1098 



SPLANCHNOLOGY 



equivalent to that portion of the right bronchus which lies on the distal side of its 

 eparterial branch. The first branch of the left bronchus arises about 5 cm. from 

 the bifurcation of the trachea, and is distributed to the superior lobe. The main 

 stem then enters the inferior lobe, where it divides into ventral and dorsal branches 

 similar to those in the right lung. The branch to the superior lobe of the left lung 

 is regarded as the first of the ventral seiies. 



Structure. The lungs are composed of an external serous coat, a subserous areolar tissue 

 and the pulmonary substance or parenchyma. 



The serous coat is the pulmonary pleura (page 1090); it is thin, transparent, and invests the 

 entire organ as far as the root. 



The subserous areolar tissue contains a large proportion of elastic fibers; it invests the entire 

 surface of the lung, and extends inward between the lobules. 



The parenchyma is composed of secondary lobules which, although closely connected together 

 by an interlobular areolar tissue, are quite distinct from one another, and may be teased 

 asunder without much difficulty in the fetus. The secondary lobules vary in size; those on 

 the surface are large, of pyramidal form, the base turned toward the surface; those in the 

 interior smaller, and of various forms. Each secondary lobule is composed of several primary 

 lobules, the anatomical units of the lung. The primary lobule consists of an alveolar duct, 

 the air spaces connected with it and their bloodvessels, lymphatics and nerves. 



The intrapulmonary bronchi divide and subdivide throughout the entire organ, the smallest 

 subdivisions constituting the lobular bronchioles. The larger divisions consist of: (1) an outer 

 coat of fibrous tissue in which are found at intervals irregular plates of hyaline cartilage, most 

 developed at the points of division; (2) internal to the fibrous coat, a layer of circularly disposed 

 smooth muscle fibers, the bronchial muscle; and (3) most internally, the mucous membrane, 

 lined by columnar ciliated epithelium resting on a basement membrane. The corium of the 

 mucous membrane contains numerous elastic fibers running longitudinally, and a certain amount 

 of lymphoid tissue; it also contains the ducts of mucous glands, the acini of which lie in the 

 fibrous coat. The lobular bronchioles differ from the larger tubes in containing no cartilage 

 and in the fact that the ciliated epithelial cells are cubical in shape. The lobular bronchioles 

 are about 0.2 mm. in diameter. 





Fio. 974. Part of a secondary lobule from the depth of a human lung, showing parts of several primary lobules. 

 1, bronchiole; 2, respiratory bronchiole; 3, alveolar duct; 4, atria; 5, alveolar sac; 6, alveolus or air cell; m, smooth 

 muscle; o, branch pulmonary artery; v, branch pulmonary vein; , septum between secondary lobules. Camera 

 drawing of one 50 M section. X 20 diameters. (Miller.) 



Each bronchiole divides into two or more respiratory bronchioles, with scattered alveoli, and 

 each of these again divides into several alveolar ducts, with a greater number of alveoli con- 

 nected with them. Each alveolar duct is connected with a variable number of irregularly 

 spherical spaces, which also possess alveoli, the atria. With each atrium a variable number 

 (2-5) of alveolar sacs are connected which bear on all parts of their circumference alveoli or air 

 sacs. (Miller.) 



