THE RETINA 19 



The first four layers listed above constitute the neuro- 

 epithelial layer and are the neurones of the first order. The 

 remaining layers are considered the cerebral portion wherein 

 there exists a complex arrangement of nervous and susten- 

 tacular elements resembling, in structure and function, those 

 of the central nervous system of which the retina represents 

 an outlying portion. The fifth layer (internal nuclear layer) 

 which contains the bipolar, horizontal, and amacrine cells, 

 and the sixth layer, comprise the neurones of the second 

 order. The seventh and eighth layers make up the neu- 

 rones of the third order, which pass centripetally to the 

 primary optic center (lateral geniculate body) of the meta- 

 thalamus. 



The thickness of the retina varies considerably in differ- 

 ent regions of the same eye, being typically thickest in the 

 fundus (except at the region of the fovea), and thinnest at 

 the ora serrata where the nervous portion of the retina 

 terminates (Figure 2). Many measurements have been 

 made of the thickness of the various retinal layers in man 

 and in animals, with not too successful agreement among 

 investigators. This is undoubtedly due to the fact that the 

 retina, which is a most delicate membrane, is easily distorted 

 by various histological techniques, so that more or less 

 shrinkage invariably follows fixation. Hence the results 

 may differ considerably. Menner (1929) made measurements 

 of the retina in a number of representative vertebrates, the 

 results of which differ widely from those of my own upon 

 corresponding animals of the same classes of vertebrates. 

 My measurements were made of the retinas of twenty- 

 seven animals from all classes of vertebrates. The eyes were 

 preserved with the same fixative (modified Held's) so that 

 the matter of differential shrinkage due to the use of different 

 fixatives can at least be discounted. The measurements were 

 made in the region of the fundus, approximately midway 

 between optic nerve and ora serrata, and near (.5 to 1 mm.) 

 the ora serrata. These measurements include the fol- 

 lowing: 



