CHAP, in.] SIGHT. 1197 



ganglionic cells. In addition to these neuroglial elements the 

 retinal blood vessels as we shall presently see carry with them 

 a small amount of actual connective- tissue. 



740. The Nervous Elements. As we have said, there is at 

 least a functional continuity between the rods and cones on the 

 outside and the optic fibres on the inside. The structures of the 

 outer nuclear layer are on the one hand closely and obviously 

 continuous with, indeed form part of, the rods and cones, and 

 on the other hand are if not actually continuous at least func- 

 tionally connected across or by means of the outer molecular layer 

 with the structures of the inner nuclear layer ; while these in turn 

 make connections, across or by means of the inner molecular layer, 

 with the optic fibres either indirectly through the nerve cells of 

 the ganglionic layer or in a more direct manner. 



The rods and cones. Each rod consists of at least two quite 

 distinct parts, of wholly different nature, called respectively the 

 outer and the inner limb. The outer limb (Fig. 145, r.o.) is a 

 cylinder, in man about 30 //, in length by 2 p in diameter, which 

 when seen in a natural condition is transparent, though highly 

 refractive, and also doubly refractive. In prepared specimens it 

 often appears fluted or grooved lengthwise and is very apt to 

 cleave transversely into discs or fragments, of varying thickness. 

 It is probably made up of a number of excessively thin discs, '6 yu, 

 or less in thickness, superimposed on each other and united by 

 some kind of cement substance. The material of which the limb, 

 as a whole, is composed stains deeply with osmic acid, but not 

 at all with carmine and similar staining reagents, and is of a 

 peculiar nature, in many respects resembling but still differing 

 from the medulla of a medullated nerve. It is sensitive to light, 

 swelling up when in the living eye it is exposed to light and 

 shrinking again when the light is removed. During life it is 

 coloured with a peculiar pink colouring matter of which we shall 

 treat later on, called visual purple, and which as we shall see is 

 bleached by the action of light. 



The inner limb (Fig. 145, r.i.) is an elongated ellipsoidal or fusi- 

 form body, about as long as, and at its broadest part slightly 

 broader than, the outer limb, being truncated at its outer end 

 where by a flat surface it lies in contact with the inner end of 

 the outer limb. It is of a wholly different nature from the outer 

 limb, staining with carmine and other staining reagents, and 

 having the ordinary optical features of protoplasmic cell-substance. 

 The outer part lying next to the outer limb exhibits a longi- 

 tudinal striation or fibrillation, but the inner part is faintly and 

 finely granular, the whole however being very transparent. This 

 slight difference of marking indicates a division of the inner limb 

 into an outer and an inner moiety differing in nature from each 

 other; and in some animals the outer part is occupied by a 

 distinctly differentiated structure, called the " ellipsoidal body." 



