OPTIC TRACTS. 



119 



.tc.. lol. 



As regards the course of the optic fibres, the results of experiment show that 

 whilst some have their cell-origin in the periphery presumably in the nerve-cells 

 of the retina others take origin in nerve-cells of parts of the brain (superior 

 corpora quadrigemina, external geniculate body, and pulvinar). Of those nerve- 

 fibres which have their cell-origin in the retina, and which, therefore, undergo 

 degeneration backwards after section of the optic nerve or tract, some end by 

 arborisations between the nerve cells of the external geniculate body and pulvinar, 

 others by arborisations in the more superficial layers of the superior corpora quadri- 

 gemina (see figs. 79, 80, and 

 87). It has been found 

 that after enucleation of 

 the eye the grey matter 

 between the cells in these 

 several parts undergoes 

 atrophy. Those fibres which 

 have their cell-origin in the 

 nerve-centres end by ter- 

 minal arborisations within 

 the retina. Their course 

 will there be followed in 

 the part of this work which 

 deals with the structure of 

 the eye. 



Finally, whilst dealing 

 with the course of the 

 optic nerve-fibres, it may 

 -be as well to mention the 

 probable intercentral con- 

 nections of the nervous 

 visual apparatus. (1.) There 

 is in all probability a double 

 connection between the cor- 

 tex of the occipital lobe 

 (which represents the higher 

 visual centre) and the lower 

 visual centres of the thala- 

 mencephalon and mesen- 

 cephalon by two sets of 

 fibres, one set arising from 

 the cortex and passing 

 through the corona radiata 

 and caudal end of the in- 

 ternal capsule, finding their 



terminal arborisation in the grey matter of the lower optic centres, and another set 

 arising in cells of those centres and finding their terminal arborisation in the 

 occipital cortex. These connections are confined to the parts of the same side of the 

 brain ; there is no evidence of any crossing of the fibres. (2.) There appears to 

 be an intimate connection between the lower optic centres and the grey matter of 

 the bulb and cord. This connection is probably mainly effected through the upper 

 fillet. (3.) The nuclei of the nerves to the muscles of the globe of the eye, both 

 external and internal, are certainly connected with the optic centres. It is possible 

 that this connection may be partly effected through the posterior commissure and 

 posterior longitudinal bundle ; it is probably both crossed and uncrossed : but the 



otorias 



cereliri 



Fig. 87. DIAGRAM OF THE PROBABLE COURSE AND RELATIONS 



OF SOME OF THE OPTIC FIBRES. 



