756 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



tion, imbibe water, and break up into a series of disks. Subsequently they 

 undergo further changes of form, the final result being a spheroidal body 

 resembling certain myelin drops. When well preserved they exhibit also a 

 longitudinal and very fine striation. Ritter, Hensen, and others think that 

 an axial fibre can be seen in the outer segment of the rods. The internal 

 segments of both rods and cones exhibit a fine longitudinal striation on their 

 surface, which are rather chanuellings or fibres, probably the latter, since 

 they both surround the base of the inner segment, and protrude from its 

 outer extremity, inclosing the base of the outer segment in a kind of cradle. 

 Heusen and some others believe that an axial fibre may be seen in the in- 

 ternal segment of the rods in some animals, but such fibres are not visible in 

 man. The nucleated cone-granule is a bipolar cell, the peripheric process 

 of which is the cone itself, whilst the central extremity arises from the ex- 

 ternal molecular layer, by the coalescence of a number of fine fibrils. The 

 rods and cones, the latter of which present in many animals, as in birds and 

 reptiles, a drop of brilliant-colored oily substance at their peripheric ex- 

 tremities, are the terminal organs of the optic nerve-fibres ; but it still re- 

 mains undecided whether the fibrils in the interior of the inner segments 

 stand in connection with the nerve-fibrils of the fibres in question. The 

 outer segment may possibly represent a non-nervous physical accessory ap- 

 paratus. It is remarkable that section of the optic nerve in the living ani- 

 mal is followed by degeneration of all the elements of the retina except the 

 bacillary layer, which thus scarcely seems to belong to the true nerve-tissues. 

 The layer of hexagonal pigment-cells, commonly termed the pigment epi- 

 thelium of the choroid, belongs both physiologically and morphologically to 

 the retina. The outer segments of the rods and cones are so closely embraced 

 by them, in w 7 ell-preserved specimens, that they will rather separate from 

 the internal segments or fracture through their substance than become de- 

 tached from them. The proper nervous structures of the retina are per- 

 meated and supported by a stroma of connective tissue, which is bounded 

 externally and internally by the membrame limitantes, the external limiting 

 membrane corresponding to the inner extremity of the rods and cones or 

 line of detachment of Jacob's membrane ; and the internal limiting mom- 

 bra ne (8) resting on the hyaloid. The supporting connective tissue ( Fig. l2(i!) ) 

 is closely allied structurally to that of the brain and spinal cord, which has 

 been named Neuroglia by Virchow. By Schultze it is termed spongy con- 

 nective tissue. It fills up the interspaces between the ganglion-cells, gran- 

 ules, and nerve-fibres, and consists of radial supporting fibres (most con- 

 stantly found in the internal granule-layer) and oval homogeneous nuclei. 



616. There are two spots in the Retina, in which the arrangement of the 

 foregoing components is essentially different ; and from these differences, 

 important physiological conclusions may be drawn. One of these is the 

 slight eminence at which the Optic nerve enters, which is a little below and 

 internal to the posterior extremity of the axis of the eye. The other is the 

 " yellow spot of Soemmeriug," which is situated in the exact centre of the 

 retina. The modifications undergone by the Retina at the macula lutea and 

 fovea ccntralis arc considerable. Nearly in the axis of vision an intensely 

 yellow pigment is deposited between the elements of the different layers, 

 with the exception of the bacillar and external granular layer. The centre 

 of the yellow spot is depressed anteriorly (Fig. 270) to form the 1'ovea cen- 

 tralis. The retina is thicker at the macula In tea, of course, excepting at 

 the fovea centralis, than in the adjoining parts, and is at the same time softer 

 and more prone to post-mortem change, probably in consequence of the de- 

 ficiency of the radial supporting connective tissue. The layer of the gan- 

 glion-cells and internal division of the external granule-layer, are the layers 



