454 EESPIRATION 



is effected by the corresponding veins, but some of this blood also finds its way 

 into the pulmonary veins by anastomoses. 



The diameter of the alveoli varies between 120 and 380 fj,, their average diam- 

 eter being 120 fj,. Inasmuch as from 300 to 400 millions of alveoli are contained 

 in each lung, and inasmuch as one of them possesses an area of about 0.321 mm. 2 , 

 the total respiratory surface must amount to 130 m. 2 in men and to 104 m. 2 in 

 women. If these values are now compared with the size of the body-surface, it 

 will be seen that the latter is 100 to 125 tunes smaller than the respiratory surface 

 of Our lungs. It must be evident, however, that the area formed by the pulmonary 

 blood is somewhat smaller than the alveolar surface, because the capillaries do 

 not occupy the entire extent of the alveolar surface. Thus, it has been estimated 

 that, if all the blood present in the lungs of a man, could be made to form a single 

 layer measuring 10 ju in thickness, it would cover an area of 120 m. 2 . These 

 figures, very naturally, are only approximately correct and are not intended to 

 be memorized but simply to permit us to form an idea regarding the enormous 

 surface of blood brought into relation with the outside air. 



CHAPTER XXXVII 

 THE MECHANICS OF THE RESPIRATORY MOVEMENTS 



General Topography. The lungs of the mammal are contained in 

 the cavity of the thorax which forms the fore part of the general 



FIG. 233. ILLUSTRATION TO SHOW THE POSITION OF THE LUNGS IN THEIR RELATION TO 

 THE WALL OF THE THORAX. 



cavity of the trunk. They are surrounded on all sides by relatively 

 solid walls, made so by a copious inlay of bony laminae. From the 

 abdominal cavity they are completely separated by a muscular sep- 

 tum, the diaphragm, but communicate with the pharyngeal, nasal 



