GENERAL STRUCTURE OF BIRDS. 



93 



nest. The nutritive functions are performed with extra- 

 ordinary activity in Birds, that the means may be supplied 

 for the maintenance of their locomotive activity. Their blood 

 is particularly rich in red particles, and its heat is usually 

 considerably above that of Mammals. Its circulation is very 

 energetically carried on ; and although the lungs themselves 

 are constructed upon a type inferior to that of Mammals, 

 and the mechanism of respiration is less complete, yet, by an 

 extension of the respiratory organs through the whole fabric, 

 the aeration of the blood is carried on with unequalled 

 energy ( 326). 



80. The arrangement of the organs contained in the cavity 

 of the trunk of Birds differs from that which has beeD 

 described in Mammals, chiefly 

 in this, that there is usually 

 no diaphragm to separate the 

 chest from the abdomen, and 

 that although the lungs them- 

 selves are confined to the upper 

 part of this cavity, they are con- 

 nected with a series of air-sacs 

 which are distributed through 

 the whole of it. In the accom- 

 panying figure, which repre- 

 sents the internal organs of the 

 Ostrich, the heart is seen at a, 

 the stomach at 6, and the in- 

 testinal tube at c. The windpipe, 

 d, opens into the lungs, e, which 

 are themselves small, and are 

 attached to the ribs, instead of 

 lying freely in the cavity of the 

 chest : but the space they would 



Fig. 31. LUNGS OF THE OSTRICH. 



, 



; ///, air-cells,in which are also 

 seen tne tubes b ^ which these air ' 



cellg communicate W uh the lu-ngs. 



. the lieart ; b > the stomach ; c c, the 



.. . , j fm j intestines; d, the trachea; e, the 



Otherwise have OCCUpied IS filled lungs; 

 n-n Vnr fhp IflTfTP flir rpll<3 f f 



up by tne large air- eiis, /,/, 



which communicate freely with 



the lungs and with each other, and which even occupy a large 



part of the cavity of the abdomen, as seen in the figure. 



81. In the class of EEPTILES we find a variety of form so 

 remarkable, that, if we were influenced by this alone, we 

 should scarcely regard the animals it contains as belonging to 



