PIGMENTATION OF LATERAL EYES OF VERTEBRATES 77 



pigment often in fine streams resembling nerve-fibres, along the course 

 of the vessels, may indicate the route which is taken by secretions leaving 

 the gland, it being suggested that these make their way from the spaces 

 of the reticulum into the perivascular spaces and veins along the course 

 of the pigment granules. 



The pigment which occurs in eyes, whether vertebrate or invertebrate, 

 is concerned not only in the reflection or absorption of light, or, as in 

 the iris, the screening off of superfluous rays, but it may also be developed 

 in other situations than the eye or its immediate neighbourhood as a means 

 of attraction or sometimes detraction. An example of the latter is found 

 in the spots of dark pigment known as " false eyes " seen on the sides of 

 certain fishes near the tail, which probably serve to distract attention from 

 the true eye and more vital parts of the body. Pigment, moreover, is 

 not always developed for a useful purpose, but may be formed as a waste 

 product in a degenerating organ, as is the case in the lens of the 

 pineal eye of certain lizards, e.g. the blind-worm {Anguis fragilis). More- 

 over, the deposition of pigment in the retinal cells of the pineal organ 

 may occur as an inherited character, although the need for this pigment 

 has, according to geological evidence, ceased millions of years ago. The 

 clinical importance of the occurrence of pigment in the pineal body in 

 the human subject lies, as previously mentioned, in the possibility of a 

 melanotic sarcoma arising in this organ. The presence of light brown or 

 yellow pigment granules in cells of glial type or the branched connective 

 tissue cells around the vessels and in the capsule or the interlobar and inter- 

 lobular septa appears to be one of many indications of the vestigial nature 

 of the organ, more especially as the amount of this type of pigment is 

 greater in specimens obtained from aged individuals than in the pineal 

 bodies of young children or infants. 



Pigmentation of the Lateral Eyes of Vertebrates 



The situation and types of pigment found in the lateral eyes of verte- 

 brates are so well known that we do not propose to do more than allude 

 to this aspect of the general question. Broadly speaking, the two types 

 of cell in which pigment is deposited or formed, namely (1) epithelial 

 and (2) mesodermic, resemble the same two types of cell in the pineal 

 organ and in the eyes of invertebrates. There are some points of interest, 

 however, with reference to the development of pigment in the lateral eyes 

 which have a bearing on pigmentation in the pineal organ, and we shall 

 therefore give a brief account of the development of pigment in the human 

 eye. This subject has been recently studied by Ida Mann. 1 Minute 



1 Ida Mann, Development of the Human Eye, Cambridge University Press, 

 1928. 



