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ANATOMY AND PHYSIOLOGY 
however in some circumstances, by which it is probably 
better adapted to see in the medium through which the 
light is to pass. It is upon the whole small for the size 
of the animal, which would lead to the supposition that 
their locomotion is not great ; for I believe animals that 
swim are in this respect similar to those that fly ; and as 
this tribe come to the surface of the medium in which 
they live, they may be considered in the same view with 
birds which soar, and we find birds that fly to great 
heights and move through a considerable space in search - 
of food, have their eyes larger in proportion to their size. 
The eyelids have but little motion, and do not consist 
©f loose cellular membrane as in quadrupeds, but rather 
of the common adipose membrane of the body. The 
tunica conjunctiva, where it is reflected from the eyelid 
to the eyeball, is perforated all round by small orifices 
of the ducts of a circle of glandular bodies lying behind 
it, and the secretion from them all I believe to be a 
mucus similar to what is found in the turtle and croco- 
dile : there are neither puncta nor lachrymal ducts, so 
that the secretion, whatever it be, is washed off in the 
water. The muscles which open the eyelids are very 
strong ; they take their origin from the head, round the 
optic nerve, which in some requires their being very 
long, and are so broad as almost to make a circular mass 
round the whole of the interior straight muscle of the eye 
itself. They may be divided into four : a superior, an 
inferior, and one at each angle ; as they pass outwards 
to the eyelids they diverge and become broader, and are 
inserted into the inside of the eyelids almost equally all 
