THE SENSES 



that there will have been quite profound changes in its visual apparatus ; 

 and in addition there must be alterations to enable the eye to with- 

 stand the enormous pressure to which it is at times subjected. 



Putter (1902) has investigated this subject more thoroughly than 

 anyone else and it is upon him that we must rely for our facts. He 

 found that pinnipeds and sirenians also have the eyes importantly modi- 

 fied, but his material representing the latter group was not altogether 

 satisfactory. The changes which he found to have taken place in the 

 eyes of pinnipeds, sirenians, mysticetes and odontocetes he has sum- 

 marized as follows: 



Optic adaptations 



1. Lens almost spherical. 



2. Refractive index higher than in any terrestrial mammal; almost 

 as great as in fish. 



3. Relationship of the neural elements of the retina; unusual num- 

 ber of rods connected with a single ganglion cell. 



4. Superfluous ganglion cells in outer granular layer of retina. 



5. Extensive tapetum lucidum. 



6. Enlargement of fundus at the expense of the pre-equitorial zone. 



Thermal adaptations 



1. Diminution of the cornea in proportion to size of the bulbus. 



2. Form and number of lymphatics in cornea propria; large and 

 relatively few in number. 



3. Unusual development of choroid and of the perichoroidal lymph 

 spaces. 



4. Form of optic orifice: is so much reduced that only the cornea 

 is visible. 



5. Tremendous development of musculature, with immovable bul- 

 bus. 



Hydrostatic adaptations 



1. Curvature of the cornea so as to receive support laterally. 



2. Lateral thickening of the cornea. 



3. Epitheleal cornification of cornea; cornified substance unites di- 

 rectly with elastica anterior. 



4. Thickening of sclera; tremendous at equator and above fundus, 

 slight at corneal sulcus. 



5. Thick optic sheath ; supporting bulbus like a column. 



6. Arterial and venous network of the ciliary blood vessels (retia 

 mirabilia) . 



[65] 



