers. Portions of three fibers 

 are shown, with their peculiar 

 cross stripes, and the fine blood 

 vessels, or capillaries 



