THE CEREBRUM. 



549 



surface known as the angular gyrus. He found that extirpation of the an- 

 gular gyrus alone was followed by a temporary blindness of the opposite eye, 

 which was, however, not hemianopsic in character. 1 He also found that 

 destruction of the occipital lobe together with the angular gyrus gave rise 

 to a more or less enduring hemianopsia, in addition to the transient 

 blindness of the opposite eye. From these and similar facts he concluded 

 that the angular gyrus is the area of representation for the macular or central 

 region of the retina, and the occipital lobes for the corresponding halves 

 of the peripheral portions of the retina. 



It was, however, found by Munk, Schafer, and others that the angular 

 gyrus was not concerned in any way with vision; that extirpation of the 



FIG. 251. DIAGRAM OF THE MOTOR AND SENSOR AREAS ON THE MESIAL SURFACE OF THE MON- 

 KEY BRAIN. (After Horsley and Schafer.) 



occipital lobe alone, especially if the line of division be carried a little further 

 forward on the mesial and inferior surfaces, was followed by homonymous 

 hemianopsia. Additional experiments lead to the conclusion that the 

 area for macular vision is near the anterior extremity of the calcarine fissure, 



1 In a consideration of this subject certain facts connected with visual perception, both hi 

 physiologic and pathologic conditions, must be kept in mind. Thus, visual sensation may arise 

 from stimulation of either the central portion, the macula, or the peripheral portion of the retina 

 or both. In the first instance the vision is termed central or macular; in the second instance, 

 peripheral or retinal. Macular vision is clear, sharp, and distinct; retinal vision progressively 

 indistinct from the center toward the periphery. Division of one optic tract is followed, in con- 

 sequence of the partial decussation of the optic nerve-fibers at the chiasma, by a loss of function 

 in the outer two-thirds of the retina of the same side, both in the central (macular) as well as in 

 its peripheral portions, and the inner one-third of the retina of the opposite side. To this condition 

 the term hemianopsia has been applied. As a result of this want of functional activity of these ret- 

 inal portions on the side of the lesion, rays of light emanating from objects situated in the opposite 

 side of the field of vision will not be perceived when both eyes are directed to the fixation point. 

 To this " blindness " in the opposite half of the field of vision the name hemianopsia is given. In 

 the lesion under consideration (division of one optic tract) the hemianopsia is bilateral, and as 

 it affects the corresponding portions associated in normal vision it is of the homonymous variety. 

 Division of the right optic tract is followed by left lateral homonymous hemianopsia, indicative of the 

 fact that objects in the field of vision to the left of the binocular fixation point are invisible. 



