386 RESPIRATION. [CHAP. XXIX. 



They form a double organ, with a single common air-tube, the 

 trachea, and a single common pulmonary artery, supplying the 

 venous blood. These vessels branch first into a right and left, and 

 then into many subordinate ramifications up to the ultimate air- 

 cells and capillaries. Four veins carry off the aerated blood to the 

 left side of the heart. Being penetrated by the air, the lungs are 

 the lightest organs in the body. In the fostus, before breathing, 

 they are small, and transmit only so much blood as is requisite 

 for their own growth, but when the air enters, their volume 

 augments, their absolute weight increases in consequence of the 

 greater afflux of blood, while their specific gravity diminishes. 

 Krause estimates the average absolute weight of the lungs in men 

 to be three pounds and a half, in women two pounds and three- 

 quarters, and the left to be smaller than the right by one-tenth. 

 The weight, as compared with that of the whole body, is as one to 

 forty or fifty. 



In shape the lungs are adapted to that of the cavity in which 

 they are lodged; their apices rise into the neck, their bases rest 

 on the diaphragm, between them lies the heart with the great 

 vessels. They are invested by a serous covering, the pleura, which, 

 after lining the thoracic walls, is reflected over them at their root, 

 and dips into those fissures, which serve to subdivide them im- 

 perfectly, the right into three, the left into two, lobes. 



The trachea descends in the middle line from the larynx (which 

 is a complicated development of it for the protection of the 

 orifice of the respiratory organ, and for the production of sound, 

 and which will be afterwards described), as far as opposite the 

 second or third dorsal vertebra, being straight, sub-cylindrical, 

 flat behind, and about three-quarters of an inch wide. It is held 

 permanently open by from sixteen to twenty cartilaginous rings, 

 flattened in the direction of the wall in which they are imbedded, 

 and deficient behind to an extent of one-third. Of these the 

 highest is the thickest and the lowest is adapted by its shape to the 

 bifurcation of the trachea into the two bronchi. The free ends 

 of these cartilages are sometimes forked, and contiguous ones are 

 now and then joined. They are immediately invested with peri- 

 chondrium, a dense, white fibrous, inelastic, membrane, and are 

 connected by a continuation of it extending between their borders 

 and ends. This inelastic membrane, by its toughness, resists undue 

 extension in the longitudinal direction. 



Looking on the trachea behind, we observe the space between 

 the ends of the cartilages covered with irregularly interwoven 



