AND PHYSIOLOGY. 253 



tion, but as the period occupied by expiration is longer than 

 that of inspiration, consequently they rest during a longer 

 time than they are in action. 



FUNCTIONS OF THE LUNGS. 



446. The First Object of Breathing —The immediate 

 object of breathing is to bring air into the lungs and carry 

 it out again after it has performed its office ; and the function 

 of the lungs is to expose as large a surface of blood as possible 

 to the air inhaled. The ultimate objects of this arrangement 

 are to remove waste products from the body, which exist in 

 the form of carbonic acid and water, and also to generate the 

 animal heat necessary. A third, but by no means an incon- 

 siderable value of this function, is to convert the gluten of 

 vegetable food into fibrin. 



447. Another Use of Breathing, Purifying the Blood 

 — Endosmose and Exosniose — The impurities of the blood 

 — carbonic acid and water — are exchanged for the oxygen of 

 the atmosphere. This is not effected by actual contact of 

 blood and air, but through an intervening membrane, the 

 wall of the air-cells. It is a remarkable property of mem- 

 branes, both animal and vegetable, called Imbibition, or En- 

 dosmose and Exosmose, that allows fluids and gases to pass 

 through them in opposite directions at the same time. 

 Through the membrane of the lungs, the carbonic acid, which 

 has more affinity w r ith pure air than for the blood, passes out- 

 wards, while the pure air, containing oxygen, has an affinity 

 for the blood, and passes inwards. 



448. Amount of Air Used in Breathing — La uses of its 

 Variation — C n p n c i t y of the Chest. — The average amount 

 of air which p.isses in and out of the lungs at each inspiration 



446. What is the object of breathing:, and what is the function of the lungs? What 

 three processes are accomplished by this process? 447. How does breathing purify the 

 blood? Describe the process of imbibition. 



