220 HUMAN PHYSIOLOGY. 



two thirds of the retina of the same side and the nasal third of the 

 retina of the opposite side. The fibers of the optic tract arborize 

 around nerve-cells in the external geniculate body, the pulvinar, and 

 the anterior quadrigeminal body. By means of the optic radiation, 

 the nerve-cells in these different ganglia are brought into relation 

 with the visual center, the cuneus. 



Properties. The optic nerves are insensible to ordinary impres- 

 sions, and convey only the special impressions of light. Division 

 of one of the nerves is attended by complete blindness in the eye of 

 the corresponding side. 



Hemiopia and Hemianopsia. Owing to the decussation of the 

 fibers in the optic chiasm, division of the optic tract produces loss of 

 sight in the outer half of the eye of the same side, and in the inner 

 half of the eye of the opposite side, the blind part being separated 

 from the normal part by a vertical line. The term hemiopia is 

 applied to the loss of function or paralysis of the one half of the 

 retina ; as a result of this, there will be an obliteration of the field 

 of vision on the opposite side to which the term hemianopsia is 

 given. If, for example, che right optic tract be divided, there will 

 be hemiopia in the outer half of the right eye and inner half of the 

 left eye, thus causing left lateral hemianopsia, and as the two 

 halves are affected which correspond in normal vision, the condition 

 is known as homonymous hemianopsia. Lesion of the anterior part 

 of the optic chiasm causes blindness in the inner half of the two 

 eyes. 



Functions. Governs the sense of sight. Receives and conveys to 

 the brain the nerve impulses made by ether vibrations and which 

 give rise to the sensation of light. 



The reflex movements of the iris are called forth by stimulation 

 of the optic nerve. When light falls upon the retina, the nerve im- 

 pulse developed is carried back to the tubercula quadrigemina, where 

 it is transformed into a motor impulse, which then passes outward 

 through the motor oculi nerve to the contractile fibers of the iris and 

 diminishes the size of the pupil. The absence of light is followed by 

 a dilatation of the pupil. 



Third Pair. Motor Oculi Communis. 



Origin. From several groups of nerve-cells situated in the gray 

 matter beneath the aqueduct of Sylvius. 



