PIGMENT LAYER OF THE RETINA. 271 



part to be not spheroidal, but elliptical and rod-like* are, accord- 

 ing to the statements of A. Frisch, small crystals, which when 

 perfectly fresh are recognizable under very high powers by their 

 sharp' angles and borders.! They are placed with their long 

 diameter at right angles to the surface of the retina, and 

 therefore, when seen from this surface in profile, are of a rod- 

 like form. Rosow and Frisch found that the largest measured 

 from four to five micromillimeters in length. 



The pathological pigmentation of the retina, known to ophthalmo- 

 logists under the name of Retinitis piymentosa, which is accompanied 

 by diminished sharpness of vision, and leads ultimately to loss of sight, 

 is highly remarkable. In typical cases of such pigmentary degenera- 

 tion we have probably to deal with a partial degeneration of the pig- 

 ment epithelium, and a more or less extended pigmentation of the other 

 layers of the retina, together with a degeneration of the rods and cones, 

 and finally with atrophy of the nervous constituents of the retina. 

 The granular pigment set free by the breaking of some of the pigment 

 epithelial cells makes its way into the other layers of the retina. This, 

 however, is clearly only possible after previous destruction of certain 

 parts of the bacillar layer and of the limitans externa, as well as of 

 the external granule layer. Having reached the deeper layers of the 

 retina, the granular pigment follows the adventitia of the bloodvessels, 

 and thus probably the perivascular lymph sheaths, and thus becomes 

 deposited far and wide in the form of diffused masses. 



Inasmuch as this either occurs as a congenital condition, or takes 

 place in early infancy as a result of hereditary influence, especially in 

 the offspring of blood relations, who, as is well known, furnish a rich 

 contingent of malformations, we may conclude that we have- here a 

 defective development in the outer lamina of the primary eye vesicle, 

 which undergoes conversion (see below, "Development of the Retina,") 

 into the pigment epithelium of the retina. Owing to the intimate 

 relation existing between the pigment cells and the rods and cones, 

 it is inevitable that processes of softening taking place in the pigment 

 cells, and, implicating the bacillar layer, will by regressive extension 

 also come to affect the other layers of the retina. Precise anatomical 

 investigations upon this kind of degeneration, which has been admira- 



* Rosow in Grafe's Archiv, Band ix., Heft, iii., p. 65. 



t A. Frisch, Gestatten des Chorioidalpigmentes, "Forms presented by 

 the choroidal pigment." Sitzungsberichte d. AcacL zu Wien, 18G9, 

 Juli heft. 



