RESPIRATION. 91 



the blood. Thus, two sparrows were found by Lavoisier to 

 require as much for their existence as a Guinea-pig, an animal 

 many times as large. Another use ascribed to this arrange- 

 ment by Mr. Hunter, is that of acting, in some degree, as a 

 reservoir of air, to prevent the necessity of frequent respira- 

 tion, which may be supposed inconvenient to birds while mov- 

 ing rapidly on the wing. 



' The voice of birds is more remarkable and beautiful than 

 that of any other animal except man ; and on account of the 

 large quantity of air which they have at command, it is very 

 much more powerful. But the sounds uttered by man and 

 quadrupeds are produced by an organ situated at the top of 

 the windpipe, called the larynx, with the assistance of the 

 mouth, at the top of the windpipe ; whilst, in birds, the organ 

 of voice, or larynx, is situated at the spot where it divides into 

 two parts to go to the lungs on each side, that is to say, at 

 the bottom of the throat. The variations of note are pro^ 

 duced by a little membrane in the tube of the windpipe, which 

 is made to vibrate by the air ; and by means of a number of 

 little muscles, which either tighten or relax it, it is made to 

 give the various notes. Hence, in singing, birds seldom close 

 or make any motions with their beaks. That the voice is 

 produced at this place, has been proved by cutting off the 

 necks of some birds, which still retained the power of uttering 

 their notes. The other parts of the windpipe are not, how- 

 ever, without their use. Some changes of tone are produced 

 by shortening or lengthening it, and others by contracting or 

 enlarging its upper opening into the mouth. The instrument 

 of voice, in'fact, resembles in many respects a musical instru- 

 ment, and the excellence and beauty of the notes of birds 

 depend very much upon imitation and education. The night- 

 ingale, if secluded in a cage when young, never sings so per- 

 fectly as in the wild state, unless exposed in a place where it 

 can hear the song of those which are at liberty. Many birds 

 are capable of imitating a great variety of sounds, and some 

 have been taught to sing very accurately tunes of human 

 composition, merely by playing them upon some instrument 

 in their hearing.' 



The lungs of Reptiles do not consist, like those of the 

 Mammalia and Birds, of a solid organ penetrated in every 

 direction by the air tubes, but of a number of bags of a mem- 

 branous texture, into which the air is conveyed. In some, 

 this is effected by the motion of the ribs and muscles of the 

 abdomen, as in serpents and lizards. In others, as in frogs 



