454 



THE NERVOUS SYSTEM. 



FIG. 122. 



\ 



field of vision will be obliterated ; since it is the left half of each retina 

 which receives rays coming from the right side, and vice versa. 



On the other hand a lesion situated at the front of the chiasma, and 

 on the median line, as at C, Fig. 122, will interrupt only the crossed 

 fibres, which supply the inner portion of both eyes. This will produce 

 a hemiopia in which the left eye is blind for the left half of the visual 

 field, and the right eye is blind for its right half. The whole field of 

 vision is therefore perceptible when both eyes are used; but when 

 either of them is covered, the defect becomes apparent. 



Exactly the opposite condition is that in which the left half of the 

 field of vision is obliterated for the right eye, and the right half for the 



left eye. This may be caused 

 by injuries affecting simultane- 

 ously the outer border of the 

 chiasma on both sides, as at D, 

 D, Fig. 122 ; by which the di- 

 rect fibres going to each eye are 

 interrupted, while the cross fibres 

 remain intact. According to 

 the citations of Charcot,* all 

 these forms of hemiopia have 

 been observed in company with 

 the corresponding lesions. 



Lastly, unilateral blindness, 

 that is, blindness of one eye 

 (amblyopia), may be produced 

 by cerebral lesions r indepen- 

 dently of any injury to the optic 

 nerves or tracts. It has already 

 been seen (page 430) that in 

 the dog unilateral blindness, on 

 the opposite side, may result 

 from destruction of a portion 

 of the cortical layer of the hemi- 

 sphere. In man, as shown by 

 Charcot, f hemianaesthesia, from 

 lesion of the posterior part of 

 the internal capsule or corona 

 radiata, is accompanied, as a 

 rule, by impairment of vision in 



the opposite eye. This is no doubt due to interruption of the nerve 

 fibres between the central terminations of the optic tract (corpora 

 geniculata, optic thalamus, and tubercula quadrigemina) and the cere- 



LESIONS OF THE OPTIC NERVES AND TRACTS. A. Le- 

 sion of Eight Optic Nerve ; blindness of Bight Eye. 

 B. Lesion of Left Optic Tract; hemiopia of left 

 side, both eyes. C. Lesion of Decussating Fibres 

 of the Chiasma; internal hemiopia, both eyes. D, 

 D. Double Lesion of Direct Fibres ; external hemi- 

 opia, both eyes. 



* Lefons sur les Localisations dans les Maladies du Cerveau. Paris, 1878, pp 

 124, 125, 126. 

 t Ibid., pp. 119, 120, 121, 129. 



