Observatious on the structure of the central foveii ol the human eye. 15 



but it is slightly more (0'133 mm.) if reckoned to a line following 

 the general curve of the outer surface of the retina. 



3. As to the relative size of the cones over the fovea. 



A point which is apparently shown by our specimens and photo- 

 graphs, viz. that the dip of the external fovea is not fully compensated 

 by the extra length of the cones as we approach its centre, cannot, 

 for reasons which we have already entered into (p. 10) be regarded as 

 settled satisfactorily. The appearance however of a general cupping in 

 of the external surface is so very well marked (Fig. 1), and the cones 

 would have to be so enormously long to compensate for the depression, 

 that we are of opinion the general external surface including the 

 pigmentary layer must also have been somewhat cupped in. ^) We are 

 unable however to offer satisfactory evidence on this point against 

 the positive statement of Max Schultze ^) that the dip of the limitans 

 externa is fully compensated by the elongation of the cones. In any 

 case there is certainly some compensation, for the outer segments of 

 the cones unquestionably become gradually longer towards the centre 

 of the fovea. But on the other hand the inner segments are decidedly 

 shorter, measuring 0'020 mm. as compared with a length of 0'028 mm., 

 at a distance of 0*3 mm. from the centre. At the very edge of the 

 external fovea they are again reduced to 0-021 mm. The total length 

 of the cones at the centre we are for the before-stated reasons unable 

 to give, but at 0*3 mm. from the centre, where the outer segments 

 are fairly well preserved, the length of these is 0-05 mm. and at the 

 edge of the external fovea 0*03 mm. The diameter of the inner seg- 

 ments at the centre of the fovea we find to be 0*002 1 mm. which is 

 almost exactly the measurement obtained by H. Müller. At the edge 

 of the external fovea the diameter of the inner segments is somewhat 

 greater viz 0'0028 mm. Schultze was of opinion that the measurement 

 given by H. Müller was smaller than it should be in consequence of 

 a shrinkage of the retina from the mode of preparation, but there is 

 no evidence of shrinkage of the retinal elements in our preparations, 



If this is the case the choroid is probably slightly bulged opposite the 

 fovea. 



-) Archiv f. mikr. Anatomic. Bd. II. 



