ORGANS OF RESPIRATION. 



tissue and fixed connective-tissue cells. Elastic fibers are, however, 

 numerous, forming networks beneath the basement membrane. 



The work of Miller has given a clearer conception of what may 

 be regarded as the units of lung structure, namely, the lobules. 

 Such a unit or lobule is composed of a terminal bronchiole or 

 alveolar duct, with the air-spaces atria, air-sacs, and air-cells 

 connected with it, and their blood- and lymph-vessels and nerves. 

 The general arrangement of these structures may be observed in 

 Fig. 253, which gives a diagram of a lung lobule. The shape of the 

 atria, air-sacs, and air-cells may be seen in Fig. 254, which is from 

 a wax reconstruction of these structures. 



The blood-vessels of the lung, including their relation to the 

 structures of the lung lobules, have been investigated by Miller; his 

 account is closely followed in the following description : The pul- 

 monary artery follows closely the bronchi through their entire 

 length. An arterial branch enters each lobule of the lung at its 

 apex in close proximity to the terminal bronchiole. After entering 

 the lobule the artery divides quite abruptly, a branch going to e^ich 

 atrium ; from these branches the small arterioles arise which supply 

 the alveoli of the lung. " On reaching the air-sac the artery breaks up 

 into small radicals which pass to the central side of the sac in the sulci 

 between the air-cells, and are finally lost in the rich system of capil- 

 laries to which they give rise. This network surrounds the whole air- 

 sac and communicates freely with that of the surrounding sacs." This 

 capillary network is exceedingly fine and is sunken into the epithelium 

 of the air-sacs so that between the epithelium and the capillary there is 

 only the extremely delicate basement membrane. Only one capillary 



network is found between any 

 two contiguous air-cells or air- 

 sacs. The atria, the alveolar 

 ducts and their alveoli, and the 

 alveoli of the respiratory bron- 

 chioles are supplied with similar 

 capillary networks. The veins 

 collecting the blood from the 

 lobules lie at the periphery of the 

 lobules in the interlobular con- 

 nective tissue, and are as far dis- 

 tant from the intralobular arteries 

 as possible. These veins unite to 

 form the larger pulmonary veins. 

 The bronchi, both large and 

 small, as well as the bronchioles, 

 derive their blood supply from 

 the bronchial arteries, which also 

 partly supply the lung itself. 

 Capillaries derived from these ar- 

 teries surround the bronchial system, their caliber varying according 



Fig. 252. Scheme of the respiratory 

 epithelium in amphibia : The upper figure 

 gives a surface view : b, Basilar portion ; a, 

 the thin process. The lower figure is a sec- 

 tion : , Respiratory epithelial cell ; b, blood- 

 vessel ; c, connective tissue around the al- 

 veoli. 



