326 



THE SPECIAL SENSES. 



muscle have a very different path. They arise also in the brain, 

 most probably in the midbrain, although their exact origin has 

 not been determined satisfactorily, and pass down the spinal 

 cord to terminate in the lower cervical region. From this point 

 the path is continued by spinal neurons which leave the cord in the 

 eighth cervical and the first and second thoracic spinal nerves and 

 pass by way of the corresponding rami communicantes into the 

 sympathetic chain at the level of the first thoracic ganglion. From 

 this point the fibers pass upward in the cervical sympathetic with- 

 out terminating until they reach the superior cervical ganglion near 

 the base of the skull. From this ganglion the path is continued 

 by sympathetic (postganglionic) fibers which pass to the Gasserian 

 ganglion and unite with its ophthalmic branch. Subsequently 

 they leave the ophthalmic nerve in the long ciliary branches. These 

 fibers under normal conditions are in constant (tonic) activity, so 

 that if the path is interrupted at any point by section of the cervi- 



Gasserian. 

 ffeoyuott* 



SufienorCe* 

 feuyttotj g- 



'nal 

 *ord. 



Ganylwn. <$fwrt Ciliary heroes 



wal 

 vjmpatketie 



Fig. 136. Schema showing the path of the preganglionic and postganglionic fibers 

 to the ciliary muscle and to the sphincter and dilator muscles of the iris. (Modified from 

 Schultz.) The course of the long ciliary nerves is represented very diagrammatically. 



cal sympathetic, for instance the pupil is seen to contract. This 

 constant activity may be referred directly to the activity of the 

 spinal neurons whose cells lie in the spinal cord in the lower cervical 

 and upper thoracic region. The cells in question constitute what 

 is sometimes called the lower ciliospinal center of Budge. 



The Accommodation Reflex and the Light Reflex of the 

 Sphincter Muscle. When the eye is accommodated for a near 

 object by the contraction of the ciliary muscle there is always a 

 simultaneous contraction of the sphincter pupillae whereby the 

 pupil is narrowed. The act is one of obvious value in vision, since 

 by diaphragming down the lens the focus is improved and more 

 exact vision, such as is needed in close work, is obtained. The act 



