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The Fovea. By J. K. Stonaker. 



In a brief discussion of a subject like this one can but touch upon a few of 

 the many interesting and important points which present themselves in a careful 

 study. The following is mainly an abstract of a paper that will appear soon in 

 a number of the "Journal of Morphology" under the heading of "A Compara- 

 tive Study of the Area of Acute Vision in the Vertebrates," to which any one is 

 referred who may desire a more thorough and systematic treatise on this subject. 



The term Fovea comes from the Latin, and means a pit or depression. It is 

 in this sense that I have used it, and not as the point of acute vision in any eye. 

 The significance of this statement will be readily seen when you consider the fact 

 that many animals do not possess such a pit or fovea, but do have an area of 

 most acute vision. However, when a fovea is present it is the point of most 

 acute vision. 



Before giving a minute description of the fovea a few words concerning its 

 embryological development may be desirable. 



J. H. Chievitz, a German investigator, has done a great deal on this subject. 

 He finds that there is first develoiied, in tlie place where the fovea afterwards 

 appears, a thickening of the retina. This thickening he terms the "Area cen- 

 tralis." It is present in the human foetus about the sixth month, after which 

 time the fovea begins to appear. This increase in thickness is due largely to an 

 accumulation of cells in three layers, viz.: the nerve cell layer and the inner 

 and outer nuclear layers. Then follows a gradual pitting in of the vitreal sur- 

 face, due to a thinning out or pushing toward the periphery of the elements of 

 all the layers of the retina excepting the rod and cone and the pigment layers. 

 This development has proceeded in some animals only to the formation of an 

 area, in others to a very shallow fovea ; while many have a very deep and well- 

 defined depression. In a very shallow fovea all the layers may be present in the 

 center, though somewhat thinner; but in the center of a deep fovea some of the 

 layers will be entireW absent, and those which remain very much reduced, ex- 

 cepting, of course, the rod and cone and the pigment layers. The layer which 

 disappears first is the nerve fibre layer. Then follow the nerve cell layer, inner 

 molecular layer, inner nuclear layer, and, in a very deep fovea, the outer molec- 

 ular and nuclear layers may also be wanting. This is readily seen in the fovese 

 of the turkey, pigeon, robin, hawk or human. We thus have a fovea developed 

 which is always surrounded by an area, or, in the terms of human physiology, a 

 macula lutea. 



