HISTORY OF WHALES 



*97 



light rays are brought to a focus in front 

 of the retina. Thus when a whale tries 

 to view objects above the water level 

 (Diagram 5) it is troubled not only with 

 astigmatism because of differences in 

 curvature of the cornea in the two direc- 

 tions (Diagram 4), but also in being unable 

 to change the curvature of the lens for 

 optical accommodation because of de- 

 fective ciliary muscles. According to 

 Matthiessen (1886-93) the cornea of a 

 whale living in salt water functions like 

 a weak dispersive lens and, although the 

 curvature of the cornea has little optical 

 significance, an increase in size brings 

 more light through the pupil to the 

 peripheral parts. From an optical stand- 

 point it would be to the advantage of a 

 whale to have a large cornea, for they 

 have to see with a weak light when 

 feeding below the surface. The deeper a 

 whale dives the weaker the light it 

 has to see by. There may be some 

 question, however, as to the limits of 

 vision in the inky darkness that prevails 

 in the depths of the sea. The bottlenosed 

 beaked whale (Hyperoodoti), which hunts 

 for cuttlefish in the depths of the ocean, 

 has an eye with a more pronounced 

 corneal curvature, which permits more 

 light to reach the peripheral parts of the 

 retina, and there is a corresponding 

 thickening of the rim and its abutment, 

 the sclera. A large cornea is disadvan- 

 tageous to the whale for architectural and 

 thermal reasons. The toothed whales 

 have larger corneas than the whalebone 

 whales, but this enlargement has been 

 compensated by the thickening of the 

 cornea itself. 



Fossil casts or impressions of soft parts 

 like the eyes are of very rare occurrence, 

 so that for evidence in regard to the 

 visual powers of the extinct Eocene 

 zeuglodonts one must examine natural 

 and artificial casts of the cranial cavities 



of their skulls. These endocranial casts 

 furnish data on the proportions of the 

 several parts of the brain and the relations 

 of the nerves, and also afford much infor- 

 mation about the sensory capacities of the 

 animals in question. A number of these 

 brain casts have been described by Fraas, 

 Stromer, Elliot Smith, Andrews, and 

 Dart. The most complete endocranial 

 cast known to us at present was figured 

 and described by Stromer in 1908. 



Brains of late Eocene zeuglodonts 

 appear shrivelled and shrunken in com- 

 parison with the earlier forms, and there 

 is reason to believe that these animals 

 had not such keen sight as some of their 

 contemporary land dwelling allies. A 

 relative loss of the faculty of sight appears 

 to be indicated by the smallness of the 

 cerebrum and its lack of growth in later 

 forms. The side to side contraction of the 

 area between the olfactory peduncles and 

 the tuber cinereum demonstrates the relative 

 atrophy of the optic chiasma in the 

 zeuglodont brain. A reduction of the 

 faculty of sight and possibly of smell 

 involves a compensatory dependence by 

 the zeuglodonts upon the sense of touch 

 in the muzzle. This interpretation, ac- 

 cording to Dart (19x3), is suggested by 

 the trigeminal specialization in the brains 

 of zeuglodonts. One might infer from 

 the known braincasts that these zeuglo- 

 donts experienced some difficulty in adjust- 

 ing their eyes for under-water vision and 

 that a defective eyesight may have been 

 one of the causes that contributed to the 

 final extinction of this widely distributed 

 group. 



In its general features, the cast of the 

 brain of Prosqualodon (Dart, 19x3) shows 

 a remarkable resemblance to the zeuglo- 

 dont type, particularly in the cerebellar 

 enlargement and hypertrophy of the 

 trigeminus. On the other hand the 

 brain of Prosqualodon has a well defined 



