Specialized Cells: Muscles, Nerves and Sense Organs 115 



whole surface. Others, such as starfishes, have specially sensitive eye 

 spots, and illumination brings about movement either towards or 

 away from the light.^ 



Out of these primitive responses, living things have developed 

 during the ages the most astonishing sense organs. In a world in which 

 the competition for food was severe, the possession of even slightly 

 more effective means of finding it must have had great survival value. 

 It is not surprising that the struggle for existence has produced a 

 steady improvement until the beautiful arrangements possessed by 

 the higher animals at the present time were elaborated. Special cells 

 have been developed which have in many cases a most extraordinary 

 sensitiveness to particular kinds of stimulus. The olfactory cells of 

 the nose of some animals is capable of detecting extremely small 

 quantities of some chemical compounds. It is well known that dogs 

 can follow a human being by smell the day after he has gone along 

 the track. It has been demonstrated that in the nose of the dog a single 

 molecule of a fatty acid must be capable of stimulating the smell 

 organ.^ 



Insects also have an extraordinary smell ability. Female gypsy 

 moths can, it is said, attract the male from distances up to two miles 

 by means of an odour. The male will respond to the excised scent 

 gland of the female or even to a piece of blotting paper which has 

 been touched by the scent gland but if the antennae of the males 

 are removed, they are quite unresponsive. Parasitic wasps can locate 

 their larval hosts, even when buried several centimetres in wood. 



The human eye is a most fantastic example of fitness and efficiency. 

 It is incredibly complicated. The retina of each eye contains about 

 120 million sensitive elements in the form of rods and about 6 million 

 cones. About a million nerve fibres take their responses to stimula- 

 tion into the brain. It is obvious that many of these sensitive elements 

 must share a single nerve fibre. 



The purpose of this arrangement is to provide both night and day 

 vision. The rods, which provide for night vision, are extremely sensi- 

 tive and many of them share a single nerve fibre. This means that the 

 nerve fibre has a large number of terminations, which are spread out 

 so as to catch all the available light. It has been shown by Dr M. H. 

 Pirenne that the sensitiveness of the eye approaches the maximum 

 possible, which could be reached if every quantum of light impinging 

 on the retina produced a sensation. Careful measurements by Pirenne 



IN. Millott, 'Animal Photosensitivity', Endeavour. XVI, 19, 1957. 

 ^ One molecule of the fatty acid, when applied to about 10* sense cells, can 

 be detected. The chance of two molecules reaching one cell in the smell organ 

 under these circumstances is remote. 



