836 THE NERVOUS SYSTEM 



spinal series and these differ from the somatic sensory neurons of the dorsal 

 root ganglia only in that they convey sensations from the viscera instead of 

 the body wall and cutaneous surfaces. On the other hand, the efferent system 

 of neurons is unlike that of the cerebrospinal series in that two neurons are 

 involved in conveying the efferent nerve impulse instead of one as in the 

 cerebrospinal series. The body of one of these two neurons, the preganglionic 

 neuron, lies within the brain or spinal cord, whereas the cell body of the other, 

 the postganglionic neuron, is associated with similar cell bodies within cer- 

 tain aggregations called sympathetic ganglia (fig. 358A). The axons of the 

 postganglionic neurons run to and end in the cardiac and blood vessel mus- 

 culature, gland tissue and smooth musculature in general throughout the body. 

 According to Ranson, '18, p. 308, "The autonomic nervous system is that 

 functional division of the nervous system which supplies the glands, the heart, 

 and all smooth muscle, with their efferent innervation and includes all general 

 visceral efferent neurones both pre- and postganglionic." 



b. Divisions of the Autonomic Nervous System 

 There are two main divisions of the autonomic system, viz.: 



( 1 ) The thoracicolumbar autonomic system, also called the sympathetic 

 division of the autonomic system, and 



(2) The craniosacral autonomic system, also called the parasympathetic 

 division of the autonomic system (see figure 358D). 



The thoracicolumbar outflow of efferent fibers has preganglionic fibers 

 which pass from the spinal cord along with the thoracic and upper (anterior) 

 lumbar spinal nerves, whereas the preganglionic fibers of the craniosacral 

 outflow depart from the central nervous system via cranial nerves III, VII, 

 IX, X and XI, and in the II, III and IV sacral nerves. 



c. Dual Innervation by Thoracicolumbar and Craniosacral Autonomic 



Nerves 



Most structures innervated by the autonomic nervous system receive a 

 double innervation, one from the sympathetic and the other from the para- 

 sympathetic division, both, in many instances, having opposite functional 

 effects upon the organ tissue. 



Examples of this dual innervation are: 



1) Autonomic Efferent Innervation of the Eye. Preganglionic cell bodies in 

 oculomotor nucleus, fibers passing with nerve III to ciliary ganglion. Post- 

 ganglionic cell bodies in ciliary ganglion; postganglionic fibers by way of 

 short ciliary nerves to ciliary muscle and circular muscle fibers of iris. Func- 

 tion: Accommodation of eye and decrease in diameter of pupil. The foregoing 

 innervation is a part of the cranio-sacral autonomic outflow. A parallel inner- 



