/ ORGANS OF THE SENSES. 



liquid vitreous humour. The bright coloured and highly 

 moveable iris, with its black uvea, appears sometimes de- 

 tached from the free anterior margin of the choroid, and 

 its coloured surface presents aggregations of minute globules 

 like those of the choroidal pigment. The number of folds 

 in the pecten varies from three or four observed in the 

 goat-sucker to nearly thirty observed in several singing birds. 

 The eyes of birds are moved by the ordinary four straight 

 and two oblique muscles, they are provided with lachrymal 

 glands above and glandulae Harderi beneath as in many 

 reptiles, their conjunctiva forms always a large pellucid and 

 highly elastic membrana nectitans at the inner angle of the 

 eye moved by two distinct muscles, and their lower and 

 upper eye -lids are very perfect, being already provided with 

 tarsal cartilages, Meibomian glands, and even eye-lashes, 

 besides the ordinary muscles for their elevation and de- 

 pression. 



As some of the mammalia are organized to fly through 

 the air, some to walk on the earth or burrow in the interior, 

 and others to swim through the dark abyss of the ocean, 

 their organs of vision are very differently constructed to 

 adapt them for receiving visual impressions in these different 

 situations. They are generally more spherical in their form 

 and consquently more thin and membranous in their tunics, 

 and better provided with external means of motion and 

 of protection, than in the oviparous classes of vertebrata. 

 They have generally a lateral aspect to give greater extent 

 to the field of vision, but in nocturnal quadrupeds and 

 in quadrumana and man they are directed forwards with 

 more parallel axes to give greater precision and strength 

 to the visual impressions. The eyes are large in most 

 of the lower herbivorous mammalia, as the ruminatia, the 

 rodentia and most of the pachyderma, and also in most 

 of the nocturnal species, as we generally find in other classes ; 

 and they are very small in the adult state of many of 

 the burrowing quadrupeds, as moles and shrews, in the 

 larger pachyderma and cetacea, in the highest mammalia, 

 where their axes are nearly parallel, and in the cheiroptera, 

 where, as in aquatic birds, their deficiency is compensated 

 for by other organs of sense. The same circumstances 

 which modify the form of the eye and the proportions of 

 its refractive parts in other classes of animals affect the organ 



