MACULA. LUTEA AND FOVEA CENTRALIS. 281 



fovea centralis. The colouring material, which is most intense 

 in the fossa, and becomes gradually paler towards the margin 

 of the spot, dees not present a granular appearance, but is com- 

 pletely hyaline, and in consequence only so far disturbs the 

 transparency of the retina at this part, that it absorbs a consi- 

 derable portion of the violet and blue rays before these reach 

 the layer of cones.* 



With the aid of Browning's spectroscope I have very dis- 

 tinctly perceived the shortening at the violet end of the spec- 

 trum under the microscope. I have not, however, by this 

 mode of observation discovered any special absorption-bands. 

 According to Huschke,f the intensity of the colour of the 

 yellow spot is subject to variation, being brighter in blue-eyed 

 than in dark-eyed men. 



The retina is thicker at the macula lutea, of course excepting 

 at the fovea centralis, than in the adjoining parts ; but is at the 

 same time softer and more prone to post-mortem changes. It 

 is probably owing to its greater capacity for imbibition, that 

 it usually swells up at this part soon after death, forming the 

 so-called plica centralis. It is well known that the attenuated 

 centre of the yellow spot tears with great facility, and then 

 appears as a hole in the substance (Foramen centrale). The 

 high degree of softness and destructibility of the substance of 

 the yellow spot receives explanation from the circumstance 

 that the delicate nervous elements far preponderate in this 

 part over the widely distributed plexuses and fibres of the sup- 

 porting tissue found in other parts of the retina. The nervous 

 elements are very numerous and closely packed at the macula 

 lutea, which is in accordance with its physiological importance 

 as the most sensitive spot of the retina. The layer of the 

 ganglion cells and internal division of the external granule 

 layer, which Henle designated external fibrous layer, are the 

 layers that are most obviously thickened. On the other hand, 

 there is no continuous layer of nerve fibres beneath the limitans 



* According to Preyer (Pfliiger's Archiv, Band i., p. 299), the first ob- 

 servations on this point were made by Maxwell. See also Max Schultze, 

 Ueber den gelben Fleck der Eetina. Bonn, 1866. 



t Eingeweidelehre in Soemmering's Anatomie, p. 727. 



