ARE^ CENTRALES AND FOVE/E 



185 



and it is more than likely that its area centralis is an area of especial 

 sensitivity, not of acuity at all.* 



In only two genera of snakes is a fovea positively known to occur. 

 The East Indian long-nosed tree-snake, Dryophis mycterizans (Fig. 79) 

 has a keyhole-shaped pupil with the slot of the keyhole pointing forward 

 well beyond the rim of the lens, thus constituting an extensive aphakic 

 space. The fovea in Dryophis is at the outer rim of the retina on the 

 temporal or caudal side of the eye, and a line from it through the center 

 of the lens passes out through the slot in the keyhole pupil, along a 

 groove on the cheek in front of the eye, and straight forward parallel 

 to the axis of the body. It is significant that herpetologists have long 



Fig. 77 — Area centralis and fovea in fishes. 



a, portion of retina from sagittal section of eye of a shark, Mustelus mustelus. After Franz. 

 ac- area centralis (note increased length and concentration of visual cells, number of 

 ganglion cells). 



b, eye of a teleost, Serranus scriba, horizontal section; retina shown in black. After Kahmann. 

 /- fovea; n- nasal side; ^ temporal side. 



*The same suspicion falls upon the ungulates and carnivores, hardly any of which are 

 strictly diurnal. The majority of afoveate arese would in faa bear re-investigation with this 

 suspicion in mind, for it is already known that the special area of the opossum has its 

 histological peculiarities aimed at increasing sensitivity, not resolving power. There appear 

 to be circumscribed central areas of extreme sensitivity in the retinje of the echidnas and 

 some 'edentates', for these nocturnal animals are reported to wince and close their eyes in evi- 

 dent distress whenever the light-beam of an ophthalmoscope strikes the small area mentioned. 

 This, by the way, is quite a different thing from the phenomenon in the human eye which 

 has led some careless ophthalmologists to refer to the macula lutea as the 'most sensitive' 

 spot in the human retina. It is the least sensitive spot, becoming quite blind in low illumi- 

 nation — but it happens to be the pupillomotor area, the part of the retina which controls 

 reflexly the closure of the pupil when illumination is suddenly increased. The fovea of the 

 owl is also the pupillomotor area — and here, perhaps, it is extremely sensitive as well, in 

 the true sense of 'sensitive'. 



