588 TEXT-BOOK OF PHYSIOLOGY 



temporal convolution as well, there is an area in which, it is believed by 

 some clinicians, words are associated with concrete ideas of objects recog- 

 nized by one or more of the senses. To this area the term naming area 

 has been given. It is connected by association fibers with the areas for> 

 word hearing and word seeing. 



7. The Visual Area. The area for visual sensibility has been assigned to 

 portions of the occipital and parietal lobes and may be divided into 

 primary and secondary areas. 



The primary area is located in a triangular-shaped area on the mesial 

 surface of the occipital lobe, which includes the gray matter between 

 the parieto-occipital and the calcarine fissures (the cuneus), and in 

 the gray matter of the first occipital convolution on the lateral aspect 

 of the occipital lobe. The secondary areas are located partly on the, 

 lateral aspect of the occipital lobe and partly in the supra-marginal 

 and angular convolutions of the parietal lobe. 



Focal lesions of the primary area on one side are followed by lateral 

 homonym ous hemianopsia, which, however, does not involve, as a rule, 

 the fovea or macula. 1 It is, therefore, the area of homonym ous half- retinal 

 representation. The location of the area for macular or central vision is 

 uncertain. Henschen locates it in the anterior part of the area near the ex- 

 tremity of the calcarine fissure, and asserts that in each area both maculae 

 are represented. From experiments made on monkeys Schafer locates it in 

 the same region. 



The primary area is connected, on the one hand, with the basal visual 

 centers (the external geniculate body and the thalamus) by the optic radiation 

 and, on the other hand, with the secondary areas by association fibers. 



The secondary areas on the lateral aspect of the occipital lobe are rather 

 extensive, reaching down as far as the third and fourth occipital convolu- 

 tions. Clinical evidence indicates that the cortex of this entire area is 

 associated with the registration or memorization of the visual sensations 

 and perceptions of objects, though it may be subdivided into smaller areas 

 for the registration of the visual sensations of different groups of objects 

 such as geometric and architectonic forms, of persons, places and natural 

 objects. Diseased processes in this region of the brain may result in the 

 condition known as object blindness. The area on the lateral aspect of the 

 parietal lobe (the supramarginal and angular convolutions) are associated 

 with the memorization of the visual sensations and perceptions of words, 



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

 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 reti- 

 nal 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. 



