CIRCULATING SYSTEM. $49 
air on the exterior. Priestiey found the colour of blood 
changed by the air, when enclosed in a moistened bladder, 
and the same effect was observed by Hunrer, 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- 
sbrane, consisting of very distinct longitudimal 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 wmdpipe 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 Cricotd, 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. 
