228 



RESPIRATION . 



4 



&m'** : 



$*M 



fins' 



the respiratory system, and the consequent augmented amount 



of respiratory process, by 

 which a larger extent of 

 membranous surface became 

 indispensable. The bronchi 

 in birds are continued into 

 the lungs, where they divide 

 into membranous tubes, 

 which permeate their sub- 

 stance; the deeper tubes 

 stand like organ-pipes, and 

 open into the superficial 

 tubes ; and all are covered 

 with small parietal cells, 

 U P 011 which vessels are dis- 

 tributed ; the cells form very 

 elegant, delicate microscopic 

 reticulations, and generally 

 present themselves as six- 

 sided spaces. 



[389. The lungs of man 

 andthealiaarefonn- 

 ed af ter anoth er and a differ- 

 ent type ; the trachea here 

 divides and subdivides, like 

 the branches of a tree, into 

 finer and finer branches, 

 which at first contain carti- 

 lages in their constitution, 



Fig. 240.-A piece from that part of 

 the Serpent's lung which is most scan- 

 tily supplied with vessels, magnified 

 four hundred times. The vessels here 



injected with fine size and vermilion 



in 



but which by and by become 

 membranous, and finally end 

 in blind sacculi, or rather 

 in hollow berry or bud-like 

 and clustered vesicles (figs. 

 241 and 242). The pulmo- 

 nic cells of man and the 

 mammalia, consequently, 

 are not parietal, but termi- 

 nal ; they vary from the 

 6th to the 18th of a line in magnitude, the majority of them 

 measuring between the 8th to the 1 Oth of a line in diameter. 



Fig. 241. Terminal vesicles of the 

 human lung, hanging to a branch of 

 the bronchi as berries hang to their 

 stalk, and distinct from one another. 

 The figure is half a plan, and the mag- 

 nifying power used very high. 



