240 AN AMERICAN TEXT- HOOK OF PHYSIOLOGY. 



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

 ends of the afferenl optic fibres forming an optic tract arc for the most part 

 distributed between the anterior quadrigeminum, the geniculatum externum, 

 and the pulvinar of the same side. By central cells located in these latter 

 structures the pathway is continued to t In- occipital cortex of the hemisphere 

 of the same side, their axones passing in the occipital end of the internal 

 capsule and forming the optic radiation. It must he remembered, however, 

 that between the cortex and the primary centre-, and again between these 

 centres and the retina, there are pathway- conducting from the cortex to the 

 primary centres, and also from the primary centres to the retina. 2 



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

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

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

 side come together in the optic trad 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 chiasm would be, in some measure, 

 explained by this anatomical arrangement. If strictly interpreted, an ap- 

 proximately equal number of fibres would be expected for each half of the 

 retina. Such, however, has not been established as the relation between the 

 areas of the bundles. It is to be added, nevertheless, that anatomical arrange- 

 ments such as decussations are probably open to wide individual variations, 

 and hence that many more observations are required before we can say what 

 is the hsikiI 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 duv to 

 early blindness following destruction of the retina in the ease of the blind 

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

 fundamental portion, while the associated portions extend some distance on 

 to the convex suffice of the hemisphere. 4 



First Nerve. — Comparative anatomy indicates that the part- of theenceph- 

 alon mediating the sen -e of smell are most closely connected with the cerebral 

 hemispheres, in the sense that phylogenetically the firsl development of the 

 cortex was in connection with the central terminations of the olfactory tracts. 5 

 It happens in man. however, that although the cerebral hemispheres are pro- 

 portionately much more massive than in the lower mammals, yet the olfactory 

 bulbs and tract- are at the same time but poorly developed. The pathway 



1 Kolliker's Festschrift, Wiirtsburg, 1887. 



2 von Monakow: Archivfiir Psychiatric, 1890, Bd. xx. II. ■'!. 



s Henschen: Klinische und anatomische Beitrtigezur Pathologie des Qehirns, Upsala, 1890-92. 



4 Donaldson : American Journal of Psychology, 1892, vol. iv. No. 4. 



5 Sir William Turner : Journal of Anatomy, 1890; Edinger: AnaUmmcher Anzeiger, 1893. 



