Whence the Shadows? 227 



When viewed from above, the brain case of the typical shark looks 

 like a distorted hourglass whose upper half is larger than its lower. The 

 upper half of the hourglass contains the biggest parts of the shark's 

 brain— the olfactory lobes and the centers of the sense of smell. Because 

 of the enormous size of these lobes, the shark's brain has been dubbed a 

 "brain of smell." 



Curiously enough, the cerebral hemispheres of man— the seat of his 

 highest mental faculties— seem to have evolved from primitive olfactory 

 lobes, originally the major channels through which man's evolutionary 

 forebears gathered information. (This is an extremely significant fact.) 



Thanks to the "brain of smell," the ability of the shark to detect the 

 scent of food is amazing. Dr. Gilbert Percy Whitley of Australia, who has 

 made a life-long study of sharks, tells of sharks following bathers who 

 had merely scratched their legs while wading in the shallows. The sharks 

 had detected these minute traces of blood. "I also found," Whitley re- 

 ports, "that they would come very quickly to a spot in which sea-birds' 

 eggs had been broken in the water, so that they must have a keen sense 

 of smell." 



Experiments have shown that a certain species of male moth can de- 

 tect the scent of a female moth at a distance of 2 miles. Studies indicate 

 that the ability of sharks and other fish to detect smells is often similarly 

 keen, although almost nothing is known of scent diffusion in water. But, 

 as zoologist A. D. Hasler has remarked, "We are concerned here with a 

 sense of such refined acuity that it defies comparable attainment by the 

 most sensitive instruments of modern chemical analysis." 



All animals, man and Selachians included, ultimately use a liquid 

 medium to employ their sense of smell. In terrestrial animals, the odor 

 of a smellable substance travels through the air to a mucous film in the 

 nostril. There, captured in a liquid, the smell is registered and relayed 

 by the olfactory nerves to the brain, which interprets what it is. In the 

 Selachians, the odor of the smellable substance travels through the water 

 to the olfactory pits, or nostrils, on the underside of the shark's snout. 

 Almost invariably, the pits are not used for breathing-, the result being 

 that they have only one purpose: the detection of smells. The pits are 

 lined with a sensitive membrane that is usually folded into a series of 

 ridges coated with scent-sensitive tissue. As the shark or ray swims, a cur- 

 rent of water constantly passes over this olfactory tissue. Since the swim- 

 ming is more or less uninterrupted, so is the flow of smell-messages that 

 are being transmitted to its scent-oriented brain. 



If a shark's nostrils are plugged and no water is allowed to flow 

 over the olfactory membrane, the shark usually will swim over food 

 without detecting it merely by sight. When its nostrils are unplugged, 

 the shark can zero in on food even though it has been hidden. 



