MAMIMALS 



489 



the same side of the bram lays the foundation for full coordination, 

 visual and motorial, between the two eyes.^ 



The semi-decussation of fibres results in great alterations in the finer 

 structure of the lateral geniculate body, the relay station between the optic 

 nerve fibres and the cerebral cortex. It will be seen - that in the lower 

 Vertebrates this structure is insignificant but that in Mammals in which visual 

 projections on a considerable scale are first relayed to the cortex it becomes 

 inuch inore complex, particularly the dorsal nucleus to which this function 

 is assigned. In the lower Mammals this structure is relatively simple and it 









o. 







m^.M^ 



Fig. 646. — The Lamina Cribrosa of the Kitten. 

 Twenty-four hours before birtli (Wilder's stain ; ;■, 160) (Katharine Tansley 



would seem that each optic ner\e fibre connects with several cells in the geniculate 

 body which itself shows no ordered lamination. In the Australian opossum, 

 Trichosurus viilpecula, an agile arboreal animal, however, the dorsal nucleus 

 shows a four-layered structure (Packer, 1941), while in Carnivores and Primates, 

 six layers appear (Le Gros Clark, 1941-42). This system of lamination is associa- 

 ted with the partial decussation of optic nerve fibres in the chiasma — a 

 characteristic of INIammalia : in the opossum crossed fibres terminate in the 

 1st and 3rd layers, uncrossed in the 2nd and 4th ; in the Primates crossed fibres 

 terminate in the 1st, 4th and 6th layers, uncrossed in the 2nd, 3rd and 5th 

 layers (Figs. 647 and 648). In the Primates also each retinal cell is projected 

 onto the geniculate body in a point-to-point manner. The reception unit for 

 each of a pair of retinal corresponding points is thus a band of cells involving 

 three lainina?, while the projection unit onto the visual cortex is a band of cells 

 involving all six layers. 



^ See further, p. 697. 

 2 p. 541. 



