780 THE CEREBRAL CORTEX. 



According to Dejerine, 1 lesions of this prefrontal region are also followed by 

 degeneration in the stratum opticum of the thalamus, with subsequent atrophy 

 of the mesial nucleus. Lesions of the parietal lobe and angular gyrus, on 

 the other hand, cause, according to the same author, degenerations in the 

 pulvinar and posterior part of the lateral nucleus. From the base of the third 

 frontal convolution, and from the operculum, fibres pass into the mesial 

 fasciculus of the crusta. 



Connections of the visual area. To the occipital lobe both cortici- 

 fugal and corticipetal fibres can be traced. The corticifugal fibres pass chiefly 

 to the pulvinar thalami and anterior quadrigeminal body of the same side, 2 and 

 are scattered as they pass from the cortex amongst the other fibres of the optic 

 radiations. The occipital lobe also sends fibres to the crusta. 3 The corticipetal 

 fibres (optic radiations) arise from the lateral geniculate body, 4 and also from 

 the pulvinar thalami. They are traceable, according to Ferrier and Turner, 

 not only to the occipital lobe, but also to the angular gyrus, whereas, according 

 to Henschen and to Flechsig, they pass chiefly, if not entirely, to the cuneus 

 and the lips of the calcarine fissure. M. and Mnie. Dejerine have also 

 described connections of the lingual and fusiform lobules with the posterior 

 and inferior parts of the thalamus. 



The occipital lobes are said to be commissurally connected with one 

 another through the splenium of the corpus callosum ; 5 according to 

 Ferrier and Turner, the same statement applies to the angular gyri. The 

 occipital lobe of one side is also connected with the temporal lobe of the 

 opposite side by some of the fibres of the corpus callosum which form part of 

 the tapetum (Sachs). There is, further, a well-marked tract of longitudinally 

 coursing association fibres which passes forwards to connect the occipital with 

 the frontal lobe, and shorter association fibres connecting it with nearer 

 portions of the hemisphere. 



The corticipetal tract for visual perceptions passes from the peripheral sense 

 organs by way of the optic nerves and tracts to the corpora geniculata lateralia, 

 thence as just stated by the optic radiations of Gratiolet to the visual area in 

 the occipital lobes. According to Monakow, 80 per cent, of the fibres of the 

 optic tract pass to the corpus geniculatum laterale and pulvinar, where they 

 terminate, new fibres arising in these nuclei and continuing the optic path to the 

 occipital lobe. Here these (corticipetal) fibres terminate mainly in the cuneus 

 and the lips of the calcarine fissure. But, according to Flechsig, it is from the 

 macula lutea that fibres pass at least indirectly to the calcarine fissure, not 

 from the retina generally. 6 v. Monakow has shown that extirpation of the 

 occipital cortex in young animals leads to atrophy of the cells of the lateral 

 geniculate body, whereas section of the optic tract does not affect the cells, but 

 the grey matter in which they lie. 



The manner in which the fibres pass from the lateral geniculate body to 

 the visual cortex has been studied by Cajal, who finds that they are collected 

 into a bundle situated at the upper and outer part of the cerebral peduncle, 

 and continued into the internal capsule, whence they radiate into the occipital 

 lobe ; here they are continued into the grey matter, where they appear to form 

 the stripe of Gennari. 7 Other fibres of the optic tract go to the anterior corpora 



1 Compt. rend. Soc. de bioL, Paris, 2 Fev. 1897. 



2 v. Monakow, Arch. f. Psijchiat., Berlin, Bde. xx., xxii.; Ferrier and Turner, Phil. 

 Trans., London, 1898, B. 



3 Monakow, loc. cit. ; Dejerine, Compt. rend. Soc. dc bioL, Paris, 1893, p. 193. 



4 Flechsig, " Die Localisation der geistigen Yorgange," Leipzig, 1896 ; Henschen, 

 " Pathol. des Gehirns," Upsala, 1893. 



5 Dejerine, Compt. rend. Soc. de biol., Paris, 1892, p. 579; Vialet, "Les centres ce"rebraux 

 de la vision," 1893. 



6 "Gehirn und Seele," Leipzig, 1896. 



7 Cajal, "Beitr. z. Studien des Med. Obi., u.s.w.," Leipzig, 1896; also "Die Sehrinde," 

 Leipzig, 1900. 



