GENERAL STRUCTURE OF THE LUNGS. 897 



to 20 cubic inches by the presence of a full meal in the stomach. (Hutchinson, in 

 Journal of Statistical "Society, August, 1844; and in Medico-Chirurg. Transactions, 

 vol. xxix., 1846 ; also, in the article " Thorax," in Cyclopaedia of Anatomy and Phy- 

 siology, and the article " Respiration," by Reid, in the same.) 



Texture and consistence. The substance of the lung is of a light porous 

 spongy structure, and, when healthy, is buoyant in water : but in the foetus, 

 before respiration has taken place, and also in certain cases of congestion, col- 

 lapse, or consolidation from disease, the entire lungs, or portions of them, 

 sink in that fluid. The specific gravity of a healtliy lung, as found after 

 death, varies from 345 to 746, water being 1000. When the lung is fully 

 distended its specific gravity is 126, whilst that of the pulmonary substance, 

 entirely deprived of air, is 1056. (Krause.) When pressed between the fingers, 

 the luugs impart a crepitant sensation, which is accompanied by a peculiar 

 noise, both effects being caused by the air contained in the tissue. On cutting 

 into the lung, the same crepitation is heard, and there exudes from the cut 

 surface a reddish frothy fluid, which is partly mucus from the air-tubes and 

 air-cells, and partly a serous exudation, tinged with blood, and rendered 

 frothy by the admixed air. This fluid escapes in largest quantity from the 

 posterior portion of the lung. 



The pulmonary tissue is endowed with great elasticity, in consequence of 

 which the lungs collapse to about one-third of their bulk when the thorax is 

 opened, and the resistance offered by the walls of that cavity to the atmos- 

 pheric pressure on their outer surface is in this way removed. Owing to this 

 elasticity also, the lungs, if artificially inflated out of the body, contract to 

 their previous volume when the air is again allowed to escape. 



Colour. In infancy the lungs are of a pale rose- pink colour, which might 

 be compared to blood-froth ; but as life advances ,they become darker, and 

 are mottled or variegated with spots, patches, and streaks of dark slate-colour, 

 which sometimes increase to such a degree as to render the surface almost 

 uniformly black. 



The dark colouring matter found in these streaks is in the form of granules and 

 collections of granules, not inclosed in cells ; it is deposited in the interstitial areolar 

 tissue mostly near the surface of the lung, and is not found so abundantly in the 

 deeper substance. It exists sometimes in the air-cells, and on the coats of the larger 

 vessels. Its quantity increases with age, and is said to be less abundant in females 

 than in males. In persons who follow the occupation of quarriers, more especially 

 colliers, the lungs are often intensely charged with black matter. The black colouring 

 substance of the lung seems to be composed of a mixture of carbon and some animal 

 matter. A black substance of precisely the same nature is found in the bronchial 

 glands. In exceptional cases the adult lungs are found with only very slight streaks 

 of pigment. 



Root of the Lung. 



The root of each lung is composed of the bronchus and the large blood- 

 vessels, together with the nerves, lymphatic vessels, and glands, connected 

 together by areolar tissue, and enclosed in a sheath of the pleura. 



The root of the right lung lies behind the superior vena car a and part of 

 the right auricle, and below the azygos vein, which arches over it to enter 

 the superior cava. That of the left lung passes below the arch of the aorta, 

 and in front of the descending aorta. The phrenic nerve descends in front 

 of the root of each lung, and the pneumogastric nerve behind, whilst the 

 ligamentum latum pulmonis is continued from the lower border. The 

 bronchus, together with the bronchial arteries and veins, the lymphatics 



3 N 



