The true bronchi, in every part of the lung, are distinguished by their tubular form and smooth walls (a,b)—characters in which they contrast with the loculatecl aspect of the in tercellular passages.
The walls of the rninutest bronchi are composed of three coats : a rnucous, a mus cular, and fibrous. Schroeder van der Kolk has proved that unstriped muscular fibres are contained in the parietes of the smallest of these tubes. The illustrations which accom pany the theses of Adrius Adriani are drawn by S. van der Kolk himself, from his own microscopic dissections. In the whale the structural elements of the walls of the smallest bronchial tubes are of very large dimensions, and therefore readily to be detected.
The muscular fibrillm are principally dis posed circularly (b, c, b, c); and the elastic (a, a, a), longitudinally.
During the ingress of the respiratory column of air into the lungs, both these orders of fibres must be stretched ; during the egress of the air, the one must actively contract, the other must passively recoil. This constitutes an expiratory force. It is important to re member that these two elements continue to prevail in the parietes of the bronchi as long as they retain the character of bronchi pro perly so called—in other words, to the limits everywhere which denote the origins of the intercellular passages. At this point the muscular element ceases altoaether ; so also does the ciliated epithelium ; but the elastic fibres proceed, under a modified form how ever, over the walls of the intercellular pas sages and air-cells. The muscular and fibrous structures are discoverable in the walls of the bronchi 'after these latter have penetrated within the bounds of the lobuli ; but never the cartilaginous. This latter element, how ever, exists in the walls of the smallest of the extra-lobular bronchi.
The extreme end of each bronchus is the common mouth of the infundibulum of Rossig nol; the peduncle of the pulmonary vesicles of Reisseissen ; the origin of the interlobular passages of Addison.
The bronchi divide on no constant or regular plan. — Small branches sometimes proceed from a large stem, at different angles, from every point of its circumference.
Frequently they multiply dichotomously (b, fig. 210.) ; that is, a single tube divides into two of equal diameters. Sometimes the
main bronchus exhibits a zigzag outline, the branches proceeding from the alternate angles. This latter method obtains with great con stancy in the case of the intralobular bronchi. The number of branches within the lobule into which a bronchus subdivides bears a general proportion to the size of that lobule. In the smallest, the intercellular passages begin from two or three bronchial peduncles ; in the largest, from eight or ten. In some instances a second or supplementary bronchus enters a lobule at the side. It is, however, the rule, that each lobule is supplied only with a single central bronchus. The point of attachment of the bronchus is the apex (a) of the lobule (b) ; the opposite point being the base. The angle of division in the bron chial tree is, for the most part, the obtuse.
This disposition of the tubes favours, mecha nically, both the ingressing and the egressing column of air.* It has been maintained by Dr. Radcliffe Hall f, that the contractility of the bronchial tubes is called into action rhythmically in each expiratory movement, to assist in emp tying the lungs. But no evidence has been adduced in support of this doctrine. If the contraction of the bronchial tube, through muscular or any other force, occurred at the first stage of the act of expiration, it is ob vious that it would arrest rather than favour the egress of the air. It is not, however, im probable, that a certain regulated power over the outgoing column of air is exerted by the parietes of the bronchial tubes. This is more likely to consist in a shortening and length ening of the tubes. They may also serve to regulate the supply of air to the lobules, in accordance with the wants of the system, just as the contractility of the minute arteries regulates the supply of blood to the organs to which they proceed. It may possibly be through this channel that the remark able variation is effected in the amount of respiration which adapts the quantity of heat produced to the depression of the external temperature. It has been further suggested by Dr. W. Gairdner § that the contractility of the smaller bronchi may serve to expel col lections of mucus which may accumulate within them, and which neither ciliary action nor the ordinary expiratory efforts suffice to displace.