WHALES 415 



whether Physeter has extra-long rods compared with other genera of its 

 group, most of which are shallow-swimming forms. 



The most striking thing about the whale eye, as the reader's first 

 glances at the illustrations must have shown him, is its phenomenally 

 thick sclera. It results in the eye having actually a relatively small inter- 

 nal volume. Beer was unable to learn of a whale eye with an internal 

 capacity of more than 123 cc, though whale eyes may be several inches 

 in diameter; whereas the 37-millimeter eye of an ocean sun-fish (Mold 

 mold) which he measured would hold 180 cc. The thick sclerotic coat 

 and optic nerve sheath (Fig. 141) are generally assumed to be adaptive 

 to the resistance of the water pressure endured by the sounding cetacean. 

 The same thick sclera is seen in the monstrous (65-foot) whale shark 

 (Rhineodon typus) and in the (also huge) basking shark (Selache max- 

 ima) — and the latter, at least, goes to great depths. But it is again seen 

 in the elephants, which seldom get their heads wet. 



Adaptation to Water Pressure? — The generalization has been made 

 that animals which are very large for their kind, and whose eyes are 

 relatively small for their size, have extremely thick scleras. In such forms 

 as Cryptobranchus, possibly also in the European sturgeons, we can 

 shrug this off as a 'disharmony'. The eyes of elephants, large sharks, and 

 whales are too well built to make such a dismissal plausible. The thick 

 sclera is seemingly really necessary, to maintain ocular rigidity against 

 the pull and haul of the extra-ocular muscles. For, the absolute strength 

 of a muscle increases as the cube of its linear dimensions; and when an 

 object is greatly enlarged without a change of its material, its rigidity 

 declines. A plank in the proportions of a toothpick would be far more 

 supple than the toothpick itself. 



But the whale eye, though it may be as large as a grapefruit, is sup- 

 posed not to move. Does the water-pressure theory then account for its 

 extra-thick rind? The scleras of deep-sea fishes are not thick, but we tend 

 to suppose that these animals are somehow adjusted to the hideous pres- 

 sure bearing upon them (see p. 394). In actual fact, no fish needs any 

 sort of adjustment to meet great pressures, for there is an incompressible 

 fluid continuum throughout his tissues, whose pressure at all times equals 

 exactly that upon the body surface. Any surface fish which can stand 

 cold water, and whose anatomical topography will tolerate having the 

 swim-bladder completely collapsed without ripping anything among the 

 viscera, can plunge slowly for a mile or for as many miles as the deeps 

 provide. He would have to be slightly insane to do so, he might have a 



