NERVOUS CONSTITUENTS OF THE RETINA. 245 



ment, which is the fundamental condition of the visual act, 

 must take place at this spot. 



The rods are cylindrical bodies with a length at the back 

 part of the eye in Man of fifty to sixty micromillimeters, and 

 a thickness of two micromillimeters ; more anteriorly, near 

 the ora serrata, they are somewhat shorter, though equally 

 thick. They stand in close apposition, so that there is not 

 much more space between them than results from their cylin- 

 drical form. Distributed at regular intervals between the rods, 

 except at the macula lutea and the ora serrata, are the flask- 

 shaped cones. The distance between two cones amounts on 

 an average to eight or ten micromillimeters, the intervening 

 space being occupied by three or four rods in a straight line. 

 The average thickness of the cones at their base, with the 

 exception of those of the macula lutea, varies from six to seven 

 micromillimeters. Externally they diminish like a wine flask, 

 and are not unfrequently slightly distended just above the 

 base ; they terminate in a conical point, the extremity of 

 which is in a plane anterior to that of the rods, so that the 

 cones are shorter than the adjoining rods. Like the rods, the 

 cones also become shorter towards the ora serrata, and still 

 earlier increase in thickness. 



In both structures two essentially different segments are 

 distinguishable, which have been named respectively the ex- 

 ternal and the internal segment, by W. Krause.* The dis- 

 tinction is most marked, and has been longes.t known, in the 

 cones, in which the conical point characterized by its higher 

 refractive power had previously been indicated by H. Miiller 

 as the cone-rod. The rods present a similar structure, except 

 that the external segment is not conical in form, but for the 

 most part regularly cylindrical.! The junction between the 

 external and the internal segment in the rods of Man is situ- 



* Gottinger Nachrichten, 1861, No. 2. Zeitschrift fur rat. Medidn, 

 Bandxi., p. 175. 1861. 



t In Amphibia only (Frog, Triton, Axolotl) does the diameter of the 

 external segment of the rod slightly decrease towards the outer extremity. 

 This is particularly observable in young animals (A rchiv fur Mikroskop. 

 Anatomic, Band iii., Taf. xiii., fig. 14), and may in some instances ren- 

 der it impossible to distinguish the rods from the cones. 



