6oo TEXT-BOOK OF PHYSIOLOGY 



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 visua} sensations and perceptions of words, 

 letters, numbers, and perhaps objects. If the visual word area is destroyed 

 by disease, word blindness is established, and the patient is unable to under- 

 stand written or printed language because of his inability to revive memory 

 images of words. Letter and number blindness may or may not be present 

 according to the extent of the lesion. 



All the special sense areas may, therefore, be said to consist of two smaller 

 areas, a sensor and a psychic, e.g., a cutaneous sensor and a cutaneous psychic, 

 a visual sensor and a visual psychic, etc.; the former is for the development 

 of crude sensation; the latter, more complex in character, is for the correla- 

 tion and formation of sensations whereby judgments or definite conceptions 

 are formed (Campbell). 



Motor Reactions Following Electric Stimulation of Sensor Areas. 

 Electric stimulation of the sensor areas is attended by certain motor reac- 

 tions which vary in accordance with the area stimulated. Thus, when the 

 electrodes are applied to different portions of the occipital lobe the eyeballs 

 are conjugately turned upward, downward, or laterally and to the opposite 

 side; when placed on the upper portion of the superior temporal convolu- 

 tion, the ear is pricked up or retracted, the head is turned to the opposite 

 side and the pupils are dilated; when placed on the hippocampal convolu- 

 tion, there is movement of torsion of the nostril and lips of the same side. 

 It was then assumed that the movements were reflex in character inasmuch 

 as they resembled the movements caused by strong impressions made on the 

 sense-organs, and due to the development of sensations by the electric cur- 

 rents and not an evidence that the area in question is a motor area in the 

 sense that this term is applied to the area along the Rolandic fissure especially 



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. 



