52 THE VERTEBRATE RETINA 



which transmit visual impulses, though many are centrifugal. It is heavily 

 ensheathed by tendinous and vascular coats continuous on the one hand 

 with the sclera and on the other hand with the meningeal coverings of 

 the brain, and is divided by internal septa, of connective tissue and neu- 

 roglia, into many fiber-bundles. The central retinal artery and vein join 

 the nerve at some distance from the eyeball and run through its center 

 to emerge within the eye at the nerve head, where they branch over the 

 inner surface of the retina. The optic 'nerve' is called such only for 

 convenience. It is not a true nerve but, like the retina, an ectopic portion 

 of the brain itself. 



Within the cranium the two optic nerves cross through each other 

 and continue, as the 'optic tracts', into the brain. The crossing or 

 'chiasma' is especially complex in man and in all other mammals, for 

 in them only some of the fibers from each eye cross into the opposite 

 optic tract, the others going directly into the tract on the same side. 

 In other vertebrates, the crossing or 'decussation' of the fibers is com- 

 plete — that is, all of the fibers from each optic nerve enter the opposite 

 side of the brain (Fig. 21). No special advantage is gained by such an 

 arrangement — it arose mysteriously along with the numerous similar 

 decussations in the tracts of the brain, brain stem, and spinal cord; but 

 there is a special value of partial decussation which will be found ex- 

 plained in Chapter 10, section D. Even where the decussation is total, 

 the chiasma is seldom a simple anatomical crossing of one whole optic 

 nerve over the other. This is indeed the situation in most fishes; but 

 elsewhere the two nerves are interwoven to a greater or lesser extent 

 (Fig. 21b). 



(B) Types of Visual Cells 



General Types— Rods versus Cones — The visual cells of vertebrates 

 are of two general types which were long ago given the names 'rod' and 

 'cone' — though with our superior modern knowledge of their phylogen- 

 etic ramifications and physiological characteristics we might wish that 

 a more apt pair of names could be substituted for the traditional ones. 

 In a given retina containing both highly sensitive visual cells (rods) 

 and relatively insensitive ones (cones) , the high- and low-threshold cells 

 can always be told apart; but the rod of one retina may resemble struc- 

 turally the cone of another, or may give evidence of having been recently 



