416 ADAPTATIONS TO MEDIA AND SUBSTRATES 



Struggle to rise again until the refilling of the gas-bladder had restored 

 his buoyancy, and he might have to rise slowly to avoid the equivalent 

 of caisson disease, due to warmed, decompressed gases foaming his blood. 

 But he would take no harm. 



The eye is in even better position (than the body as a whole) to 

 'stand' this pressure which does not have to be stood; for it contains no 

 air-pockets whose compression to the point of obliteration would cause 

 distortion. The whole body of a sperm whale must have a tremendous 

 problem in keeping half-a-million tons from collapsing his lungs. Indeed, 

 the diaphragm may somehow permit such a collapse, the viscera coming 

 up into the chest cavity and squeezing so much oxygen into the blood 

 that the animal's ability to remain so long below, and his necessity for 

 spouting sixty times before he can sound again, are thereby accounted 

 for. But the whale's eye certainly does not need its thick sclera just be- 

 cause the beast subjects himself to a great range of pressure : 



Etmopterus can live as deeply as any whale ever goes; and in a speci- 

 men of this shark whose eye measured 17.0 mm. in diameter, an inves- 

 tigator found the sclera to be microscopically thin. In the benthonic 

 chimaeras, the sclera is actually discontinuous. In bathypelagic teleosts 

 the scleral cartilage has been reduced from the usual extensive cup to a 

 narrow ring. When all of these fishes which face great pressures, and 

 great changes of pressure, have weaker eyeball-walls than their shallow- 

 water relatives, it hardly looks as though the whales needed to thicken 

 their scleras for pressure-resisting purposes. 



The mere fact that the whale's cornea is relatively thin — though com- 

 pletely exposed to the water — is by itself enough to show that the thick- 

 ness of the sclera can have no relation to high pressure as such. But the 

 differential pressures upon various areas of the cornea, due to wave 

 action, to ordinary swimming movements, and to quick changes in speed 

 and direction, would deform so thin a cornea on so large an eyeball, 

 were that cornea not supported peripherally by an immensely stiffer 

 structure — just as a plastic watch-glass is supported by its unyielding 

 metal bezel. 



While a grape keeps its rotundity nicely while lying on a table, it 

 would flatten out and burst if it were magnified to the size of a house — 

 unless, that is, its skin were thickened out of proportion. The inordin- 

 ately thick scleras of the large whales and the biggest sharks are no 

 thicker than need be. They are a logical result of making a soft-tissued 

 optical instrument almost too large for rigidity in the face of the buffet- 



