148 LMNTS OF PHYSIOLOGY 



near the surface. If oxygen is supplied to the blood, the 

 latter conveys the oxygen to the cells; but it is seen that a 

 very efficient organ is needed to supply the blood with oxy- 

 gen sufficient for so many cells. The breathing apparatus 

 varies in different animals ; it usually consists of a device 

 for exposing to the air a great amount of thin tissue, which 

 is a specialized form of the outer skin of the animal (if 

 the animal is not a land animal, the tissue is exposed to 

 the water) ; the animal is further provided with means to 

 keep up a current of air (or water) on the outside of this 

 modified skin and a current of blood on the inside. In 

 some small animals, as the ameba and the earthworm, the 

 simple external body surface is sufficient for the purpose 

 of breathing. But large animals with many deep-seated 

 and inaccessible cells require an increase of surface, which 

 is supplied by having the oxygen-absorbing surface inside 

 of the body and by having it folded or provided with tubes 

 or branches of the greatest degree of complexity. In man, 

 it has been estimated that by the finer and finer division of 

 the air sac, a pair of human lungs presents to the air a 

 surface of at least one hundred square feet (or ten feet 

 square). The remainder of the breathing apparatus' con- 

 sists of muscles for changing the air that is in contact 

 with this great surface. 



248. Definitions. The passing of the air into the lungs 

 is called inspiration, and the passing of the air out from 

 the lungs is called expiration. The two together constitute 

 respiration, or breathing. 



249. Anatomy of the Respiratory Organs. The air 

 usually passes in at the nose and returns by the same 

 way, except during talking or singing. If you look in 

 your mouth with a mirror, you will see at the back part 

 an arch which is the rear boundary line of the mouth. 

 Just above the arch is likewise the limit for the back part 

 of the nasal passages. The funnel-shaped cavity beyond, 



