THE NERVOUS SYSTEM 183 



to each of the spinal nerves. Association cords, made up of both 

 white and gray fibers, connect each set of chain ganglia in a 

 longitudinal direction. 



The autonomic nervous system is composed of two parts; the 

 sympathetic or ortho 'sympathetic and the parasympathetic divisions 

 (Fig. 118). The sympathetic division includes all of the chain 

 ganglia, the coeliac, superior, and inferior mesenteric ganglia, the 

 rami communicantes of the thoracolumbar group of spinal 

 nerves, and their preganglionic and postganglionic continuations. 

 The parasympathetic division is made up of the visceral branches 

 of two or three of the sacral spinal nerves and visceral branches 

 of cranial nerves III, VII, IX, and X, all of which pass directly 

 to certain structures such as the ciliary ganglion of the eye, the 

 cardiac ganglion of the heart, the myenteric plexuses of the gastro- 

 intestinal tract, etc., where they connect with postganglionic 

 fibers that in turn innervate the tissue. Thus in both the 

 sympathetic and parasympathetic divisions, the preganglionic 

 fibers originate in the brain or cord and the postganglionic fibers 

 in chain or peripheral ganglia. These are the efferent pathways. 

 Afferent pathways to the central nervous system are provided 

 by fibers from spinal-ganglion neurons that accompany the pre- 

 ganglionic fibers. The white rami communicantes are composed 

 of efferent and afferent fibers originating in the spinal cord or the 

 spinal ganglion. The fibers of the gray rami originate in chain 

 ganglia of the autonomic system. 



The autonomic system is concerned with the coordination and 

 control of involuntary functions such as the beating of the heart 

 and activities of the viscera in general, including blood vessels. 

 With the exception of pain, fullness of the bladder or rectum, 

 most of these activities do not enter consciousness to any extent. 

 Actually the autonomic system is not completely independent 

 of the central nervous system since all of its preganglionic fibers 

 begin in the cord or brain and its afferent pathways end there. 

 It is autonomic in the sense that most of its functional activity 

 takes place without interference from the central nervous system. 



Reflex Action. — A reflex action is an immediate involuntary 

 response to a sensory stimulus. Such responses are invariable 

 and can be predicted as a consequence of applying a definite 

 stimulus at a certain point. The simplest kind of reflex arc 

 accounting for such a result would consist of a sensory neuron 



