THE EE8PIRATORT MECHANISM. 381 



them taking up for themselves from their environment the 

 oxygen which they need, and passing out into it their car- 

 bon dioxide waste; in other words, there is no differen- 

 tiation of the external and internal respirations. When, 

 however, an animal is larger many of its cells are so far from 

 a free surface that they cannot transact this give-and-take 

 with the surrounding medium directly, and the blood, or 

 some liquid representing it in this respect, serves as a mid- 

 dleman between the living tissues and the external oxygen; 

 and then one usually finds special respiratory organs devel- 

 oped, to which the blood is brought to make good its oxygen 

 loss and get rid of its excess of carbon dioxide. In aquatic 

 animals such organs take commonly the form of gills; these 

 are protrusions of the body over which a constant current of 

 water, containing oxygen in solution, is kept up; and in 

 which blood capillaries form a close network immediately be- 

 neath the surface. In air-breathing animals a different ar- 

 rangement is usually found. In some, as frogs, it is true, the 

 skin is always moist and serves as an important respiratory 

 organ, large quantities of venous blood being sent to it for 

 aeration. But for the occurrence of the necessary gaseous 

 diffusion, the skin must be kept very moist, and this, in a 

 terrestrial animal, necessitates a great amount of secretion by 

 the cutaneous glands to compensate for evaporation; accord- 

 ingly in most land animals the air is carried into the body 

 through tubes with narrow external orifices and so the drying 

 up of the breathing surfaces is greatly diminished; just as 

 water in a bottle with a narrow neck will evaporate much more 

 slowly than the same amount exposed in an open dish. In 

 insects (as bees, butterflies, and beetles) the air is carried by 

 tubes which split up into extremely fine branches and ramify 

 all through the body, even down to the individual tissue ele- 

 ments, which thus carry on their gaseous exchanges without 

 the intervention of blood. But in the great majority of 

 air-breathing animals the arrangement is different; the air- 

 tubes leading from the exterior of the body do not subdivide 

 into branches which ramify all through it, but open into one 

 or more large sacs to which the venous blood is brought, and 

 in whose walls it flows through a close capillary network. 

 Such respiratory sacs are called lungs, and it is a highly de- 

 yeloped form of them which is employed in the Human 

 Body. 



The Air-Passages and Lungs. In our own Bodies some 



