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TEXT-BOOK OF PHYSIOLOGY. 



part of the reflex nerve mechanism by which the circular fibers of the 

 iris (the sphincter pupillae) are excited to contraction. These fibers 

 arborize around nerve-cells in the anterior quadrigeminal body, from 

 which axons descend in the posterior longitudinal bundle. In their 

 course they give off collateral branches to the nuclei of origin of the 

 oculo-motor nerve, from which nerve-fibers pass to the iris. 



Light falling on the retina generates nerve impulses which, when 

 conducted through the afferent and efferent pathways just mentioned, 

 stimulate the circular fibers to contraction. The extent of the con- 

 traction will depend on the in- 

 tensity of the light. In the 

 absence of all light the muscle 

 completely relaxes. 



Hemiopia and Hemian- 

 opsia. Division of the optic 

 nerve between the eyeball 

 and the optic chiasm is fol- 

 lowed by complete blindness 

 in the eye of the corresponding 

 side. Owing to the partial 

 decussation of the fibers in the 

 chiasm, division of an optic 

 tract is followed by a loss of 

 sight in the outer two-thirds of 

 the eye of the same side and in 

 the inner third of the eye of the 

 opposite side. To this loss of 

 visual power in the retina the 

 term hemiopia is given. In 

 consequence of this loss of vis- 

 ual power in the retina there is 



a corresponding obscuration or total obliteration of nearly one-half of 

 the visual field, to which the term hemianopsia is given. If, for 

 example, the rightogticjzad is divided there will be rumifigiaTn the 

 outer two-thirds of the right eve ancl the inner third of the left eye^with 

 left lateral hemianopsia, and as the portions of the retina whicri are 

 affected are associated in vision the loss of the visual fields is spoken 

 of as homonymous hemianopsia (Fig. 241). A destructive lesion of 

 the cerebral visual area, the cuneus and the adjacent gray matter on 

 the right side, is also followed by left lateral hemianopsia.* 



FIG. 242. DIAGRAM TO SHOW THE EXIST- 

 ENCE OF HEMIANOPSIA. 



* It should be borne in mind that in both instances the retina itself is unaffected. 

 The impact of light generates, as usual, nerve impulses which proceed as far back- 

 ward as the point of division or destruction. In consequence those portions of the 

 cerebral cortex stimulation of which evokes the sensation of light remain unaffected 

 and the individual does not become aware through sensation, of the presence of a 

 luminous body in the left side of the visual field. 



