IVIAMMALS 



485 



retina of the rabbit and the pure-cone (or virtually so) retina of the 

 squirrel (Figs. 640 and 641). In the rod-doniinated retina the outer 

 limbs of the rods are long, the outer nuclear layer is thick, there are few 

 ganglion cells and few optic nerve fibres. In the retina of the squirrel, 

 on the other hand, the visual cells themselves are rather unusual and 

 somewhat atj^ical, being arranged in two layers, one outside the 

 other. Those of the inner layer have long striated outer limbs, while 

 in those of the outer layer this structure is shorter and buried in the 

 pigment epithelium. The inner nuclear layer is unusually thick as also 

 is the ganglion cell layer ; there are only 2 to 4 visual cells to each 

 ganglion cell and therefore to each optic nerve fibre, so that the latter 

 layer is again unusually prominent (Arden and Tansley, 1955). 

 According to Vilter (1954) the ratio of cone nuclei to ganglion cells in 

 the souslik, Citellus, is 200.000 : 90,000 for the whole retina. 



An area centralis specifically elaborated for acute vision is found 

 among Placentals, but not commonly (Chievitz, 1891 ; Slonaker, 

 1897 ; Ziirn. 1902) ; most require no specific differentiation for their 

 panoramic vision. When it does occur it may take one of two forms 

 — a band stretching across the posterior part of the fundus or a cir- 

 cular area lying temporal to the optic disc ; occasionally both are 

 combined. 



A band-shaped area is seen in Rodents, most jDronounced in the temporal 

 region ; in the rabbit it is a broad streak 3-4 mm. wide in its central part running 

 just vinderneath the optic disc, and throughout its extent the retina is thicker 

 than elsewhere particularly in its rod-and-cone layers and in the layer of ganglion 

 cells (Chievitz, 1891). According to Krause (1895) the content of visual purple 

 is greater within this area than elsewhere ; and external to it the choroid is 

 thickened (Davis, 1929). The sciuirrel has a similar (pure-cone) band but less 

 well defined. Amonsr the Ungulates, some Artiodactyls (Ruminants such as 

 the ox) have a similar band-shaped area running horizontally above the disc 

 and the lower part of the tapetum. associated with a round area centralis in 

 the temporal region. 



Such a temporal round area is common in Ungulates (sheep, goat, horse, etc.) ; 

 it is also typical of the Carnivores, particularly the Felidse, lying lateral to the 

 optic disc. In this family, particularly in the cat, the tiger and the lion, the 

 area centralis becomes highly differentiated ; the visual elements (cones, 

 according to Thieulin, 1927) are closely packed and ganglion cells are accumulated 

 in several layers, while there is an external depression (an " external fovea ") on 

 the choroidal aspect (Borysiekiewicz, 1887, tiger ; Ziirn, 1902, cat ; Briickner. 

 1949, lion).^ In the dog also there are said to be no rods in the central area 

 (Ziirn, 1902). Among the Primates a central area is present in the Prosimians 

 (Lemur catta, L. macaco, etc.) and among the Simians in the nocturnal 

 Nyctipithecus. In Tarsius, one of the Prosimians, the macvilar region shows a 

 sudden increase in the number of percipient elements : the number of bipolar 

 and ganglion cells also increases, showing that the elements, although still 

 retaining the morphological characteristics of rods, are assuming the physiological 



1 According to Wolfflin (1047), who examined a h\'pnotized lion, the macula is not 

 ophthalmoscopically visible. 



Tarsier 



