EFFERENT PATHS AND REFLEX ARCS 333 



chiasma; synapses in the superior colliculus of the corpora quadrigemina ; fibers 

 of the tectobulbar tract ending in the nucleus of Edinger-Westphal (visceral 

 efferent portion of the oculomotor nucleus); the visceral efferent fibers of the 

 oculomotor nerve, ending in the ciliary ganglion; and the postganglionic fibers 

 extending from the ciliary ganglion to iris. 



The pupillary-skin reflex is a dilatation of the pupil following scratching of 

 the skin of the cheek or chin. This is but one example of the fact that dilatation 

 of the pupil can be induced by the stimulation of many sensory nerves and con- 

 stantly occurs in severe pain. The path includes the following parts : the fibers 

 of these sensory nerves and their central connections in the brain stem and spinal 

 cord; preganglionic visceral efferent fibers, which arise from the cells of the inter- 

 mediolateral column of the spinal cord and run through the upper white rami 

 and the sympathetic trunk to the superior cervical sympathetic ganglion; and 

 postganglionic fibers, which arise in that ganglion and run through the plexus on 

 the internal carotid artery to end in the iris (Fig. 247). 



We have in the case of the pupillary reactions an illustration of the double 

 and antagonistic innervation, which, as we shall see in the next chapter, is a 

 rather characteristic feature of the autonomic nervous system. 



