384 



CHAPTER XXIX. 



ON RESPIRATION. COMPARATIVE ANATOMY OF THE RESPIRATORY 



ORGANS. ANATOMY OF THE HUMAN LUNGS. TRACHEA. BRON- 

 CHI. BRONCHIA. ULTIMATE PULMONARY TISSUE. MOVEMENTS 



OF RESPIRATION. FREQUENCY OF RESPIRATIONS, AND RATIO TO 



THE PULSE. AERIAL CAPACITY OF THE LUNGS, AND AMOUNT OF AIR 



BREATHED. CHANGES IN THE RESPIRED AIR, CARBONIC ACID EX- 

 HALED. OXYGEN INHALED. CHANGES IN THE BLOOD. NATURE 



OF THE RESPIRATORY PROCESS. 



RESPIRATION is that function by which an interchange of gases 

 takes place between the interior of an organized being and the 

 external medium ; and in the animal kingdom oxygen is the gas 

 received, and carbonic acid the gas given out. Every part of the 

 surface to which the outer medium (whether air or water) has 

 access may be considered to share in respiration, but in all, except 

 some of the lower animals, special organs are provided in which 

 the interchange can be more readily effected. These organs in all 

 cases consist of a membranous surface, adapted for contact with the 

 surrounding medium and capable of exposing the fluids of the body 

 in an especial manner to the action of the air. The interchange of 

 the gases through this respiratory membrane is essentially a purely 

 physico-chemical phenomenon, and must be studied as such. The 

 very great variety of structures with which different animals are 

 furnished for this function merely present us with modifications of 

 the elementary conditions, whereby its activity and extent are 

 governed in the several instances. The contact of air with the 

 blood may be influenced (1) by atmospheric concentration or dilu- 

 tion. In water-breathing animals, the air breathed is that held in 

 solution in the water, and is of course in very small quantity. The 

 density or rarity of the air, according to temperature and baro- 

 metric pressure, may perhaps affect the activity of respiration in 

 air-breathers. (2) The extent of the respiratory surface, (3) the 

 thickness of the tissue between the air and the blood, and (4) 

 the more or less complete manner in which the general mass of 

 the blood is brought from the tissues to the respiratory surface, 

 all these exert much influence on the activity of respiration. 



