Human physiology : designed for colleges and the higher classes in schools and for general reading . vessels. The object, therefore, of respiration is to in-troduce the air freely into these cells. The air enters throughthe windpipe, and this branches out into tubes called bronchi,which divide and subdivide, till they become very minute, andthen end in the air-cells. These cells are estimated to be aboutthe TtToth of an inch in diameter. Some calculations havebeen made in regard to the extent of surface which they wouldall make if they could be spread out in one sheet. There is ofcourse no gre
Human physiology : designed for colleges and the higher classes in schools and for general reading . vessels. The object, therefore, of respiration is to in-troduce the air freely into these cells. The air enters throughthe windpipe, and this branches out into tubes called bronchi,which divide and subdivide, till they become very minute, andthen end in the air-cells. These cells are estimated to be aboutthe TtToth of an inch in diameter. Some calculations havebeen made in regard to the extent of surface which they wouldall make if they could be spread out in one sheet. There is ofcourse no great accuracy in such calculations; but we can readilysee that the aggregate surface must be immense, and, therefore,the blood is thus very extensively exposed to the action of the RESPIRATION. 87 ^Air-tubes. Relative situation of the lungs and the heart. air. In Fig. 36 is represented the lung of oneside, d; thebranches of the bronchi of the other lung, c, at the lower partot which, e, they are represented as they branch out minutelyto open into the air-cells; b is the trachea or windpipe, and a. LUNGS AND AIR-TUBES. is the larynx at the top of it. It is through a chink called theglottis, in the larynx, that all the air passes as it goes into andout from the lungs. This will be particularly described here-after. 132. In Fig. 33, in the last chapter, you see represented therelative situation of the heart and lungs, the lungs being some-what separated, however, from the heart, to the right and left,in order to show that organ fully. In their natural positionthey are close to the heart, and cover up all of it, except asmall portion in front and to the left side, where its beating isso plainly felt. Both the heart and the lungs are suspended inthe chest to the upper part of the walls of this cavity, and arefastened also to the spinal column in the rear. The large vesselsof the heart, and the bronchi of the lungs, serve as the princi- HUMAN PHYSIOLOGY. Pleura. Mechanism of breathing.
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