322 



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 through the 

 Gasserian ganglion and are distributed with its ophthalmic branch. 

 Subsequently they leave the ophthalmic nerve in the long ciliary 



nckofSt^. Lyy cilii 



Dilalor 

 Jsupillae. 



^hhinetef 

 ■ -llae. 



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. — (Mo 

 SchuUz.) The course of the long ciliary nerves is represented very diagrammaticaUy. 



-(Modified from 



branches. These fibers under normal conditions are in constant 

 (tonic) activity, so that if the path is inteiTupted at any point — by 

 section of the cervical sympathetic, for instance — the pupil is seen 

 to contract. 



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 of the iris 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 

 is usually spoken of as the accommodation reflex, but, in reality, it 

 is rather what is known as an associated movement. The act 



