800 SPECIAL PHYSIOLOGY. 



746, water being 1000. The color of the lungs, varies at different 

 periods of life ; in the newly born infant, they are of a pinkish white ; 

 in the adult they are darker, and become mottled with deep slate-col- 

 ored spots, patches, or lines, which increase in number and assume a 

 deeper hue as life advances, becoming in some individuals, even black. 



The surface of each lung is invested by a thin, smooth, transparent, 

 elastic, serous membrane, named the pleura, which passes into certain 

 fissures upon the surface of the lungs ; at the root of the lungs it is 

 reflected upon the inner surface of the corresponding pleura! chamber 

 of the thorax, the whole of which it lines, forming a closed sac (see 

 p. 31). The parietal portion of this membrane partly lines the sides 

 of the thorax, where' it forms the costal pleura; but in the middle 

 line, it covers a portion of the pericardium, and other parts, and 

 touches the opposite pleura ; above, it closes in the upper opening of 

 the thorax, reaching higher than the first rib ; below, it lines the dia- 

 phragm. A triangular duplicature, forming the broad ligament of the 

 lung, passes down from the root of the lung to the diaphragm, and 

 serves to retain the lower portion of the lung in position. The free 

 and moist surfaces of the parietal and pulmonary portions of the 

 pleura, touch each other. If serous fluid, blood, pus, or air should 

 collect between them, the cavity of the pleura becomes evident, and 

 the diseases known as hydrothorax, hgemothorax, empyema, or pneu- 

 mothorax are developed. The right and left pleurae are quite distinct 

 from each other. The place where they come into contact, is just 

 above the middle of the sternum; here they are connected together 

 by areolar tissue, but in other parts of the middle line, they are sep- 

 arated from each other by an interval, called the mediastinum, in 

 which all the parts contained in the thorax, except the lungs, are 

 lodged. The right pleural chamber is shorter and wider than the 

 left. 



Each lung, as well as the pleural chamber into which it accurately 

 fits, is of a conical shape, and presents an apex, a base, an outer and 

 inner surface, and an anterior and posterior border. The apex is 

 blunt, and projects into the neck, from an inch to nearly an inch and 

 a half above the first rib. The base is broad and concave, and rests 

 on the convexity of the diaphragm ; its margin is thin, and passes be- 

 tween that muscle and the wall of the chest ; it reaches much lower 

 down at the outer side and behind than it does in front. The outer 

 surface, smooth and convex, is in contact with the walls of the thorax, 

 and is of greater depth behind, than in front. The inner surface is 

 concave, being adapted to the pericardium and heart, behind which it 

 presents a depression, called the hilus, where the bronchi and pul- 

 monary vessels forming the root of the lung, pass in and out. The 

 anterior borders of the lungs are thin, and partly overlap the peri- 

 cardium, that of the right lung reaching to the middle of the whole 

 length of the sternum, that of the left lung doing so only as low as 

 the fourth costal cartilage. Above this point, the anterior borders of 

 the two lungs are merely separated from each other by their pleural 

 membranes; but below it, the anterior border of the left lung forms, 

 over the pericardium and apex of the heart, a V-shaped notch, the 



