. Animal physiology. Physiology, Comparative; Physiology, Comparative. RESPIRATORY APPARATUS OF MAMMALIA. 257 a muscular partition of which the use in respiration is very- important. An imperfect diaphragm is found in some Birds, which approach most nearly to the Mammalia in their general structure. The lungs are suspended, as it were, in this cavity, by their summit or apex; and are covered by a serous mem- brane termed the pleura^ which also lines the thorax, being reflected from one surface to the other precisely in the manner of the pericardium. Thus the pleura of the outer surface of the
. Animal physiology. Physiology, Comparative; Physiology, Comparative. RESPIRATORY APPARATUS OF MAMMALIA. 257 a muscular partition of which the use in respiration is very- important. An imperfect diaphragm is found in some Birds, which approach most nearly to the Mammalia in their general structure. The lungs are suspended, as it were, in this cavity, by their summit or apex; and are covered by a serous mem- brane termed the pleura^ which also lines the thorax, being reflected from one surface to the other precisely in the manner of the pericardium. Thus the pleura of the outer surface of the lung is continually in contact with that which forms the inner wall of the thorax; they are both kept moist by fluid secreted from them ; and they are therefore so smooth, as to glide over one another with the least possible friction. The lungs themselves are very minutely subdivided; a and thus expose a vast extent of surface in proportion to their size. The air-cells of the human lung, into which the air is conveyed by the branches of the wind-pipe, and on the walls of which the blood is distributed, do not ave- rage above the l-100th of an inch in diameter. In the accompany- ing figure is represented, on one side, the lung, d, presenting its natural appearance; and on the other, the ramifications of the air-passages or bronchial tubes, c, e, by which air is conveyed into every part of the lungs. The trachea or windpipe, b, opens into FlG-156—Air-tubes and lung of Man. the pharynx or back of the mouth by the larynx, a. The con- struction of this is especially destined to produce the voice; and will be explained under that head (Chap, xiii.) ; but it may be here mentioned that the entrance from the pharynx into the larynx consists of a narrow slit, which is capable of being en- larged or closed by the contraction of muscles. These muscles are made to act by a process corresponding with that which is. Please note that these images are extracted from scanned page images that m
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