804 SPECIAL PHYSIOLOGY. 



municate with each other (Waters) ; but this is not universally admitted, 

 at least as regards the human lung. The cells situated beneath the 

 pleura, are four- or six-sided. No direct communication exists between 

 the air-cells of adjacent lobules. The number of air-cells in both lungs 

 has been calculated to be about 6,000,000. The walls of the air-cells 

 are thin and transparent, and are composed of areolar, mixed with 

 fine elastic tissue, lined by an exceedingly delicate mucous membrane, 

 consisting of a thin transparent basement membrane, covered with a 

 polygonal squamous non-ciliated epithelium. 



The pulmonary arteries and veins are the functional, or respira- 

 tory, vessels of the lungs. The pulmonary arteries, unlike the systemic 

 arteries, convey venous blood from the right ven- 

 113. tricle of the heart to the lungs. The trunk of 



the pulmonary artery, having passed obliquely 

 upwards and to the left, for about 2 inches, 

 reaches the concavity of the aortic arch, and 

 there divides into the right and left pulmonary 

 arteries ; each of these enters the corresponding 

 lung, and divides into as many primary branches 

 as there are lobes : these branches rapidly sub- 

 divide, in company with the bronchia, and finally 

 end in the pulmonary capillaries. These last- 

 named vessels, placed beneath the mucous mem- 

 ceils, with the capillary net- brane of the air-cells and intercellular passages, 

 work injected ; the puimon- form a delicate and close network, composed of a 



ary capillaries are seen to j } } f j, vegse ] s fc av i n g exce edingly 



be wider than the meshes . fe Y . . .. . '. fe i 



between them. thin walls ; their width varies from g^^tn to 



^uflth of an inch ; that of the meshes between 

 them, is*much less; at the sides of adjoining cells, the capillary net- 

 work is placed between their adjacent walls, so that it is acted on by 

 the air on both sides. The venules which arise from the capillary net- 

 work, quickly join to form larger trunks, which generally pursue a 

 different route to that of the arteries ; they finally end in four pul- 

 monary veins, two from each lung, which convey the arterialized blood 

 to the left auricle of the heart. The pulmonary veins are destitute of 

 valves ; their capacity is said to be about equal to, or even less than, 

 that of the pulmonary arteries. Within the lungs the pulmonary ar- 

 teries are usually situated above the bronchial tubes, and the pulmon- 

 ary veins below. The bronchial arteries are the nutritive vessels of 

 the lung; they are given off from the aorta, or from an intercostal ar- 

 tery, and are distributed to the walls of the bronchia and pulmonary 

 vessels, to the interlobular areolar tissue, and other neighboring parts; 

 they usually end in the bronchial veins, but the branches which sup- 

 ply the smallest bronchia end in the pulmonary capillary network. 

 The lymphatics are superficial and deep, and terminate in the bronchial 

 glands at the root of the lung. The nerves are derived from the vagus 

 and sympathetic; connected with the latter are numerous minute 

 ganglia. (Remak.) 



