1200 THE RODS AND CONES. [BOOK m. 



some creatures, such as birds, the two are not even in apposition, 

 being separated from each other by the intercalation of a spherical 

 globule of a fatty nature often coloured. We may infer that the 

 outer limb serves in some way or other as a purely physical 

 dioptric apparatus, and that the strictly physiological changes, 

 those which initiate the visual nervous impulses, begin in the 

 inner limb. 



A cone, like a rod, consists of an outer and an inner limb. 

 The outer limb (Fig. 145, c.o.) is conical, not cylindrical in form, 

 and, though much shorter than the outer limb of the rod, being 

 in man about 10 /x in length, is in nature in every way similar to 

 it save that it contains no visual purple. The inner limb c.i. is 

 almost in all ways like the inner limb of a rod, save that it is 

 usually broader, the diameter of the cones (outside the fovea) 

 being about 6 /*. Piercing the external limiting membrane it 

 narrows to a fibre, cone fibre, c.f., which however is broader than 

 a rod fibre, and the oval nucleus c.n. which it bears, usually close 

 under the external limiting membrane, is not banded and contains 

 a conspicuous nucleolus. On reaching the outer molecular layer 

 the cone fibre, expanding into a sort of foot, breaks "up into a 

 number of fibrils, which like the corresponding end of the rod 

 fibre cannot be satisfactorily traced beyond their entrance into the 

 layer. 



Over the retina (we are now, it will be remembered, excluding 

 the macula lutea) the rods are much more numerous than the 

 cones, there being about two or three rods in the line joining two 

 cones, and since the outer limbs of the cones are much shorter than 

 those of the rods, a surface view of the retina seen from the outside 

 shews nothing but rods when the tops of the rods are in focus ; if 

 the focus be carried lower down, inwards, the cones will come into 

 view and a mosaic of rods with cones interspersed between them 

 will be seen. Towards the extreme periphery of the retina the 

 cones become relatively more numerous, and close to the ora 

 serrata are alone present. The total number of cones has been 

 calculated to be more than three million. 



To complete the account of the layer of rods and cones, we may 

 add that the outer limbs and also, to a certain extent and under 

 certain conditions of which we shall speak later on, the inner 

 limbs are imbedded in the layer of pigment epithelium, and that 

 between the inner limbs a number of fine acicular cilia-like 

 processes, apparently of cuticular nature, start up from the external 

 limiting membrane, forming a sort of basketwork support for 

 those structures ; on the other side of the limiting membrane, 

 as we have already said, all the nervous structures of the outer 

 nuclear layer, that is to say the rod fibres and cone fibres with 

 their respective nuclei, are supported by neuro-keratinal sponge- 

 work proceeding from the fibres of Muller. 



741. The nuclei of the inner nuclear layer, with the 



