THE RESPIRATIOiV 



73 



both plants and animals. To supply the deeper cells large 

 animals require a breathing' surface greater than the area 

 of the skin. This is supplied by having the oxygen-absorb- 

 ing surface folded inward to form folds, tubes, and cavities 

 of great complexity. If the lungs of a man were unfolded 

 and all their tubes and cavities spread upon one surface, 

 an area of more than one hundred square feet (or ten feet 

 square) would be covered. 



Each respiration, or breath, consists of the passing in 

 of the air, or inspiration, sending it out, or expiration, 

 and a pause after 

 one but not after 

 both of the other 

 stages. 



The Air Passages. 

 The air usually 

 passes in at the 

 nose and returns 

 by the same way, 

 except during talk- 

 ing or singing. Ob- 

 serve your mouth 

 with a mirror (Fig. 

 68); at the back 

 part, an arch is 

 seen which is the 

 rear boundary line 

 of the mouth (Exp. 

 i). Just above the 

 arch is likewise the rear boundary line of the nasal pas- 

 sages. The funnel-shaped cavity beyond, into which both 

 the mouth and nasal passages open, is called the pharynx 

 (far'inks), or throat (see Fig. 68, also Fig. 83). Below, 



FIG. 67. CIRCULATION THROUGH LUNGS (sche- 

 matic) : " venous " blood (in pulmonary artery) 

 black ; " arterial " blood (in pulmonary veins) 

 white. 



