XIV INTRODUCTION. 



for in the great Bustard of India, Otis nigriceps, the male 

 is, at least, a third larger than the female, and the differ- 

 ence between the sexes of the Woodcock does not extend 

 to the Snipe. 



The heart has two distinct chambers. Arterial plexuses 

 are often present in various parts of the system. The 

 lungs are undivided, and are attached to the ribs and the 

 dorsal vertebraj. The enveloping membrane is pierced 

 with large holes, communicating with apertures in the 

 bronchial tubes, through which the air inspired passes into 

 large air-sacs in the thorax and abdomen, and even into 

 the interior of many of the bones. In the Hornbills, 

 the very phalanges of the toes are hollow, and communi- 

 cate with the lungs. The oxygen of the air is thus brought 

 into contact, not only with the sub-divisions of the 

 pulmonary artery, but with those of the aortic system. A 

 high and rapid aeration of the blood is thus maintained, 

 and the great energy and irritability of the muscular sys- 

 tem of Birds is a direct consequence of this amount of re- 

 spiration. The trachea, long in all, forms curvatures in 

 some within the sternum, in the male only or in both 

 sexes ; and one or two expansions of the middle portion 

 of the trachea occur in a few birds. The rings of the 

 trachea are entire. 



The organ of voice is situate at the bifurcation of the 

 bronchi, or above it. At the bifurcation is a glottis, furnisli- 

 ed, in singing birds, with several sets of peculiar muscles, and 

 named by some the lower larynx. The true or upper larynx 

 has very little to do with the production of the voice. Where 

 there is only one pair of muscles, the voice is not capable 

 of inflection ; and some few birds want even these, and 

 are perfectly dumb. Those birds that have an extensive 

 musical scale are able to shorten the tracheal tube, the 



