CHAP, ii.] THE BEAIN. 787 



shew any sign of atrophy, and the part of the optic tract which 

 does not degenerate is the inferior commissure connecting the 

 two median corpora geniculata. Obviously these parts are asso- 

 ciated with functions of the brain other than those of sight. 

 The lateral corpora geniculata, the pulvinar and the anterior 

 corpora quadrigemina, are, we may repeat, alone to be regarded 

 as the chief central parts in which the optic nerves end. We 

 may also repeat that owing to the peculiarity of the optic decus- 

 sation each optic nerve thus finds its endings in both sides of 

 the brain. 



497. Though the above three bodies are undoubtedly the 

 chief endings of the optic nerve, three primary visual centres, 

 if we may so call them, it is also believed that some fibres of 

 the optic tract, making connections with neither of these three 

 bodies, pass by the crus cerebri straight to certain parts of the 

 cerebral hemisphere (Fig. 138, d) ; but this fourth ending is by 

 no means so clearly established as are the other three. And 

 undoubtedly the main connection of the cerebral hemisphere 

 with the optic tract is not a direct one, but an indirect one, 

 through the three bodies in question. A number of fibres pro- 

 ceeding from the occipital cortex and reaching the thalamus 

 through the hind limb of the internal capsule form what is 

 called the ' optic radiation,' These fibres beginning (or ending) 

 in the cortex of the occipital region, end (or begin), (Fig. 138, 

 op. racl) to a large extent, in the pulvinar and in the lateral 

 corpus geniculatum, but also in the anterior corpus quadrigem- 

 inurru reaching it by the anterior brachium. When in a new- 

 born animal the occipital cortex, or even a certain part of the 

 occipital cortex is removed, these three bodies atrophy, and 

 the atrophy extends to the optic tract and nerve ; conversely 

 removal of both eyes in a new-born animal leads not only to 

 atrophy of the three bodies in question, but also to imperfect 

 growth of the occipital lobes. Not only so, but even in the full- 

 grown animal removal of the occipital cortex entails a degenera- 

 tion and atrophy of these three bodies, the details differing in 

 different animals according to their kind, the degeneration in 

 some animals reaching into the optic tract and nerve. And 

 even in man degeneration of the external corpus geniculatum, of 

 the pulvinar, and to some extent of the anterior corpus quadri- 

 geminum and of the optic tract has been observed to result from 

 disease of the occipital lobe. This is a remarkable circum- 

 stance ; for we may assume that the fibres carrying impulses 

 from say the corpus geniculatum to the occipital lobe, are axis 

 cylinder processes starting from cells in the former structure 

 to end in the latter, and therefore having their trophic centres 

 in the former. The degeneration occurring in the corpus gen- 

 iculatum as a result of the removal of or disease in the occipital 

 cortex is a different thing from the degeneration which takes 



