THE MUSCULATURE OF THE BRONCHI AND LUNGS

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Abstract
This is a review of published material relating to the smooth muscle of the bronchi and lungs, with correlated subjects, found in some 492 articles, with a digest of the views on the anatomy of this motor mechanism and its physiology in respiratory movements and in other pulmonary activities. Evidence from recent techniques, as roentgenography, bronchoscopy, etc., is added. This muscle-system extends uninterruptedly throughout the entire bronchial tree from the larynx to the outermost ends of the air passages, and forms a branched, tubular, contractile network of fibers embracing the mucosa of the airway. The meshes of this network open up and the fibers relax during inspiration in adaptation to the attendant lengthening and widening of the air passages. The form of the bronchial musculature adapts it to expulsion of air from the lungs during expiration, for contraction of its fibers has the effect of shortening and narrowing the air tubes. Thus it may be looked upon as a factor in lung deflation, acting in cooperation with the stretched elastic fabric to diminish the volume of the bronchial tree, or the "dead space." An anatomical interpretation of the "dead space" is given, showing it to be variable and to be synonymous with "bronchial capacity." The musculature of the peripheral "respiratory" part of the tree is fundamentally the same as that of the purely "conducting" part, but its meshes encircle the numerous mouths of the respiratory air-chambers, which lead off from it, the structure here resembling the lung of the frog. This is the true "bellows" part of the lung, as Keith pointed out. The muscular mechanism of this "bellows" action and an explanation of the operation of ventilating the alveoli without deleterious stretching of their walls are given. The innervation of the bronchial muscle, by vagus and sympathetic fibers, is reviewed, and an hypothesis suggested for nervous control in respiratory movements, permitting of appropriate relaxation in inspiration and contraction in expiration. The action of a host of drugs on this muscle is examined and classified. The bronchial musculature is capable of manifesting certain peristaltoid movements, which are looked upon as operating to eject thick exu-dates; indeed such a movement may be going on continually, and may act like a perpetually moving escalator to waft liquids or gases toward the outer world. The relation of the bronchial musculature to certain pathological conditions is discussed. In asphyxia, for instance, the liberation of adrenalin is regarded as protective in that it favors a broncho-dilatation. The various factors engendering asthma are scrutinized and this condition discussed with relation to the bronchial musculature. It is pointed out that the muscle in the walls of the terminal bronchi is in a position to produce most effective interference with the air-flow during spasm. The involvement of the bronchial muscle in anaphylaxis and other abnormal conditions is studied. The pulmonary muscle lying in the interstitial tissue, though not so conspicuous as that in the air-way walls, yet collectively forms a motor system which is doubtless of functional importance, though no specific role can now be assigned to it. Emphasis is placed on the conception of the lung as a muscular organ.

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