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iuii<' has been inflated with air it will float on water so long as healthy. 

 This fact is made useful iu deciding whether or not an animal was born 

 dead or died after its birth. Each lung is divided into a certain num- 

 ber of lobes, which are subdivided into numberless lobules (little lobes). 

 A little bronchial tube terminates in every one of these small lobules. 

 The little tube then divides into minute branches which open into the 

 air-cells (pulmonary vesicles) of the lungs. The air-cells are little sacs 

 having a diameter varying from one-seventieth to one-two-huudredth 

 of an inch ; they have but one opening, the communication with the 

 branches of the little bronchial tubes. Small blood-vessels ramify- 

 in the walls of the air-cells. The walls of the air-cells are extremely 

 thin and consist of two layers ; the blood-vessels, being between the 

 layers, thus expose a surface to the air in two different cells. The 

 air-cells are the consummation of the intricate structures forming the 

 respiratory apjiaratus. They are of prime importance, all the rest be- 

 ing complementary. It is within these cells that is accomplished all that 

 constitutes the ultimate result of the function of respiration. It is here 

 the exchange of gases takes place. As before stated, the walls of the 

 cells arc very thin ; so, also, are the walls of the blood-vessels. Through 

 these walls escapes from the blood the carbonic-acid gas that has been 

 absorbed by the blood in its circulation through the different parts of 

 the body ; and through these walls is absorbed by the blood, from the 

 air iu the air-cells, the oxygen gas which is the life-giving element of 

 the atmosphere. 



Throughout the substance of the lungs the bronchial tubes terminate 

 in the air-cells. Blood-vessels are in every part ; likewise lymphatic 

 vessels and nerves, which require no more than mention iu this paper. 



To give a rude idea of the arrangement of the respiratory apparatus 

 as a whole, we may profitably compare it to a tree, for the want of a 

 better illustration. Say that all the respiratory tract from the nasal 

 openings back to the pharynx are the roots in the ground ; the larynx 

 may be compared to the large part of the trunk just above the earth ; 

 the windpipe will represent the body or the bole of the tree ; the right 

 and left bronchi may be considered the first two or largest limbs ; 

 the bronchial tubes are then comparable to all the rest of the limbs, 

 branches and twigs, gradually becoming smaller and branching out 

 more frequently, until the smallest twigs terminate by having a leaf 

 appended; and to complete the comparison we may as well say the 

 leaf will do to represent the air-cells. Now, imagine all parts in and 

 around the limbs, branches, twigs, and leaves filled in with some ma- 

 terial (say cotton or tow) and the whole covered over with silk or satin 

 to about the beginning of the branches on each of the first two large 

 limbs. The cotton or tow represents the lung-tissue and the silk or 

 satin covering will represent the pleura. It must be admitted that this 

 is rather a rough way to handle such delicate structures, but if the 

 reader is possessed of a good imagination he may gain, at least, a com- 

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