208 Anatomij and Physiology 



not be entering too much into detail, if I point out a few of 

 the principal varieties which obtain in the tunics in different 

 animals, before we pass on to the humours. As to size, the 

 eyeball is very irregular, and seems not at all to depend upon 

 the bulk of the animal ; for instance, the whale has a very 

 small one in proportion. The mole was supposed, until very 

 recently, to be blind, the eyeball being scarcely perceptible.* 

 Birds, again, have a very large one for the size of the body. 

 The extent of the cornea is equally various in proportion to 

 the relative size of the sclerotica. In birds, the sclerotica, 

 at its junction with the cornea, is armed with a bony rim, 

 scaly and flexible, by which the axis or depth of the eyeball is 

 lengthened or diminished />?'0 re natci. In another class the same 

 object is effected by one part of the sclerotica being thinner 

 than another, and a muscular apparatus is furnished to press 

 upon the yielding surface at pleasure. The choroid (about the 

 subdivision of which so much has been said) presents a very 

 striking feature in some animals, such as are called night 

 animals. A part of the choroid, instead of being covered 

 with black paint, is in these animals of a shining greenish 

 blue, and thus the light is not absorbed, but reflected on to 

 the retina at the other corner of the eyeball ; this tapetum 

 lucidum (shining carpet) being spread on the side of the 

 globe nearer the temples than the nose. The skate is the 

 only fish, I believe, in which it has been observed. Whales 

 have this apparatus in common with other mammiferous 

 animals, such as the dog, cat, cow, &c. In albinoes, which 

 are to be found in all classes of warm-blooded animals (Mr. 

 W. Lawrence states they are not existing in any cold-blooded 

 animals), the black paint is altogether wanting, and hence the 

 individuals are impatient of light. The shape of the iris is 

 different in different animals, being in some oblong instead 

 of round ; and placed vertically in some cases, as in the cat ; 

 or horizontally, as in the horse, in others. 



Within the tunics are contained the humours, 1st, The 



aqueoiis [Jig, S3, a), which is very clear and fluid like water 



.g (in Latin, aqua). It 



/^^^^^^x I ^ ^^ -Tn4r^ni fills the snare be- 



^•ii lliii iliilfl / '"'"^ "'i^y^^^S^fe^ ^"^ crystalline lens 



.z^^^^^^^^i ?.c,...-||f^^^^g^^^ jjj |-]^jg liquidj and di- 



's \sn I PI \ vides it into two com- 



partments, called the 



* See Edinburgh New Philosophical Journal, vol. vii., p. 340. on this 

 subject. 



