RESPIRATION. 571 



fore, exposed on both sides to the atmosphere. As the capillary net- 

 work is spread over several cells, aeration of the blood is thus 

 thoroughly -secured, and in this locality the venous blood is converted 

 into arterial. The calibre of these capillaries is extremely small, bein°- 

 only in diameter equal to the thickness of a red blood-corpuscle. It has 

 been estimated, nevertheless, that the pulmonary capillaries in man are 

 capable of containing about two liters of blood, and it has been further 

 calculated that this amount is renewed ten thousand times in twenty-four 

 hours. And, even making allowances for error in these calculations, it 

 is evident how large is the surface for the interchange of gases between 

 the air and blood. 



The diameter of the air-vesicles is from one two-hundredth to one- 

 seventieth of an inch. Their number is almost infinite. It has been 

 calculated that about the termination of each bronchus in a mammal are 

 collected seventeen thousand seven hundred and ninety air-cells, and 

 their total number has been computed to be at least six hundred millions. 

 It has been further calculated by Lieberkiihn that the whole extent of 

 respiratory surface of both lungs in man is fourteen thousand square 

 feet, or two hundred square meters, and this surface is attained through 

 the reduplications of the membrane, so occupying the least possible 

 bulk. The air-vesicles gradually increase in number from infancy to 

 adult life, when they remain stationary for a time, after which they 

 decline, so that there is less respiratory surface in infancy and old age 

 than in adult life. 



The walls of the air-vesicles are highly elastic, from the presence of 

 elastic fibres, which form a close net-work with very fine meshes. Through 

 the presence of this elastic tissue the air-vesicles, therefore, tend contin- 

 ually to contract, — a phenomenon which, as will be later demonstrated, 

 is of the greatest importance for the process of expiration. All the 

 different parts of the lungs are held together by delicate elastic tissue, 

 and outside of this by a serous membrane, termed the pleura, which 

 covers the external surface of the lungs and is reflected on to the internal 

 surface of the thorax. The pleural membrane thus forms a shut sac, 

 and the lung lies on the outside of it. 



The thorax is composed of a closed cavity in the form of a truncated 

 cone, of which the sides, back, and a portion of the interior surfaces 

 are formed by the ribs and costal cartilages with their intervening 

 muscles. Its base is oblong, more or less flattened laterally in quad- 

 rupeds and antero-posteriorly in man. The ribs are alwaj-s more or less 

 curved, with their concavity directed internally. In general, the first rib 

 is the shortest, is less curved, and less inclined to the vertebral column. 

 As a rule, it may be stated that in animals in whom the thorax is short 

 the ribs are more curved than in those where the thorax is longer. 



