976 THE ORGANS OF VOICE AND RESPIRATION. 



bronchiole now becomes enlarged, and is termed the atrium or alveolar passage ; 

 from it are given off, on all sides, ramifications, called infundibula, which are 

 closely beset in all directions by alveoli or air-cells. Within the lungs the bron- 

 chial tubes are circular, not flattened, and present certain peculiarities of structure. 



Changes in the Structure of the Bronchi in the Lungs. 1. In the Lobes of the 

 Lungs. In the lobes of the lungs the following changes take place. The cartilages 

 are not imperfect rings, but consist of thin laminae, of varied form and size, scat- 

 tered 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 diameter of which is 

 only one-fourth of a line. Beyond this point the tubes are wholly membranous. 

 The fibrous coat is 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, and consists of the 

 unstriped variety of muscular tissue. The mucous membrane lines the bronchi 

 and its ramifications throughout, and is covered with columnar ciliated epithelium. 



In the Lobules of the Lung. In the lobular bronchial tubes and in the 

 infundibula the following changes take place : The muscular tissue begins to dis- 

 appear, so that in the infundibula there is scarcely a trace of it. The fibrous coat 

 becomes thinner, and degenerates into areolar tissue. 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 among the 

 columnar ciliated epithelium ; then these patches of non-ciliated flattened epithe- 

 lium 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 flat- 

 tened 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 

 in their cavity are to be seen under the microscope granular, rounded, amoeboid 

 cells (eosinophile leucocytes), often containing carbonaceous particles. The air- 

 cells are well seen on the surface of the lung, and vary from -j^th to T ^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 in front of a bronchial 

 tube, the vein below. 



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; 1 their walls are also exceedingly thin. The arteries of neigh- 

 boring lobules are independent of each other, but the veins freely anastomose 

 together. 



The pulmonary veins commence in the pulmonary capillaries, the radicles 

 coalescing into larger branches, which run along through the substance of the 

 lung, independently from the minute arteries and bronchi. After freely com- 

 municating with other branches they form large vessels, which ultimately come 

 into relation with the arteries and bronchial tubes, and accompany them to the 

 hilum of the organ. Finally they open into the left auricle of the heart, con- 

 veying oxygenated blood to be eventually distributed to all parts of the body 

 by the aorta. 



The bronchial arteries supply blood for the nutrition of the lung ; they are 

 derived from the thoracic aorta or from the upper aortic intercostal arteries, and, 



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). 



