BIRDS. 177 



eye, which, by the assistance of a remarkable muscular apparatus, 

 may be drawn before the eye like a curtain. The cornea is 

 very convex. Although birds have no external cartilaginous 

 ear, all appear to enjoy the faculty of hearing. Some of the 

 nocturnal birds have the auditory opening surrounded by fea- 

 thers. The organs of smell are concealed in the base of the 

 bill, and the breadth of the nasal openings determine their form. 

 The sense of smell in the vulture and the raven has been said 

 to be so very acute, that they can smell carrion at an immense 

 distance ; but this from recent observation seems doubtful. The 

 tongue in birds is supported by a production of the hyoid bone. 

 The taste is not very delicate. 



Digestion in birds is in proportion to the activity of their 

 life, and the quantity of their respiration. The stomach is com- 

 posed, first of the crop, which is a dilatation of the rcsophagus at 

 the base of the neck. The food remains in this duct for some 

 time, and there imbibes a fluid analogous to the saliva, which is 

 secreted from the inside of the canal. When softened by the 

 action of heat and moisture it passes by little and little into a 

 strong muscular bag called the gizzard, where the food is tri- 

 turated the more easily, that many species swallow little stones to 

 increase the effect. This gizzard, it has been remarked, is strong- 

 est in birds which have slender bills, and which are of course 

 unable to break down their food ; in those which feed on flesh or 

 fish the muscles are much weaker, and the stomach almost mem- 

 branous. By the outlet of this stomach the food, reduced to 

 a species of chyme, flows through the remainder of the intesti- 

 nal canal, where the nutritious parts are absorbed, and the re- 

 mainder expelled by a cloaca, an orifice common to the urinary 

 and genital organs. 



Birds, such as the partridge and common fowl, whose young 

 are able to walk and feed themselves on their departure from 

 the egg, do not generally live in pairs. One male serves many 

 females, and the young are entirely trusted to the maternal 

 care. The greater part of birds, however, are blind and help- 

 less at their birth, and their parents are therefore under the ne- 

 cessity of providing for their subsistence. Pigeons disgorge half- 

 digested grains to feed their young ; and linnets bring them larva 

 of insects or the soft parts of other animals. These live always 



VOL. i. M 



