248 PHYSIOLOGY. 



into two branches, one for each lung, and again subdivides, 

 and ramifies through the organ in a manner precisely simi- 

 lar to the bronchial tubes./ Every bronchus, or branch of 

 the trachea, thus has a corresponding blood-vessel, which 

 tracks it throughout its entire course until it reaches the air 

 vesicles, upon the surface of which the minute vessels ex- 

 pand and ramify, forming a net-work so beautiful, that the 

 anatomist who first observed it, called it the rete mirabile, or. 

 the wonderful net-work. Thus the air is on one side, and 

 the blood on the other, of an immense surface of membrane, 

 finer than the most delicate lace or gauze ; and as such 

 membranes are permeable to air and other gases, the oxygen 

 of the air penetrates it and unites with the blood, while a 

 portion of carbon and water are given off by exhalation. 

 Thus does the blood lose its dark venous character, and as- 

 sume a florid, arterial hue, and become fitted to carry life 

 and vigour to every part of the system. 



14. Thus we see that the lungs and all their complicated 

 machinery of bones, ligaments, muscles and cartilages, were 

 formed for the sake of these little air-cells ; for it is through 

 their agency that the blood undergoes the necessary changes 

 and alterations. When we reflect upon the relative extent 

 of the actual respiratory surface, compared with the dimen- 

 sions of the lungs themselves, that a stratum of blood seve- 

 ral hundred feet in surface, is exposed to a stratum of air 

 still more extensive, and all compressed within the corn-pass 

 of a few inches, we are filled with admiration and astonish- 

 ment at the wisdom displayed in such a structure, and search 

 in vain, among all the contrivances of human skill and 

 genius, for a counterpart ! 



15. We are now prepared to trace the successive acts of 

 respiration, accomplished^through the agency of the me- 

 chanism just described. [About one second and a half after 

 expiration, the muscles of inspiration begin to act, the inter- 

 costals contract, and, by elevating the ribs, increase the dis- 

 tance between the spine and sternum. As the ribs rise, the 



