RESPIRATION, ORGANS OF. 



is not the result of atmospheric pressure, nor 

 of the action of the thoracic inspiratory mus- 

 cles : it is due to the elastic property of the 

 cartilaginous pieces. A " membranous inter- 

 val " in the walls of the mtra-pulmonary 

 bronchi would obviously expose the air-pas- 

 sages to injurious pressure. The muscular 

 fibres which always belong to the unstriped 

 kind in this class of bronchial tube are ar- 

 ranged chiefly circularly, but partly longi- 

 tudinally. They are thus enabled, not only to 

 contract the calibre, but to diminish the length, 

 acting, therefore, as important expiratory 

 powers. As the dissector advances towards the 

 branches of the " bronchial tree," the cartila- 

 ginous fragments in the walls of the tubes be- 

 come thinner and smaller, and more and more 

 distantly placed, until eventually they cease 

 altogether, the walls of the tubes being com- 

 posed of nothing but fibro-membrane. Here 

 the tubes are maintained in the open state in 

 part by the expansive force of the contained 

 column of air. The collapse of these pas- 

 sages leads to the atalectasis of that portion 

 of the lung to which they lead. At this 

 point the gravest impediment to respiration, 

 in bronchitis, occurs ; here also is chiefly 

 seated the obstacle to expiration in some 

 forms of emphysema ; and these musculo- 

 membranous tubules are the scene of 

 spasm during the paroxysms of spasmodic 

 asthma. The parietes of the bronchial tubes 



Fig. 208. 



A section of a minutely injected piece of human lung. 



(After Rainey.) 



The section cuts longitudinally a branching 

 bronchial tube. At a, the ciliated epithelium of 

 this tube is seen to terminate by an abruptly de- 

 nned border, illustrating the doctrine held by Mr. 

 Rainey, which contends for the complete absence 

 of all epithelium from all parts of the lung beyond 

 this limit, b marks the sudden manner in which 

 the intercellular passages arise from the extreme 

 bronchi. 



203 



within the substance of the lungs bear glands 

 like those of the trachea, and, like the latter, 

 they are lined internally by a ciliated cylinder 

 epithelium. This epithelium terminates, by 

 abruptly-defined borders, at the commence- 

 ment of the true pulmonary tissue, (ft) This fact 

 was first shown by Mr. Rainey*: subsequent 

 observers have, however, proved, contrary to 

 the opinion of Mr. Rainey, that the epithelial 

 layer docs not wholly cease at these points (b). 

 It is prolonged over the respiratory segment 

 under the character of pavement-epithelium. 



The true bronchi, in every part of the lung, 

 are distinguished by their tubular form and 

 smooth walls (a, /;) characters in which they 

 contrast with the loculated aspect of the in- 

 tercellular passages. 



The walls of the minutest bronchi are 

 composed of three coats : a mucous, 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- 



Fig. 209. 



DO 



Sub-mucous tissue from a small bronchial tube in the 



human lung. (After Hurting.') 

 a, a, a, elastic longitudinal fibres blended with 

 muscular bands ; b, b, muscular fascicles disposed 

 circularly ; c, c, extremities of the same, showing 

 the absence of tendons. 



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 fibrillaj are principally dis- 

 posed circularly (b, r, b, c) ; and the elastic 

 (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 altogether ; so also 

 does the ciliated epithelium; but the elastic 



* Mcd. Chir. Trans, vol. xxviii., 1845. 

 s 4 



