HISTORY OF WHALES 



195 



external flap of fibr o-elastic cartilage that 

 covers the external nostril. Returning 

 to the whalebone whales we observe that 

 the tubular narial passages slope forward 

 and upward, and are furnished with 

 spiral folds that fit into one another 

 when pressure is exerted from without. 

 Although the closing mechanism is con- 

 structed along different lines in the two 

 groups, in both the nostrils are effectually 

 closed when the animal is submerged. 



VISUAL SENSE 



Most whales have such acute powers of 

 hearing that it is an open question whether 

 they see or merely hear objects that 

 attract their attention. Nevertheless we 

 have indubitable evidence that the eyes 

 of cetaceans are adjusted to the specific 

 functions they have to perform. In its 

 gross features the whale eye differs from 

 that of a land mammal in having the 

 eyeball immovable, eyelids without eye- 

 lashes, no tarsus or supporting cartilage 

 in the eyelid, no Meibomian glands, and 

 a. downward direction of the eye axis. 

 As a result of an aquatic mode of life 

 whales have acquired a more spherical 

 lens and a greatly thickened sclera. 

 The ciliary processes and their muscles 

 are reduced in size and have lost their 

 original function of controlling the shape 

 of the lens. The tension of the suspensory 

 membrane (the zonula Tjnnii) is not great 

 enough to flatten the anterior surface of 

 the lens, and as a general rule the latter 

 retains a more or less spherical shape. 

 Whales thus lack the power of accommo- 

 dation. 



Since the functional irritant for the 

 dioptrical or recipient apparatus of any 

 mammal is light, to function correctly 

 the eye must be constructed so that clear 

 cut pictures are formed on any spot of the 

 retina. Inasmuch as the refractive index 

 .of sea water is not the same as that of 



air, the eyes of whales must be adjusted 

 to function when immersed. There is so 

 little difference in the refractive indices 

 of sea water and of the aqueous humor 

 that the cornea is dioptrically inoperative 

 when the eye is submerged, and the 

 dioptric apparatus is thus reduced to the 

 lens. To compensate for this (fig. zo, 

 diagram 6), however, the curvatures of 

 the anterior and posterior surfaces, as 

 well as the axis or height of the lens and 

 the refractive indices of the liquid media, 

 have been corrected so that light rays 

 passing through sea water are brought to 

 a focus on the retina. 



A number of investigators have pointed 

 out that the eyes of many of our living 

 land mammals do not possess the power 

 of accommodation, and it is not unlikely 

 that similar conditions prevailed among 

 the animals of the past. Diagram z 

 represents a normal or emmetropic eye, 

 in which parallel rays of light reaching 

 the eye through the air are brought to 

 a focus on the retina. Now if a land 

 mammal with an eye of this sort should 

 be transferred to a pelagic habitat, its 

 vision would be impaired, for only the 

 outlines of objects viewed under water 

 would be recognizable, because the light 

 rays reaching the eye through sea water 

 would be focussed beyond the retina 

 (Diagram 3). Even with the maximum 

 accommodation, in which the anterior 

 and posterior surfaces of the lens have 

 the same radius of curvature, parallel 

 rays of light reaching the eye through 

 sea water would be focussed beyond the 

 retina. Unless the axis or height of the 

 lens were increased and the refractive 

 indices of the surrounding liquid media 

 were altered slightly, the eye would 

 function imperfectly when immersed in 

 sea water. 



Under atmospheric conditions the eyes 

 of most whales are myopic, that is the 



