46 THE VERTEBRATE RETINA 



called fiber-baskets, fused with the surfaces of the bases of the visual 

 ceils and anchoring them very firmly in place. 



On the inner side of the limitans lie the nuclei of the rods and cones. 

 The diameters of these are ordinarily much greater than those of the 

 cytoplasmic parts which protrude outward through the limitans. This 

 results in the nuclei piling up into several rows (forming the 'outer 

 nuclear layer') the number of which in a given retina will be roughly 

 equal to the quotient of the square of the diameter of the nucleus divided 

 by the square of the diameter of the predominant type of visual cell. 

 Cones are usually so plump at their bases that there is room for their 

 nuclei to lie up against the limitans or even above it; but a rod nucleus 

 may lie far below its rod, being connected with the latter by a slender 

 thread which winds its way up among the intervening rows of nuclei. 



The Bipolar Layer — From each visual-cell nucleus a short thread-like 

 *foot-piece' travels inward (toward the vitreous) until it clears the other 

 visual-cell nuclei, and then expands into a terminus which may be either 

 smoothly rounded, or branched like a bird's foot (Figs. 19, 22, 23, 24). 

 This is related, as in a handclasp, to a similar arborization at the outer 

 end of a 'bipolar' neuron, whose cell body lies deeper in the retina 

 toward the vitreous. A bipolar dendrite may embrace several or a great 

 many visual-cell termini, so that the number of bipolar cells in a retina 

 is always less than the number of visual cells. The branched process of 

 the bipolar cell which joins to the visual cells, and the similar process 

 from the bipolar cell-body which travels in the opposite direction (to- 

 ward the vitreous) are however much more slender than the cell-body. 

 The bipolar nuclei are consequently piled up in several layers like those 

 of the visual cells, and this second band of nuclei forms the 'inner nu- 

 clear layer' of the retina (Fig. 19). In this layer, along with the nuclei 

 of bipolar cells, are a (usually) smaller number of nuclei belonging to 

 several types of cells which will be mentioned later. 



Some of the bipolar cells each connect with but one cone. Such are 

 the numerous 'midget bipolars' of the primate retina (Fig, 19, b). Other, 

 'diffuse' bipolars (Fig. 19, db) of several types may each embrace a great 

 number of rods, and some cones as well. In many retinae there are diffuse 

 bipolars which connect only with rods, or only with cones; but such 

 elements appear to be lacking in man. 



The inner nuclear layer is separated from the outer nuclear layer by 

 a feltwork of the delicate nerve fibers which make the connections 



