SENSITIVITY VS. ACUITY 69 



equal, a rod should be more sensitive to light than a cone — several times 

 as much photosensitive material is traversed by a pencil of light, when 

 it stimulates a rod, as when it stimulates a cone. Thus in dim light 

 sufficient chemical change may take place in a rod for an effective im- 

 pulse to reach the bipolar; but the same amount of light will not lead to 

 activity in a cone-bipolar alongside. The rod, then, will have the lower 

 threshold of stimulation — it will take less light to set off its transmission 

 of an impulse. Rods can lower their thresholds in evolution (thus in- 

 creasing their sensitivity) by lengthening their outer segments as long 

 as this does not interfere with the nutrition of the rest of the retina from 

 the choriocapillaris. Cones could of course also increase their sensitivity 

 by elongating and by approaching a cylindrical form; but they have not 

 often done so, except as a part of the process of transmuting into rods. 



The second factor influencing sensitivity is the extent of summation. 

 If several visual cells are hammering at the door of a single bipolar, it 

 is more likely to be aroused than if a single visual cell has to try to evoke 

 a bipolar response without aid from others. Nerve cells carry impulses 

 in obedience to the 'all-or-none law', which means that if a given fiber 

 conducts an impulse at all, it transmits it at full strength. The visual 

 cells, however, are not nerve cells (see Chapter 5, section B) and there 

 is no evidence that their foot-pieces obey the all-or-none law. We are 

 consequently free to suppose that when even a little light strikes a 

 rod, something happens photochemically, and that several feeble im- 

 pulses travelling down several rod foot-pieces and impinging upon one 

 bipolar dendrite can start an impulse flowing in that bipolar. In the same 

 weak illumination, a single cone or even a rod would not carry an im- 

 pulse strong enough to awaken a private bipolar. 



Indeed, unless the function of the multiple connections of rods to 

 bipolars is to promote the sensitivity of the whole rod-mechanism in this 

 way, the inward convergence of the retina becomes quite meaningless. 

 Summation tends to destroy visual acuity, and no animal needs or wants 

 diffuse vision for its own sake — he only tolerates it if he must do so in 

 order to gain the sensitivity which happens to be more important to him. 



Bulky visual cells and extensive summation promote sensitivity, but 

 it is inevitably at the expense of visual acuity. Sensitivity and resolving 

 power are thus on the two ends of a see-saw, and whatever sends one up, 

 sends the other down. This relationship holds as well for extra-retinal 

 structures as for the retina itself; for the big lenses and wide pupils of 

 some vertebrates, which produce small bright images and lower the 



