720 ORGANS OF VOICE AND RESPIRATION. 



lobule (lobular bronchial tube), and, again subdividing, ultimately terminates in 

 the intercellular passages and air-cells of which the lobule is composed. Within 

 the lungs the bronchial tubes are circular, not flattened, and their constituent 

 elements present the following peculiarities of structure. 



The cartilages are not imperfect rings, but consist of thin laminae, of varied 

 form and size, scattered irregularly along the sides of the tube, being most distinct 

 at the points of division of the bronchi. They may be traced into tubes the dia- 

 meter of which is only one-fourth of a line. Beyond this point, the tubes are 

 wholly membranous. The fibrous coat and longitudinal elastic fibres are con- 

 tinued into the smallest ramifications of the bronchi. The muscular coat is dis- 

 posed in the form of a continuous layer of annular fibres, which may be traced 

 upon the smallest bronchial tubes ; they consist of the unstriped variety of muscular 

 fibre. The mucous membrane lines the bronchi and its ramifications throughout, 

 and is covered with columnar ciliated epithelium. 



According to the observations of Mr. Eainey, 1 the lobular bronchial tubes, on 

 entering the substance of the lobules, divide and subdivide from four to nine times, 

 according to the size of the lobule, continuing to diminish in size until they attain 

 a diameter of ^th to ^ c th of an inch. They then become changed in structure, 

 losing their cylindrical form, and are continued onwards as irregular passages 

 (intercellular passages), through the substance of the lobule, their sides and ex- 

 tremities being closely covered by numerous saccular dilatations, the air-cells. 

 This arrangement resembles most closely the naked eye appearances observed in 

 the reticulated structure of the lung of the tortoise, and other reptilia. 



The air-cells are small, polyhedral, alveolar recesses, separated from each other 

 by thin septa, and communicating freely with the intercellular passages. They 

 are well seen on the surface of the lung, and vary from ^^th to 7 Vth of an inch 

 in diameter, being largest on the surface, at the thin borders, and at the apex, 

 and smallest in the interior. 



At the termination of the bronchial tubes, in the intercellular passages, their 

 constituent elements become changed ; their walls are formed by an interlacing of 

 the longitudinal elastic bundles with fibrous tissue, the muscular fibres disappear, 

 and the mucous membrane becomes thin and delicate, and lined with a layer of 

 squamous epithelium. The latter membrane lines the air-cells, and forms by its 

 reduplications the septa intervening between them. 



The Pulmonary Artery conveys the venous blood to the lungs ; it divides into 

 branches which accompany the bronchial tubes and terminates in a dense capillary 

 network upon the walls of the intercellular passages and air-cells. From this 

 network, the radicles of the pulmonary veins arise ; coalescing into large branches, 

 they accompany the arteries, and return the blood, purified by its passage 

 through the capillaries, to the left auricle of the heart. In the lung, the branches 

 of the pulmonary artery are usually above and in front of a bronchial tube, the 

 vein below. 



The Pulmonary Capillaries form plexuses which lie immediately beneath the 

 mucous membrane, on the walls and septa of the air-cells, and upon the walls of 

 the intercellular passages. In the septa between the cells, the capillary network 

 forms a single layer. The capillaries are very minute, the meshes being only 

 slightly wider than the vessels ; their walls are also exceedingly thin. 



The Bronchial Arteries supply blood for the nutrition of the lung ; they are 

 derived from the thoracic aorta, and, accompanying the bronchial tubes, are dis- 

 tributed to the bronchial glands, and upon the walls of the larger bronchial tubes 

 and pulmonary vessels, and terminate- in the deep bronchial veins. Others are 

 distributed in the interlobular areolar tissue, and terminate partly in the dee]), 

 partly in the superficial, bronchial veins. Lastly, some ramify upon the walls of 

 the smallest bronchial tubes, and terminate in the pulmonary veins. 



The Superficial and Deep Bronchial Veins unite at the root of the lung, and 



1 Medico-Chirurgical Transactions, vol. xxviii. 1845. 



