332 



THE NERVOUS SYSTEM 



(see p. 309). Thence the path follows the tectospinal and tectobulbar tracts 

 to the primary motor neurons of the cerebrospinal nerves (see p. 167). The 

 visual reflex arc begins in the retina, follows the optic nerve and optic tract with 

 partial decussation in the chiasma, to the superior colliculus of the corpora 

 quadrigemina (p. 226) ; thence it is continued by way of the tectospinal and tecto- 

 bulbar paths to the primary motor neurons of the cerebrospinal nerves (Fig. 162). 

 Pupillary Reactions. The iris is innervated by two sets of sympathetic 

 nerve-fibers derived from the ciliary and the superior cervical sympathetic ganglia 

 respectively. Impulses reaching the iris through the latter ganglion induce 

 dilatation of the pupil; those through the ciliary ganglion cause constriction. 

 The latter reaction always accompanies accommodation. When vision is fo- 



N.II 



Ciliary ganglion 



N. 



Sup. colliculus 

 Sensory nuc. N. V 



Pons- 



Upper thoracic segments of < 

 spinal cord 



N. V 

 \ Carotid plexus 



Sup. cervical sympatltetic ganglion 

 "- Cervical sympatlielic trunk 



Fig. 247. Pupillary reflex arcs. 



cused on a near object, contraction of the ciliary muscle results in accommoda- 

 tion; and at the same time contraction of the two internal rectus muscles brings 

 about a convergence of the visual axes. These two movements are always 

 associated with a third, the contraction of the sphincter pupillae. In addition 

 to this constriction of the pupil, which accompanies accommodation, two other 

 pupillary reactions require attention (Fig. 247). 



The Pupillary Reflex (Light Reflex) When light impinges on the retinae 

 there results a contraction of the sphincter pupillae and a corresponding constric- 

 tion of the pupil. The reflex circuit, which is traversed by the impulses bringing 

 about this reaction, begins in the retina and includes the following elements: 

 the fibers of the optic nerve and tract, with a partial decussation in the optic 



