THE STRUCTURE AND FUNCTION OF THE ELEMENTARY LUNG 453 



The smaller bronchioles are not in possession of a cartilaginous framework, 

 but consist merely of fibrous and elastic tissue and a scanty layer of smooth muscle 

 cells. The larger tubes, on the other hand, are equipped with rings of cartilage 

 to render them more resistant against the variations in pressure to which they 

 are subjected during each respiratory act. This entire tract is lined with a layer 

 of epithelium which, in the trachea, bronchi and bronchioles, is of the ciliated 

 columnar variety and, in the outer parts of the infundibulum, cuboidal in shape. 

 The effective stroke of the cilia is in the direction of the mouth, so that much of 

 the foreign material carried in with the air is again expelled without extra efforts. 

 The specialized respiratory epithelium is restricted to the alveolar walls. These 

 are composed of connective tissue containing a large number of elastic fibers and 

 an external lining of very flat and large cells. The elastic tissue, as we shall see 

 later, is responsible for the traction which the lung constantly exerts upon the in- 

 ternal surface of the chest wall. 



Fio. 232. — Diagram Illustbating the Arrangement of the Infundibula. 

 B, Bronchiole; D, infundibular duct; J, infundibulum; A, alveolus; S, interinfun- 

 dibular space, occupie.d by capillaries. 



The blood-vessels ramify in all directions through the interalveolar walls; 

 moreover, as the infundibular vesicles are packed close together, the blood is 

 brought into intimate relation with the air, being separated from it merely by the 

 lining cells of the capillaries and alveoli. In fact, in some of the higher animals 

 (birds) the alveolar walls seem to be devoid of lining cells. It should also be 

 remembered that by far the largest quantity of blood furnished by the pul- 

 monary artery, serves respiratory purposes only. Thus, if a man weighing 70 

 kilos, possesses 4.5 kilos of blood, not less than 700 grams of this amount are 

 contained in the pulmonary blood-vessels. Furthermore, if the circulation-time 

 in the lesser circuit is reckoned at 13 seconds, it will readily be seen that close to 

 200 kilos of blood traverse the lungs in an hour and 4500 kilos in a day. ^ A very 

 small portion of this blood is required for the nutrition of this organ, but it seems 

 that this amount is derived directly from the aorta by those arterial branches which 

 are distributed to the bronchi, interlobular septa, pleural membranes and the 

 trunks of the blood-vessels leaving the heart. The venous return from these parts 



