RESPIRATION: THE MECHANISM OF BREATHING 349 



simplest creatures, indeed, there is even no blood, the cell or cells 

 composing them taking up for themselves from their environ- 

 ment the oxygen which they need, and passing out into it their 

 carbon dioxid waste; in other words, there is no differentiation 

 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 surround- 

 ing medium directly, and the blood, or some liquid representing 

 it in this respect, serves as a middleman between the living tissues 

 and the external oxygen; and then one usually finds special 

 respiratory organs developed, to which the blood is brought to 

 make good its oxygen loss and get rid of its excess of carbon 

 dioxid. 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 arrange- 

 ment 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 com- 

 pensate for evaporation; accordingly 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 elements, which thus carry on their gase- 

 ous exchanges without the intervention of blood. But in the 

 great majority of air-breathing animals the arrangement is dif- 

 ferent; 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 



