714 THE RESPIRATORY TRACT. 



connective tissue, and the polygonal fields thus produced are 

 distinctly marked on the surface of the lung. 



The bronchi having subdivided at acute angles, the carti- 

 laginous rings gradually disappear, and in the small bronchi 

 are reduced to a few irregular plates. The muscle-layer, on the 

 contrary, increases in size, and the smaller bronchi exhibit a 

 very distinct circular layer of smooth muscle-fibers. The finest 

 bronchi have only scanty circular muscle-fibers. The fibrous 

 connective tissue composing the terminal bronchi is freely sup- 

 plied with elastic substance, and so long as the bronchi course 

 between the lobules they are attached to the lobules by loose 

 connective tissue, having a capillary system of its own. The 

 hyaline basement-layer is more marked the finer the bronchi, 

 and is covered by a stratified columnar, ciliated epithelium. In 

 the bronchioli of less than 1 mm. diameter, the stratification of 

 the epithelia is nowhere marked, though they remain ciliated to 

 the point, where the bronchiolus (alveolar duct) widens into the 

 alveoli of the lung. The columnar epithelia gradually decrease 

 in height, and finally blend with the flat, non-ciliated epithelia, 

 covering the inner surface of the alveoli. All bronchi are sup- 

 plied with acinous mucous glands, which are located above the 

 muscle-layer, and become fewer and smaller as the bronchi 

 decrease in size. In the bronchioli only simple acinous glands 

 are found, which disappear altogether when the bronchiolus 

 enters a lobule. 



Each lobule of the lung is composed of a number of alveoli, 

 which are semi-globular protrusions, separated from each other 

 by folds or septa of connective tissue. The septa, together with 

 the delicate inclosing connective tissue, constitute the wall of an 

 alveolus. The connective tissue is marked by a rich reticulum 

 of elastic fibers. The separating folds are never entirely effaced, 

 even in an expanded lung ; though by the inspiratory act they 

 become more shallow, as is demonstrated by the examination of 

 lungs, into the aerial passages of which air or gelatine had been 

 injected before hardening in chromic acid. 



The vascular supply of the lungs is very abundant. The 

 arteries which carry the venous blood are characterized by their 

 straight course ; while the veins which carry the arterial blood 

 have an irregular, slightly sinuous appearance. Both accompany 

 the bronchioli, the arteries being surrounded, besides the mus- 

 cle-layer, by a marked adventitial coat of fibrous connective 

 tissue. The bronchi have an independent system of blood- 



