CIRCULATING SYSTEM. 



air on the exterior. PRIESTLEY found the colour of blood 

 changed by the air, when enclosed in a moistened bladder, 

 and the same effect was observed by HUNTER, when it was 

 covered with goldbeaters' skin. Need we be surprised, 

 then, at the air having the necessary access to the blood in 

 the lungs or gills, since these organs have been constructed 

 for the particular purpose ? 



In those animals which possess cellular lungs, and which 

 belong chiefly to quadrupeds and birds, the air is convey- 

 ed to them by means of a tube, termed the Windpipe, 

 Trachea or Aspera arteria. This tube is composed of an- 

 nular cartilages, united by a ligamentous elastic substance. 

 On its peripheral surface, it is invested with a strong mem- 

 brane, consisting of very distinct longitudinal fibres. The 

 central surface is covered with a thin, delicate, extremely 

 irritable membrane, which is kept continually moist by a 

 mucous liquor which exhales from it. 



Where the windpipe terminates in the lungs, it subdi- 

 vides into two or more branches, termed Bronchia, which, 

 by farther subdivision, at last terminate in the larger, and 

 these again in the smaller cells of the lungs. 



The upper extremity of the windpipe, or glottis, termi- 

 nates in the pharynx by a peculiar arrangement of cartila- 

 ges, denominated the Larynx. These are moveable, and 

 connected together by membranes, which suffer them to 

 vary their position. The summit of the windpipe ends in 

 a broad annular cartilage, termed the Cricoid, on which 

 the others rest. Immediately above this, on the lateral 

 and sternal sides, is the broad angular cartilage, termed the 

 Thyroid, or Pomum Adami, Adam's Apple, from an ab- 

 surd allusion to the first transgression of our first parent. 

 Two processes, termed horns, connect this cartilage with 

 the bones of the tongue. 



