Few medical practitioners will be found who do not know that the bronchial tree contains smooth muscle, but probably not many realize that this muscle forms a motor organ extending uninterruptedly from the larynx to the outermost terminations of the air-carrying tubes, and infiltrating the entire lung substance. Indeed, on account of the thickness and uniformity of its distribution, one recent worker, Baltisberger,1 has averred that hardly a cubic millimeter of lung substance in the human being could be found which would not contain smooth muscle. There are reasons why the presence of this abundance of pulmonary muscle has not been observed earlier. One undoubtedly is the fact that in ordinary microscopic sections of the lung so much space is occupied by air that the tissues are spread apart, and the muscle elements, thus dispersed, do not bulk so large as they would if all the air could be
MACKLIN CC. FUNCTIONAL ASPECTS OF BRONCHIAL MUSCLE AND ELASTIC TISSUE. Arch Surg. 1929;19(6):1212–1235. doi:10.1001/archsurg.1929.01150060274016
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