ORGANS OF THE BODY. 



643 



The yellow spot, or macula lutea (fig. 597), from the fact of its being 

 the most sensitive portion of the retina, and also from its peculiar texture, 

 possesses for the histologist the highest interest. 



Fig. 597. Diagram of the macula lutea and fovea ccntralis of the human retina, In vertical section 

 (after Schultze). a, pigmentary; b, columnar; and c, external granular layer; d, inferior fibrous 

 portion of latter; /, internal granular layer ; g, molecular stratum ; A, stratum cellulosum ; , layer 

 of optic fibres. 



If we examine the different layers of this locality (in which the susten- 

 tacular matter is in general but ill developed), in order from within out- 

 wards, we remark that the layer of the optic fibres (i) disappears very 

 early, so that even at a considerable distance from the fovea centralis the 

 stratum of the ganglion corpuscles is in contact (its six or seven layers of 

 cells (h) accommodated to one another like epithelium) with the mem- 

 brana limitans interna. The latter stratum, also, is much thinned, as it 

 passes into the fovea centralis, so that only about three rows of cells (h) 

 are to be seen at the border (H. Muller). These, moreover, are for the 

 most part bipolar over the macula lutea (Merkel, Schultze). The central 

 portion of the fovea is destitute of ganglion cells according to Schultze 

 (and before him Bergmanri). The molecular layer (<?) diminishes like- 

 wise considerably in depth here, and in the very centre possibly disap- 

 pears. This is certainly the case with the internal granular layer (/). 



The remarkable change in the proportion existing between the rods 

 and cones as we approach the macula lutea, and in the latter itself, has 

 been already alluded, to ( 316). In fig. 597, b, we see that here the rods 

 fall in number more and more, until eventually, in the yellow spot, cones 

 alone are to be found (Henle), which increase regularly in length towards 

 the centre of the fovea, up to upwards of O100 mm., diminishing at the 

 same time in thickness. 



We must now deal with these facts, however, somewhat more in detail. 



]n man the bodies of the cones, in most parts of the retina, present a 

 diameter of 0'007-0'006 mm., falling, however, at the edge of the macula 

 lutea to -005-0 -004 mm. More towards the centre of the latter, where 

 rods no longer exist (fig. 598, a, V), they become still narrower, and in 

 the fovea centralis their diameter is only '002 '002 5 mm., probably 

 only 0-0028-0-0033 mm. when quite fresh (Schultze, with H. Mutter and 

 Welcker). Here, then, we have cones almost of the same thickness as 



