92 . THE HUMAN BODY. 



and to the shape of which they are moulded (fig. 23, p. 81), 

 they represent two irregular cones, resting their bases on the 

 diaphragm, filling with their apices the two conical spaces 

 lined by the pleura at the top of the chest, and separated by 

 the heart and the mediastinum. The right lung is divided 

 in its length into three lobes by two oblique clefts, and is 

 shorter and larger than the left, which has but two lobes. 

 The internal face of the lungs is concave; about the middle 

 the bronchia unite with the pulmonary vessels to form the 

 root of the lungs ; their base takes the form of the convexity 

 of the diaphragm; their edges, thin in front and at the bottom, 

 thick and rounded behind, partially cover the heart, and fill 

 the space which separates the diaphragm from the walls of 

 the thorax, as well as the groove between the ribs and the 

 vertebrae. The entire surface of the lungs is smooth and 

 moistened with serous secretion. 



The tissue proper of the lungs, or pulmonary parenchyma, 

 is of a grayish rose colour, soft, spongy, elastic, crepitating 

 under pressure in consequence of the air it contains. It is 

 divided into polyhedral lobules, very variable in form and in 

 the disposition of their facets, which permit exact juxtaposi- 

 tion without intervals; and they are separated by partitions of 

 cellular tissue, independent and without communication with 

 each other. Each one of these lobules is formed of a cluster 

 of little cavities called pulmonary vesicles, constituting a cul- 

 de-sac, and receiving the air from the bronchial ramifications 

 of which they are the terminal expansions. The diameter of 

 these pulmonary vesicles is from one-seventieth to one hun- 

 dred and fiftieth of an inch; from this we can judge of the 

 tenuity of their walls, in the substance of which notwithstand- 

 ing ramify the capillary vessels. Each lobule represents a little 

 lung, a diminutive of the entire organ. A bronchial twig and 

 minute artery run into it, veins and lymphatic vessels leave it. 

 On the surface the lobules appear bounded by their interme- 

 diate partitions, and they form a mosaic of which the mottled 

 colouring varies from rose to black. These black particles 

 are principally composed of a carboniferous substance which 

 penetrates the lung either with the air or with the blood, and 

 which is called pulmonary carbon. 



