A 



302 THE SPECIAL SENSES. 



is rather what isjmown as an associated movement. The voluntary 

 ejffpijt inattgyi*^ted in the brain affects the cranial centers for both 

 "" rl'iuyi'les,' and under normal conditions they always act together, 

 a fact which implies a close connection of their centers. An example 

 of a similar associated action is seen in the effect of the respiratory 

 movements on the rate of heart beat, the inspiratory discharge 

 from the respiratory center being accompanied by an associated 

 effect upon the cardio-inhibitory center whereby the heart rate 

 is quickened. In the particular case that we are dealing with 

 three muscular acts, in fact, are usually associated, for every act 

 of accommodation under normal circumstances is accompanied 

 not only by a constriction of the pupil, but also by a convergence 

 of the eyeballs, due to a contraction of the internal rectus muscle 

 in each eye. 



The light reflex is observed when light is thrown into the eye. 

 As is well known, the pupil dilates in darkness or dim lights and 

 contracts to a pin-point upon strong illumination of the retina. 

 The value of this reflex is also obvious. In the dim light the total 

 illumination and therefore the visual power of the retina is aided 

 by an enlarged pupil, but in strong lights the illumination may be 

 diminished with advantage by diaphragming, since the optical 

 image on the retina is thereby improved on account of the diminu- 

 tion in spherical aberration. The reflex arc involved in this act is 

 known in part. The afferent path is along the optic nerve; the 

 efferent path back to the sphincter is through the third nerve 

 and ciliary ganglion ; injury to either of these paths diminishes or 

 destroys the reflex. The reflex is also lost in some cases in which 

 neither of these paths seems to be involved. In tabes dorsalis 

 (locomotor ataxia) and general paresis, for instance, the pupil 

 of the eye is constricted and does not give the light reflex, but 

 still shows the accommodation reflex. Such a condition is known 

 as the Argyll Robertson pupil. Some question exists, there- 

 fore, as to the nature of the connections in the brain between the 

 afferent impulses and the motor center in the nucleus of the third 

 nerve. According to some authors (Gudden, v. Bechterew), the 

 afferent light reflex fibers are a set of fibers distinct from the visual 

 fibers proper. They arise in the retina and pass backward in the 

 optic nerve, but leave the optic tracts at the chiasma to enter the 

 walls of the third ventricle and thus reach the nucleus of the third 

 nerve. This view, however, finds no support in the histological 

 structure of the retina. Under normal conditions the light reflex 

 is bilateral, that is, light thrown upon one retina only will cause 

 constriction of the pupil in both eyes. In those of the lower ani- 

 mals whose optic nerves cross completely in the chiasma the light 

 reflex, on the contrary, is unilateral, affecting only the eye that 



