STRUCTURE OF BIRDS. 51 



properly so called, unless in the Owls, in which the external 

 ear is excessively large, and generally furnished with lateral 

 flaps or opercula, by which it may be closed. The want of 

 this part however is compensated by a circle of feathers, which 

 are capable of being erected or depressed, Fig. 13. The mea- 

 tus externus is a very short membranous canal, in many cases 

 scarcely appreciable ; the membrana t}Tnpani is generally of 

 an elliptical form, somewhat convex externally, and having 

 attached to its inner surface a single ossiculum ; the cavity of 

 the t}^npanum is of moderate size, and communicates with 

 numerous cells, analogous to the mastoid cells in the mammi- 

 fera, but w^hich extend over the greater part of the skull. The 

 eustachian tubes, wdiich communicate w^ith this cavity, are 

 large, entirely osseous, and open, either separately or united, 

 into the aperture of the posterior nares or back part of the 

 mouth. The vestibule and cochlea are small ; and there are 

 three semicircular canals, of wdiich the bony walls are easily 

 separated from the surrounding cellules. 



The nostrils. Fig. 12, e^ are generally small and placed apart, 

 being separated by the intermaxillary bones. They are of vari- 

 ous forms, sometimes open, sometimes covered by feathers, and 

 in other cases having an arched horny or membranous opercu- 

 lum. The nasal cavities are situated at the base of the upper 

 mandible. Their septum is generally complete, but sometimes 

 perforated. They have three turbinated laminae on their 

 outer side, and are covered by a pituitary membrane of delicate 

 texture, bedewed with a lubricating fluid. The posterior nares, 

 Fig. 14, open in the form of a longitudinal slit margined wdth 

 papillae upon the roof of the mouth. It is very doubtful whe- 

 ther the sense of smell be acute in any order of birds, for it has 

 been most satisfactorily proved by Mr. Audubon, that in the 

 Vultures, at least in those of the genus Cathartes that occur in 

 North America, wiiich were supposed to possess it in the 

 greatest perfection, it is so inefficient as not to indicate to them 

 the existence of putrid flesh in their immediate vicinity. 



As birds generally swallow their food w^ithout mastication, 

 and as their tongue is small and sheathed with a horny case. 

 Fig. 15, rt, it is not probable that they possess the sense of 



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