120 AVES. 



drawn apart by means of the expansions of the ribs, to which the 

 lung is firmly fixed, and the contraction of those fasciculi of the 

 diaphragm that are inserted upon the free surface of the lung. By 

 this action not only are the tubes expanded and lengthened out, but 

 in like manner also the interspaces included between them. The 

 air must of necessity therefore penetrate and distend the terminal 

 bronchial ramifications situated between these interspaces. The 

 lungs derive their supply of air, which they receive by the process 

 just mentioned, partly from the trachea and partly from the air- 

 sacs, the latter forming pneumatic reservoirs, from and into which 

 the- lungs both inspire and expire. Now as each of these reservoirs 

 is in communication with the bronchus through a wide tube, the air 

 they contain is always in a respirable condition, for while the thorax 

 is being expanded that portion also of the pneumatic sac which lies 

 concealed beneath it is expanded also, and sucks in the air upon the 

 one hand through the trunk of the trachea, upon the other, from 

 that part of the air-sac that projects out of the cavity of the thorax, 

 and which may accordingly be seen to collapse during the act of 

 inspiration. When, however, the capacity of the thorax becomes 

 narrowed during expiration, that portion of the air-sac covered by 

 it is compressed and drives out its contained air upon the one hand 

 into the tracheal trunk, upon the other into the projecting part of the 

 air-sac which is then observed to dilate. There appears, moreover, 

 to exist a special provision whereby, when the wings are elevated 

 during flight, and their pressure is consequently removed from the 

 great pneumatic sacs situated in the axilla and between the pectoral 

 muscles, that the sacs become distended with air, which, when the 

 wings are depressed, is driven out of them into the lungs, so that a 

 bird, such as the Lark, while mounting perpendicularly upward to a 

 great altitude in the air, is still enabled to sing without at the same 

 time getting out of breath. 



A pair of small glandular bodies devoid of excretory ducts occur 

 in Birds, and from the situation they occupy may be regarded as 

 Thyroid glands. They are very generally of a rounded form and of 

 a reddish color, richly supplied with vessels, and lie upon either side 

 of the lower part of the trachea, where they are more or less attached 

 by cellular tissue and an arterial ramuscule to the carotid, or else to 

 the jugular vein. Immediately beneath and united to them, there are 

 found in many Birds small corpuscles of a denser texture, and whitish 

 or yellowish color. Both thyroid glands are separated from each 

 other in the middle line by a wide interval. 



