234 BKSPIBATiON. 



swallowing movement, and is after a time regurgitated and dis- 

 charged, in order to make room for a fresh supply. 



In frogs, turtles, serpents, &c., the structure of the lung is a 

 little more complicated, since respiration is more active in these 

 animals, and a more perfect organ is requisite to accomplish the 

 arterial ization of the blood. In these animals, the cavity of the 

 lung, instead of being simple, is divided by incomplete partitions 

 into a number of smaller cavities or " cells." The cells all commu- 

 nicate with the central pulmonary cavity ; and the partitions, which 

 join each other at various angles, are all composed of thin, pro- 

 jecting folds of the lining membrane, with bloodvessels ramifying 

 between them. (Fig. 68.) By this arrangement, 

 Fig. 68. tne ex t en t O f surface presented to the air by the 



pulmonary membrane is much increased, and the 

 arterialization of the blood takes place with a 

 corresponding degree of rapidity. 



In the human subject, and in all the warm- 

 blooded quadrupeds, the lungs are constructed 

 on a plan which is essentially similar to the 

 above, and which differs from it only in the 

 greater extent to which the pulmonary cavity is 

 subdivided, and the surface of the respiratory 

 membrane increased. The respiratory apparatus 

 (Fig. 69) commences with the larynx, which 

 communicates with the pharynx at the upper part of the neck. 

 Then follows the trachea, a membranous tube with cartilaginous 

 rings ; which, upon its entrance into the chest, divides into the right 

 and left bronchus. These again divide successively into secondary 

 and tertiary bronchi; the subdivision continuing, while the bron- 

 chial tubes grow smaller and more numerous, and separate con- 

 stantly from each other. As they diminish in size, the tubes grow 

 more delicate in structure, and the cartilaginous rings and plates 

 disappear from their walls. They are finally reduced, according to 

 Kolliker, to the size of 2 V f an mcn i n diameter ; and are com- 

 posed only of a thin mucous membrane, lined with pavement epi- 

 thelium, which rests upon an elastic fibrous layer. They are then 

 known as the " ultimate bronchial tubes." 



Each ultimate bronchial tube terminates in a division or islet of 

 the pulmonary tissue, about T ' 2 of an inch in diameter, which is 

 termed a " pulmonary lobule." Each pulmonary lobule resembles 

 in its structure the entire frog's lung in miniature. It consists of a 



