STRUCTURE OF FOVEA CEtfTKALIS. 



53 



the line of pigment remains level. Hulke figured the limitans externa as plane, and 

 others (e.g., Merkel, Kuhnt, Schwalbe) have formally denied this cupping of the 

 m. limitans externa. Undoubtedly, however, it exists and may, as a glance at fig. 60 

 will render evident, be as deep as that of the limitans interim or true fovea, from 

 which for purposes of description it may be distinguished by the name of fovea 

 c.rlt'rna. On the other hand, the inner segments of the cones are not longer here 

 than elsewhere, but are if anything somewhat shorter than at the edge of the fovea, 

 but this is more than compensated for by the greater length of their outer segments. 

 The cones are also more slender in the very middle of the fovea than elsewhere, here 

 measuring not more than 0'002mm., whereas at the edge of the fovea they arc 

 double this in diameter. The outer nuclear layer (fig. 00, 6) of the macula lutea is 



m.l.c. 



m.ll 



Fig. 60. DIAGRAM OF A SECTION THROUGH THE FOVKA OKNTKALIS. (The outlines of this figure have 

 been traced from a photograph.) Magnified 350 diameters. (From a preparation by C. H. 

 Golding-Bird.) 



2, ganglionic layer ; 4, inner nuclear ; 6, outer nuclear layer, the cone-fibres forming the so-called 

 external fibrous layer; 7, cones; 7/t.l.e., mernbrana limitans externa; m.l.i. membrana limitans 

 interna. 



occupied in the greater part of its thickness by the very long and obliquely 

 disposed cone-fibres ; the nuclei are only two or three deep, and take up a compara- 

 tively small portion of the layer, which was termed the outer fibrous layer by Henle. 

 Over most of the yellow spot the cone-nuclei are placed close up to the limitans 

 externa, but a short distance from the middle of the fovea they begin to be removed 

 from the limitans, and at the centre of the fovea they are close to the outer molecular 

 layer. The cells of the pigmentary layer are smaller but deeper (O'Olmm. x 

 0*01 6mm.) and more strongly pigmented in the macula lutea than in the rest of 

 the retina. The hyaloid membrane of the vitreous humour is very thin over the 

 centre of the fovea. The choroid coat is thickened opposite the fovea, the 

 thickening being due to an accumulation of capillary blood-vessels, which here 

 occupy not only their usual position but also that of the layer of larger blood- 

 vessels, and even encroach on the lamina suprachoroidea (Nue'l). 



