586 



HEMIANOPIA AND HEMIANOPSIA. 



Other observers maintain that there is complete decussation of all the fibres in the chiasma. 

 Hence, section of one optic nerve causes dilatation of the pupil and blindness on the same side, 

 while section of one optic tract causes dilatation of the pupil and blindness of the opposite eye 

 (Knoll). In osseous fishes, both optic nerves are isolated and merely cross over each other, while 

 in the cyclostomata they do not cross at all. [Total decussation occurs in those animals where 

 the eyes do not act together. ] 



Injury of the external geniculate body and section of the anterior brachium have the same 

 effect as section of the optic tract of the same side ( 359 Bechtercw). 



In very rare cases the decussation is absent in man, so that the right tract passes directly into 

 the right eyeball, and the left into the left eyeball (Vcsalius, Caldani), the sight not being 

 interfered with. 



It is quite certain that the individual fibres do not divide in the chiasma. Two commissures, 

 the inferior commissure (Gudden) and Meynert's commissure, unite both optic tracts further 

 back. 



A special commissure (C. inferior) extends in a curved form across the posterior angle of the 

 chiasma (Gudden). It does not degenerate after enucleation of the eyeballs, so that it is 

 regarded as an intercentral connection. After excision of an eye, there is central degeneration 

 of the fibres of the optic nerve entering the eyeball (Gudden), and in man about the half of the 

 fibres in the corresponding optic tract (Baumgarten, Mohr). After section of both optic nerves, 

 or enucleation of both eyeballs, there is a degeneration, proceeding centrally, of the whole optic 



tract. The degeneration extends to the origins in 

 the corpora quadrigemina, corpora geniculata, and 

 pulvinar, but not into the conducting paths lead- 

 ing to the cortical visual centre (v. Monakow) ( 

 378, IV. I.). 



[Hemianopia and Hemianopsia. When one 

 optic tract is interfered with or divided, there is 

 interference with or loss of sight in the lateral 

 halves of both retina?, the blind part being 

 separated from the other half of the field of vision 

 by a vertical line. When it is spoken of as 

 paralysis of one-half of the retina, the term 

 hemiopia, or preferably hemianopia, is applied 

 to it ; when with reference to the field of vision, 

 the term hemianopsia is used (see Eye). Suppose 

 the left optic tract to be divided or pressed upon 

 by a tumour at K (fig. 425), then the outer half 

 of the left and the inner half of the right eye are 

 blind, causing right lateral hemianopsia, i.e., the 

 two halves are affected which correspond in ordi- 

 nary vision, so that the condition is spoken of 

 homonymous hemianopsia. Suppose the lesion 

 to be at T (fig. 425), then there is paralysis of the 

 r lg. 425. inner halves of both eyes, causing double temporal 



Diagram of the decussation of the optic tracts, hemianopsia. When there are two lesions at NN, 

 T, semi-decussation in the chiasma ; TQ, which is very rare, the outer halves of both retinae 

 decussation of fibres behind the ext. geni- ft re paralysed, so that there is double nasal hemi- 

 culate bodies (CG); a' b, fibres which do not anopsia. In order to explain some of the eye 

 decussate in the chiasma ; b' a', fibres pro- symptoms that occasionally occur in cerebral 

 ceeding from the right eye, and coming disease, Charcot has supposed that some of the 

 together in the left hemisphere (LOG) ; fibres which pass from the external geniculate 

 LOG, K, lesion of the left optic tract pro- body to the visual centres in the occipital lobe 

 ducing right lateral hemianopsia ; A, lesion cr ss behind the corpora quadrigemina, and this 

 in the left hemisphere producing crossed is represented in the diagram as occurring at 

 amblyopia (right eye) ; T, lesion producing TQ, in the corpora quadrigemina. On this view, 

 temporal hemianopsia ; N"N, lesion pro- a ^ the occipital cortical fibres from one eye 

 ducing nasal hemianopsia. would ultimately pass to the cortex of the occi- 



pital lobe of the opposite hemisphere. This 

 view, however, by no means explains all the facts, for in cases of homonymous hemianopsia 

 the point of central vision on both sides, i.e., both maculae luteae are always unaffected, so that 

 it is assumed that each macula lutea is connected with both hemispheres. The second crossing 

 suggested by Charcot probably does not occur. Affections of the optic nerve, e.g., between the 

 eyeball and the chiasma, i.e., in the orbit, optic foramen, or within the skull, affect one eye 

 only; of the middle of the chiasma, cause temporal hemiopia ; of the optic tract, between 

 the chiasma and occipital cortex, hemiopia, which is always symmetrical (Gowers). 



Fig. 424, reduced from that of Gowers, shows the relation of the fields of vision of the retina, 

 tracts, and the cerebral optic centre. 



