110 
MU. J. W. HTJLKE ON THE ANATOMY OF THE 
turbed, I found the distance of the fovea from the centre of the optic nerve exactly 
equalled If'". 
In vertical sections through the artificially hardened macula the fovea centralis is seen 
to be produced by the radial divergence of the cone-fibres from a central point, and by 
the thinning and curving of the inner retinal layers towards the outer surface of the 
retina as they approach this point. The thickness of the retina decreases in a rapid 
uniform curve from the edge to the centre of the fovea, and very slowly from the foveal 
edge towards the ora retinae. Since the maximum thickness coincides with the edge, 
and the minimum with the centre of the fovea, the former is the most elevated, the 
latter the most depressed part in the macula. 
At the centre of the fovea, where the retina is thinnest, passing from the outer to the 
inner surface, we meet with the following structures : — the bacillary layer, the outer 
limiting membrane, a thin band of areolated connective tissue which transmits a few 
cone-fibres, the inner granule and ganglionic layers represented by scattered cells and 
not separated by a distinct granular layer, optic nerve-fibres, and lastly, the inner limiting 
membrane. 
Minute Structure. 
1. The bacillary layer (Plate VII. fig. 1, i) in the fovea contains cones only; and rods first 
appear midway between the centre of the fovea and the outer border of the macula*. 
The foveal cones (figs. 2 & 3) are longer and more slender than those distant from 
the macula, and there is a similar difference in the length and stoutness of the rods. In 
both rods and cones an outer and an inner segment are observable ; the outer in profile 
is a slender rectangle ; the inner is flask-shaped, and is in the cones much stouter than 
the outer segment, while in the rods it only slightly exceeds the outer segment. 
The inner segment is always associated in the usual manner, by inclusion or by a 
communicating fibre, with an “ outer granule” (figs. 2 & 3). Connexion by a fibre 
obtains (1) with the central cones whose associate outer granules are peripherally dis- 
placed, (2) with all cones associated with a distant outer granule, and (3) always with 
the rods whose smaller diameter does not allow them to include their granule. 
There is a definite outer limiting membrane which has the same relations to the 
bacillary and outer granule-layers as in other situations. 
Outer Granule-layer (fig. 1, 2 ). — Owing to the peripheral situation of the outer granules 
belonging to the central cones, this layer is absent from the centre of the fovea. It 
begins at a short distance from this latter, attains its maximum thickness near the foveal 
edge, then decreases gradually, and again increases towards the border of the macula. 
This variation is due to the presence, at the margin of the fovea, of the granules belonging 
to the central cones in addition to those connected with the cones of this part, and to 
* Kolliker says there are no rods in the macula (Handbuch der Gewebelehre, Aufl. iv. S. 664). This dif- 
ference perhaps proceeds from the difficulty in fixing the exact extent of the macula in chromic-acid preparations, 
in which the colour of the spot is destroyed. 
