. Outlines of the comparative physiology and morphology of animals. Anatomy, Comparative; Physiology, Comparative. BLOOD SYSTEM. 359 Structure.—We have already said (page 204) that two tubes go down from the throat into the body cavity —the posterior one, the gullet, to the stomach, and the anterior one, the windpipe, or trachea, to the lungs. The one is soft and flaccid, but muscular ; the other is a firm, open tube, being kept open by a series of carti- laginous rings. This is necessary in order that the air may come and go with the least resistance possible. The trachea is capped above by t
. Outlines of the comparative physiology and morphology of animals. Anatomy, Comparative; Physiology, Comparative. BLOOD SYSTEM. 359 Structure.—We have already said (page 204) that two tubes go down from the throat into the body cavity —the posterior one, the gullet, to the stomach, and the anterior one, the windpipe, or trachea, to the lungs. The one is soft and flaccid, but muscular ; the other is a firm, open tube, being kept open by a series of carti- laginous rings. This is necessary in order that the air may come and go with the least resistance possible. The trachea is capped above by the larynx, as already explained (page 204). After a course of about five inches it divides into two great branches, one to each lung, called the bronchi. These are also ringed. The primary bronchi are subdivided into secondary and these again into tertiary, and so on until the tubes become of capillary fineness and corre- spondingly numerous. This is shown diagrammatically in Fig. 240. They finally terminate in minute cells or cellulated cells (Fig. 241). The ringed structure con- tinues, except in the smallest subdivisions and the ter- minal cells. The terminal cells are y^-j- to -^ of an inch {\ to \ millimetre) in diameter, and their number has been estimated as 600,000,000.' Now conceive this mass of finely divided tubes and terminal cells, lined through-. bronchi, Fig. 240. — Diagram showing the general structure of the lungs of man : L, larynx ; 7V, trachea ; 6r, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original LeConte, Joseph, 1823-1901. New York, D. Appleton and Company
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