120 PHYSIOLOGY FOR NURSES 



and tenth with the medulla, and (3) through the spinal 

 nerves, from the first thoracic to the second lumbar, with 

 the spinal cord. The branch connecting the spinal nerve 

 and ganglion is called preganglionic, while the branch 

 passing from the ganglion to the muscle fiber is the post- 

 ganglionic. There are three autonomic ganglia in the 

 neck, but their motor fibers are derived from the upper 

 thoracic nerves and, after joining the sympathetic trunk 

 in the thorax, ascend in it to the cervical ganglia ; which, 

 however, give off gray communicating branches to the 

 cervical nerves which in turn carry them to the plain 

 muscle fibers in the regions to which they are distrib- 

 uted. Other ganglia, the collateral are found in the 

 thorax, abdomen and pelvis, and still others, the ter- 

 minal, in the walls of the viscera. 



The activities of this extensive nervous system are 

 directed to plain muscle fiber wherever situated. The 

 muscle in the walls of blood vessels, however remote 

 from the ganglia, that in the bronchi and their subdi- 

 visions, in the intestinal canal, in the iris and the geni- 

 tourinary tract, in glands and other plain muscular or- 

 gans throughout the body, all is innervated by the auto- 

 nomic system. 



Two opposed activities are characteristic of muscular 

 fiber contraction, by which its ends are brought nearer 

 together, and relaxation by which the ends are sepa- 

 rated. Most plain muscle is arranged in circular or lon- 

 gitudinal layers around some tubular body. Contrac- 

 tion of the circular fibers diminishes the size of the tube ; 

 relaxation enlarges it. Contraction of the longitudinal 

 fibers shortens the tube ; relaxation lengthens it. Apply 

 this to a blood vessel and one sees that contraction is 

 equivalent to constriction and relaxation to dilatation. 



The best known fibers of the sympathetic are those 



