MAMMALS 



481 



animals, particularly the Rodents, possess a capillary rascularization 

 on the optic nerve-head associated with a button-like projection 

 visible ophthalmoscopically, reminiscent of the papillary conns 

 of Reptiles. 1 A j^ersistent hyaloid artery arising from the disc is 

 more connnon and is normal in a large number of Rodents and all 

 Ruminants. 



The depth to wliicli the vessels jjenetrate the retina varies con- 

 siderably. In some Insectivora (the hedgehog and the mole) the large 

 vessels lie sui^erficially, each casting a shadow ophthalmoscopically 



'^^^ 



^ Ah 



¥^ 



E«?^-'. 



"• .> * 



Fig. G3S. — Section of the Retina of the Rabbit. 



Inclucliiig tlie niedullated nerve fibres. Tlie large vessels are clearl\' -pve- 

 retiiial (I. C. ]\Iichaelsoii). 



(Barrett. 1880) : similarly in some Rodents (mouse, rabbit) they are 

 also verj^ superficial and only ]jartially embedded (Fig. 638). The 

 capillaries may n(jt penetrate so deeply into the retinal tissues as in 

 man. In the horse and the rabbit they reach the nerve-fibre layer only ; 

 in the cat the ganglion layer ; but in most diurnal types with a 

 holangiotic retina the capillaries are reflected in the outer plexiform 

 laj^er as in man. In these the reticular capillary system is usually well 

 developed and consists of t\\o main networks, an internal lying in the 

 nerve-fibre layer, and an external lying in the outer portion of the inner 

 nuclear layer, the meshes of the deeper net being smaller than the 



^ Compare the Marsupials, p. 440. 



S.O.— TOL. I. 



31 



