1866.] Mr. Hulke on the Anatomy of the Fovea centralis. 191 



optic nerve-fibres they are arranged in arcades, which contain the last- 

 named fibres and the ganghon-cells imbedded in a granular matrix of 

 finely woven connective tissue. They are less visible in the outer layers, 

 apparently because many of their terminal divisions lose themselves in the 

 (connective tissue) intergranule-layer ; and hence the decussation of the ob- 

 lique bacillary and the vertically radial connective-tissue fibres within the 

 cone-fibre plexus, so conspicuous in the chameleon, is scarcely noticeable in 

 the human retina. 



Deductions. 



1 . Since the total of the eifeets of light upon living tissue will be greater 

 as the extent of tissue traversed by it is greater, and since the relative 

 common sensitiveness of a surface varies with the number of distinct sen- 

 tient elements it contains, it follows that the greater length of the cones 

 and rods, and their greater slenderness, which allows a larger number of 

 them to the superficial unit, are in harmony with the greater sensitiveness 

 of the retina at the macula lutea. Inasmuch, however, as the foveal cones 

 are stouter than the rods, a superficial unit at the centre of the fovea con- 

 tains fewer sentient (i.e. percipient) elements than the same unit near the 

 periphery of the macula lutea ; and on this ground the sensitiveness of the 

 retina at the fovea shoidd be less than that of the retina near the periphery 

 of the macula. On the other hand, the extreme thinness of the inner 

 layers of the retina at the centre of the fovea, places the bacillary layer 

 here most favourably for receiving incident light. 



2. The division of the rods and cones into an outer and an inner seg- 

 ment is natural. The facts in support of this are, the presence of the di- 

 vision in perfectly fresh specimens ; its sharpness and constant occurrence 

 at a definite place ; the constantly rectilinear figure of the outer, and the 

 curvilinear figure of the inner segment ; the different refractive powers of 

 the segments ; and their different behaviour towards staining and chemical 

 solutions. 



3. From these structural differences it is a fair inference that the seg- 

 ments have different physiological meanings. 



The higher refractive power, straight sides, and slender cylindrical or 

 prismatic figure of the outer segment may be adaptations for confining 

 within the segment light incident upon its ends, and for preventing the 

 lateral escape of light through the sides of the segment into neighbouring 

 cones and rods. These considerations incline me to adopt the opinion 

 that this segment has an optical function, an opinion which derives further 

 support from the fact that, in those animals in which the segment is so 

 wide a cylinder that a ray might be incident upon the inner surface of its 

 sides at a small enough angle not to be reflected but to pass out, the seg- 

 ment is insulated by a sheath of black pigment. 



The inner segments of the cones and rods are the specially modified 

 peripheral terminations of the optic nerve-fibres ; and at their junction with 



