SENSES AND THE CENTRAL NERVOUS SYSTEM 227 



circumferentially to the interior of the globe through the ciHary body. 

 The latter, which consists of concentric involuntary muscle fibres, also 

 connects the iris with the choroid. The iris divides the eyeball in front of 

 the lens into an anterior and a posterior chamber, both filled with a clear 

 fluid called the aqueous humour. The part of the eyeball behind the lens 

 is filled with a transparent gelatinous substance called the vitreous 

 humour. 



The eye functions very much like a camera, though its lens (together 

 with the cornea, the aqueous and vitreous humours, all of which help to 

 refract the incoming light) is far more complex. The iris can be compared 

 with the camera's diaphragm, and the retina with the photographic film 

 or plate. When at rest, our eye is focused at infinity, i.e. the light rays 

 reflected by distant objects form a clear picture on the retina. Now, we 

 all know that when close-up pictures are to be snapped, the lens of the 

 camera must be pulled out, since otherwise the image would form behind 

 the photographic plate and thus become blurred. Some fish and snakes 

 can 'screw out' their lenses in a similar way, but man, for one, cannot, and 

 to look at a nearby object he must accommodate his eye, i.e. change the 

 shape of his lens. The more spherical the lens, the nearer to the front of 

 the eye the image is formed, and the more compressed antero-posteriorly 

 the lens the further from the front of the eye is the image. Accommodation 

 for near vision is effected by contraction of the concentric ciliary muscle 

 and consequent slackening of the suspensory ligament. As the muscle 

 contracts, the choroid is drawn forward and the ciliary processes are 

 brought closer to the lens, thus relaxing its tension. 



Visibility under water is much poorer than it is on land, since a great 

 deal of light is absorbed by the upper layers of water. Thus, ofTour coasts, 

 90 per cent of white light is absorbed by the time we go down to five 

 fathoms, and only i per cent of white light penetrates below twenty 

 fathoms. Below 215 fathoms, the sea is pitch black, no matter how clear 

 the water or how bright the sunshine. Horizontal visibility is further 

 decreased by the scarcity of light-reflecting objects. The well-known 

 ophthalmologist, G. L. Walls, of the University of Michigan, who wrote a 

 book of nearly 800 pages on the vision of vertebrates, therefore assumed 

 that the maximum visibility even in shallow seas is about fifty-six feet. 

 In other words, big whales would be unable to see their own flukes. Since, 

 moieover, many Cetaceans spend part of their time below the ice where 

 only a very small amount of light penetrates, and since many of them are 

 predominantly nocturnal animals, it is not surprising that vision does not 

 play as important a part for them as for other mammals. 



Of all Cetaceans, porpoises and dolphins, which feed on fish relatively 

 near the surface, have by far the keenest vision. The Bottlenose Dolphins 



