648 



MEDULLA OBLONGATA AND THE CRANIAL NERVES 



(6) The sphincter muscle of the iris. These fibers take their origin in the 

 median colony of small cells and terminate in the ciliary ganglion. Here they make 

 connection with postganglionic fibers formed by sympathetic neurons (nervi 

 ciliares breves). 



(c) The ciliary muscle. These fibers arise in the median colony of large cells 

 and end in the ciliary ganglion. Their postganglionic continuations are formed by 

 sympathetic neurons (nervi ciliares breves). 



We shall see later that the contraction of the ciliary muscle allows 

 the lens of the eye to become more convex, a condition necessary for 

 near vision. This change is usually accompanied by a constriction 

 of the pupil. These two reactions occur simultaneously and constitute 

 accommodation reflexes. In addition, the pupil is also constricted 

 whenever a high intensity of light is permitted to strike the eye. This 



* Edinger-Westphal nucleus. 

 Principal nucleus. 

 Median nucleus. 



Nucleus of 4th nerve. 



Fig. 321. — Nuclei or Origin of the Third and Fourth Nerves. — {From 



Poirier and C harpy.) 



reflex constitutes the so-called light reflex. In accordance with the 

 preceding discussion, it must now be evident that the afferent arc of the 

 circuit for the light reflex is formed by the optic tract, and the efferent 

 arc by the oculomotor nerve. Its center lies in the reflex area of the 

 optic tract, i.e., in the colliculus and corpora quadrigemina near the 

 aqueduct of Sylvius. The constriction of the pupil associated with 

 near vision and constituting the so-called accommodation reflex, finds its 

 origin in certain sensory stimuli which are set up in the eye muscles 

 whenever the eyes are converged for a near point. The afferent arc 

 of this reflex circuit, therefore, does not encroach upon the optic tract 

 and is not directly concerned with vision. 



