12 Grolding Bild and Schäfer, 



length of the outer segments, for although the general outline of the 

 layer which they form is well marked and fairly sharply defined 

 (PL I. Fig. 2), it is evident, from other specimens of the fovea in which 

 these structures have been better preserved, that the outer segments 

 are much longer than there shown. If they are to be assumed to 

 have been in the unaltered condition long enough to compensate for 

 the dipping in of the external limiting membrane at the fovea, the 

 cones would have to measure in total length no less than 0"133 mm. 

 which is much longer than any that have been measured by previous 

 workers 1). In diameter also the outer segments are nearly if not 

 quite as thick as the inner segments, so that they appear in well 

 preserved specimens closely packed and with their outer ends some- 

 what inclined towards the centre. 



It is obvious from this figure and from the photographs upon 

 which it is based that either the accounts and delineations which have 

 been given by former observers are largely erroneous or that there 

 is, as H. Müller supposed, great individual variation in the central 

 fovea of the human eye. Whatever be the explanation it must be 

 stated that it is a much more difficult matter than would at first 

 sight appear to obtain irreproachable sections through this part. The 

 retina is singularly liable to be thrown into folds just here after death 

 due no doubt to its imbibing fluid fi'om the vitreous humour. If to 

 obviate this the vitreous is completely removed the retina is thereby 

 liable to be mechanically injured. Considering that histologists are not 

 usually in a position to obtain a normal human eye instantly after 

 removal from the ])ody, it is probably therefore only very rarely that 

 a section passing indubitably through the very centre of a well-pre- 

 served human fovea has actually been obtained. Schwalbe, who is 

 with justice regarded as the first authority on the structure of the 

 eye and who has devoted much attention to the retina in particulai-, 

 has as we have seen been compelled to fall back for the desciiption 

 of this part upon a single specimen sent to him by another observer 

 and that one a section passing according to his own statement not 



') Schultze found the central cones plus the pigment layer to measure 01 18 mm. 



