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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOCn- I 



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FIG. I. A. Types of cones from the leopard frog, Rana pipieris (c); the snapping turtle, Chelydra 

 serpentina (</); the marsh hawk, Circus hudsoniits (c); and from the circumfoveal eminence of man 

 (/). B. Types of rods from the goldfish, Carassius auratus (/); the leopard frog, a common or 'red' 

 rod ((/); the leopard frog, a so-called green' rod of Schwalbe (f); the flying squirrel, Glaucomys 

 V. volans (/); and from the temporal side of the macula lutea of man C?)- [From Walls (146).] 



von Kries (145) led to the concept of a duplex retina 

 for scotopic and photopic vision, respectively. Most 

 eyes from this point of view are 'mixed', that is they 

 contain two organs in one. When, at about the same 

 period, Boll's (24) discovery of the light-sensitive 

 visual purple (rhodopsin) was made and ably elab- 

 orated by Kiihne (loi), Konig (99) and their col- 

 laborators, this gave further support to the duplicity 

 theory. Visual purple has been found only in the 

 outer limbs of the rod. A historical review of this 

 development is available (69). 



In the external plexiform layer, between the recep- 

 tors and bipolar cells, there are lateral connections, 

 the horizontal cells (fig. 2), joining cones, each of 

 which is embraced by a dendritic basket, to a larger 

 group of rods and cones. There are several baskets 

 to each horizontal cell and axons up to 0.8 mm in 

 length have been found (123). The arrangement sug- 



gests a starting loop or positive feedback for general 

 facilitation. 



Polyak's (122) classification of bipolars is of interest 

 because it is based on primates. There is, in the foveal 

 area, the midget bipolar which is individual or private 

 for a single cone. In the periphery each midget bi- 

 polar receives impulses from a small number of cones. 

 At the opposite end it articulates with a midget 

 ganglion cell by an axodendritic synapse, yet this 

 midget system is not wholly isolated. Mop bipolars 

 also run to the midget ganglion but this contact is 

 axosomatic. These together with all the other bipolar 

 types belong to the diffuse variety which receive a 

 large number of receptors. The mop bipolars possess 

 a kind of dendritic tuft, smaller in the fovea than in 

 the periphery and forming a receptaculum for rod 

 and cone pedicles. Its axosomatic projection is a crude 

 shallow basket touching one or more ganglion cell 



