DISEASES OF THE OPTIC NEEVE. 189 



cerebri, and both may suffer together as the result of tumour 

 infiltration. The tract, bending round the crus, lies under cover 

 of the temporo-sphenoidal lobe and may be compressed by new 

 growths in this situation. It is also liable to injury by tumours 

 springing from the base of the skull. 



The visual defect which results from a lesion of the tract is 

 homonymous hemianopia since the tract contains uncrossed 

 fibres from the temporal half of the retina of its own side and 

 crossed fibres from the nasal half of the retina of the opposite 

 side ; hence the blindness involves the nasal half of the field on 

 the side of lesion and the temporal half of the field of the 

 opposite side. (Fig. 16.) 



If no light reflex can be obtained from the blind halves of the 

 retina, or if the reflex from these halves is very sluggish, the 

 hemianopic pupillary reaction is said to be present. This is of 

 value in that it localises the lesion in the chiasma or tract as 

 distinguished from the optic radiation in the hemisphere. This 

 indication is based upon the fact that the fibres which carry the 

 impulse from the eye to the pupil nucleus in all probability leave 

 the optic tracts at their partial termination in the anterior corpora 

 quadrigemina, passing thence to the third nerve nucleus. Con- 

 sequently a lesion which produces the hemianopic reaction must 

 involve the visual fibres not further back than the corpus 

 quadrigeminum. 



The anterior choroidal branch of the internal carotid artery 

 closely follows the course of the optic tract and distributes 

 branches to it. 



The apparent termination of the optic tract in the superior 

 quadrigeminal body, the external geniculatebody, and the pulvinar, 

 might lead to the expectation that disease of these structures 

 should produce defects of vision, but clinical confirmation of this 

 is still needed. 



The optic radiation consists of fibres which pass from the 

 external geniculate body and the pulvinar to the cortex. These 

 fibres lie in the retrolenticular part of the posterior limb of the 

 external capsule ; sweeping thence round the posterior cornu of 



