MACULA LUTEA AND FOVEA CENTRALIS. 283 



length. H. Miiller and Hulke have also found that the cones 

 of the fovea are longer than those of the rest of the yellow 

 spot.* The thinnest cones of the fovea are 3 ft in thickness 

 at their base. These are distributed over a circular area of 

 about 200 fji in diameter, which, as has been stated, is the 

 diameter of the fovea eentralis, if this be determined by the 

 extent of distribution of the smallest percipient elements. 

 Taking several diameters of this area, and in the perfectly fresh 

 retina of Man, I counted in each fifty cones, all of equal slender- 

 ness. According to this, each cone would have a thickness of 

 about four micromillimeters .; but the fine intervening spaces 

 between the cones ought to be deducted. In hardened speci- 

 mens, measurements made of isolated cones often appear under 

 three micromillimeters. In specimens preserved in alcohol, 

 Henle found that they did not exceed two micromillimeters. 

 Welcker, to whom we are indebted for some very exact 

 measurements of these elements from the examinations of the 

 retina of a criminal, estimated the thickness of the cones of 

 the fovea between 3'1 and 8'6 micromillimeters, or upon the 

 average 3'3.f The long conical external segments, as they run 

 outwards to the choroid, become attenuated to one micro- 

 millimeter or less. They are invested by the pigment sheaths 

 of the coloured cells of the pigment layer, which are for the 

 most part darker at the macula lutea than in the adjoining 

 part of the retina, and reach as far as the uncoloured outer 

 part of these cells. We are in consequence probably able to 

 perceive in Man, as I have already diagrammatically repre- 

 sented in an earlier work J in animals, and -especially in Birds 

 in the perfectly fresh macula lutea, still covered with undis- 

 turbed pigment cells, the natural extremities of the cones as 

 bright spots surrounded by dark pigment. 



As already stated, no remarkable difference is observable in 



* Hulke, Philosophical Transactions, 1857, p. 110. 



t Zeitschrift fur rationelk Medicin, Band xx., p. 176, 1863. For other 

 measurements the reader may be referred to Max Schultze in Reichert 

 and Dubois-Reymond's Archiv, 1861, p. 784, and H. Miiller in the Wurz- 

 burg. Nat. Zeitschrift, Band ii., p. 219, 1861. 



t Archiv fur Mikroskop. Anatomic, Band ii., Taf. xii., fig. 1. 



