14 PRACTICAL ORNITHOLOGY. 



being disposed ideally so as to constitute several sets or systems. 

 One series of organs has relation to the existence and enjoy- 

 ments of the individual, another has reference to the continua- 

 tion of the species. A perfect or full-grown animal, a Rook, 

 for example, requires to introduce into its interior certain sub- 

 stances which are to be converted into materials capable of 

 making up for the loss daily and hourly sustained by the neces- 

 sary expenditure of its fluids. These substances, whether ani- 

 mal or vegetable, are converted into a fluid, which is the source 

 of nutrition to all the organs, and possesses the chemical ])vo- 

 perties of the various parts with which it is finally to be incor- 

 porated. The change thus effected upon the substances intro- 

 duced into the body is termed Assimilation, it being in fact a 

 conversion into like matter. The bird is furnished with wings 

 to carry it abroad in search of food, with feet to enable it to 

 walk, or leap, or stand, or swim, while procuring it, with a 

 bill with which to lay hold of it, a gullet to convey it into its 

 interior, a stomach to pound and digest it, and an intestinal 

 canal, in which it is exposed to the absorbents, and by which 

 the refuse is throw^n out. The nutritious part of the food 

 having been conveyed by the thoracic duct into one of the 

 large veins, is carried into the heart, where it is more inti- 

 mately mixed with the blood, and whence it is propelled along 

 with it to be purified in the lungs, into which air is received 

 from without, and brought into contact with the blood con- 

 tained in vessels ramifying on their cells. The digestion of the 

 crude food and the absorption of its nutritious parts, constitute 

 the process of Assimilation ; the distribution of the nutritious 

 fluid to all parts w^here it is required is termed Circulation ; 

 and the continual purification of the circulating fluid by the 

 action of the atmosphere brought into contact with it, is effected 

 by Respiration ; while the application of this fluid to its various 

 uses in the different parts is termed Secretion. 



What we have at present to attend to is the Function of Fe- 

 spiration, of which the organs are the lungs, two spongy bodies 

 situated within the thorax. The air is admitted to them 

 through a long flexible elastic pipe, which opens into the 

 mouth, and communicates with the nostrils, so that a passage 



