CENTRAL NERVOUS SYSTEM. 681 



ciated by von Gudden with the position of the eyes in the head. The extreme 

 lateral position of the eyes as it occurs in the lower mammals permits of but 

 little combination of the two visual fields ; whereas the position in man, in a 

 frontal plane, permits a combination of the fields to a much greater degree. It 

 was in accordance with this principle that partial decussation of these nerves 

 was anticipated by von Gudden in the owl, although the histological evidence 

 for it was not obtained by him. 



In man the evidence from degeneration in the optic nerve points to the 

 presence of a crossed and an uncrossed bundle of fibres in each optic nerve, 

 the uncrossed being much the smaller of the two bundles. The contrary view 

 of complete decussation has been maintained by Michel. 1 The central ends 

 of the afferent optic fibres forming an optic tract are distributed between the 

 anterior quadrigeminum, the geniculatum externum, and the pulviuar of the 

 same side. By central cells located in these latter structures the pathway is 

 continued to the occipital cortex of the hemisphere of the same side, by the 

 fibres passing in the occipital end of the internal capsule and forming the 

 optic radiation. It must be remembered, however, that between the cortex 

 and the primary centres, and again between these centres and the bulb, there 

 are pathways conducting/rom the cortex to the primary centres, and also from 

 the primary centres to the retina. 8 



As the result of partial decussation it will be seen that the relations of the 

 two bulbs to the cortex is this : The nasal or crossed bundle of the contra- 

 lateral bulb and the temporal or uncrossed bundle of the bulb of the same 

 side come together in the optic tract of one side, and are associated with the 

 occipital lobe of that side. Hence it would appear that hemianopsia or 

 blindness in the corresponding halves of the two eyes following a lesion 

 of the optic pathway anywhere behind the chiasma would be, in some 

 measure, explained by this anatomical arrangement. If strictly interpreted 

 an approximately equal number of fibres would be expected for each half 

 of the retina. Such, however, has not been established as the relation be- 

 tween the areas of the bundles. It is to be added, nevertheless, that ana- 

 tomical arrangements such as decussations are probably open to wide indi- 

 vidual variations, and hence that many more observations are required before 

 we can say what is the usual relation between these two bundles. 



With a view to determining the exact location of the cortical centres in 

 man many observations have been made. The cuneus and immediately sur- 

 rounding parts of the cortex are those most concerned. Henschen 3 indicates 

 the calcarine fissure and its immediate neighborhood as the most important 

 locality. Observations on the arrest in the development of the cortex due to 

 early blindness following destruction of the bulb in the case of the blind deaf- 

 mute Laura Bridgman show the entire cuneus to be the central and funda- 

 mental portion, while the associated portions extend some distance on to the 



1 Kolliker's Festschrift, Wurzburg, 1887. 



2 von Monakow : Archivf. Psychiatric, 1890, Bd. xx. H. 3. 



3 Experimentelle und pathologische Untersuchungen iiber der Gehirn, Upsala, 1890 -92. 



