AMPHIBIOUS MAMMALS 445 



they had tube vision, and lacked what little eye mobility they do have. 

 The lids are closed by an orbicularis oculi, and there is a weakly devel- 

 oped tarsus in the upper lid only, with no Meibomian glands. The 

 corneal epithelium is strongly keratinized, and in addition an abundance 

 of protective oil is produced by the Harderian gland, which is very large, 

 the lacrimal being only one-fourth as large (though in the fetus it is tem- 

 porarily the larger of the two — ontogeny bearing out the probable course 

 of phylogeny). Associated with the Harderian gland there is a fully- 

 formed nictitans containing a stiffening cartilage. There is no trace of a 

 nasolacrimal duct, even in the embryo, and the gummy tears are con- 

 stantly in evidence when the animal is on land. 



The eyeball is large in absolute size as well as relative to the body. In 

 the common 'trained seal' or California sea-lion (Eumetopias californ- 

 icus) it is 39 mm. in diameter both horizontally and vertically, the axis 

 showing some shortening (to 35 mm.) in keeping with the fish-like optics 

 of the eye. The axis in seals varies between 81% and 91% of the vertical 

 globar diameter. In a half grown (ten-foot) elephant seal (Macrorhinus) 

 the globe was found to be 63.2 mm. in horizontal, 65.7 mm. in vertical 

 diameter, with a 55.7 mm. axis — thus, average with respect to the rule 

 just stated, but constituting one of the rare examples of vertical ellip- 

 soidality among vertebrate eyeballs. The eye of the little Phoca vitulina 

 or harbor seal is a bit larger than our own, with the horizontal and verti- 

 cal diameters equal, as is the rule in the group. 



The cornea is circular or slightly elliptical horizontally, and is only 

 slightly arched (except in Macrorhinus) over an anterior chamber which 

 is quite un-fishlike in depth (Fig. 150), periscopy being obtained dynam- 

 ically rather than statically, as mentioned above. In Eumetopias the 

 cornea measures 30 by 25 mm. and its arch is 6.5 mm. high. The pinniped 

 sclera is thickened a bit in the fundus, and the optic nerve sheath is 

 heavy; but these structures are not all out of proportion to the internal 

 ocular volume as they are in the much larger eyes of the whales. The 

 chorioid is orthodox except for the great area and great number of 

 lamellae of the tapetum cellulosum. The ciliary body is very firmly fused 

 to the sclera so that the chorioid cannot be pulled upon during the strong 

 accommodation. It has circular muscle fibers as well as long, powerful 

 meridional bundles, and bears moderately long processes which usually 

 just reach the lens. 



The lens is spherical in Phoca, and it never has the equatorial diameter 

 more than 1.14 times the axial. It is proportionately much larger than in 



