THE LUNGS. 1119 



dichotomously, or rather bipinnately, throughout the entire organ. Sometimes 

 three branches arise together, and occasionally small lateral branches are given off 

 from the sides of a larger. Each of the smaller subdivisions of the bronchi enters 

 a pulmonary lobule, and is termed a lobular bronchial tube or bronchiole. Its wall 

 now begins to present irregular dilatations, air-cells or alveoli, at first sparingly 

 and on one side of the tube only, but as it proceeds onward these dilatations 

 become more numerous and surround the tube on all sides, so that it loses its cyl- 

 indrical character. The lobular bronchiole now becomes enlarged, and is known as 

 the atrium or alveolar passage. From the atrium are now given off in all direc- 

 tions somewhat elongated blind pouches (1 mm. in diameter), the infundibula. 

 Each infundibulum is, in its turn, closely beset with alveoli. Within the lungs 

 the bronchial tubes are circular, not flattened, and present certain peculiarities of 

 structure. 



Changes in the Structure of the Bronchi. As the bronchial tubes become 

 smaller and smaller the following changes take place : The cartilages consist of 

 thin lamina, of varied form and size, scattered irregularly along the sides of the 

 tube, being most distinct at the points of division of the tubes. They may be 

 traced into tubes the diameter of which is only one-fourth of a line. Beyond this 

 point the tubes are wholly membranous. The fibrous coat and the longitudinal 

 elastic fibres are continued into the smallest ramifications of the bronchi. The 

 muscular coat is disposed in the form of a continuous layer of annular fibres, which 

 may be traced upon the smallest bronchial tubes. The mucous membrane lines 

 the bronchi and its ramifications throughout, and is covered with columnar ciliated 

 epithelium. 



In the lobular bronchial tubes and in the infundibula the following changes 

 take place : The muscular tissue begins to disappear ; the longitudinal elastic fibres 

 begin to break up, so that in the infundibula they form an interlacement around 

 the mouths of the air-cells. The epithelium becomes non-ciliated and flattened. 

 This occurs gradually ; thus, in the lobular bronchioles patches of non-ciliated 

 flattened epithelium may be found scattered amongst the columnar ciliated epithe- 

 lium ; then these patches of non-ciliated flattened epithelium become more and 

 more numerous, until in the infundibula and air-cells all the epithelium is of the 

 non-ciliated pavement variety. In addition to these flattened cells, there are small 

 polygonal granular cells in the air-sacs, in clusters of two or three, between the 

 others. 



The air-cells are small, polyhedral, recesses composed of a fibrillated connec- 

 tive tissue and surrounded by a few involuntary muscular and elastic fibres. Free 

 within their cavities are granular leucocytes, often containing carbonaceous parti- 

 cles. The air-cells are well seen on the surface of the lung, and vary from gr^th 

 to 7o-th of an inch in diameter, being largest on the surface at the thin borders 

 and at the apex, and smallest in the interior. 



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. In the lung 

 the branches of the pulmonary artery are usually above and behind a bronchial 

 tube, the vein below and in front. 



The pulmonary capillaries form plexuses which lie immediately beneath the 

 mucous membrane in the walls and septa of the air-cells and of the infundibula. 

 In the septa between the air-cells the capillary network forms a single layer. The 

 capillaries form a very minute network, the meshes of which are smaller than the 

 vessels themselves ; * their walls are also exceedingly thin. The arteries of neigh- 

 boring lobules are distinct from each other, and do not anastomose, whereas the 

 corresponding venous anastomosis is extremely free. 



The radicles of the pulmonary veins commence in the pulmonary capillaries, 

 and coalesce into larger branches, which accompany the arteries and return the 



1 The meshes are only 0.002'" to 0.008'" in width, while the vessels are 0.003'" to 0.005'" 

 (Kolliker, Human Microscopic Anatomy). 



