150 EXAMINATION OF HORSES 
body. In order that this vast volume of blood may be 
purified by the air it is essential that the air should get to 
it, and it is only separated from it by the thinnest possi- 
ble layer of epithelim lying upon a delicately-thin base- 
ment membrane. The blood has therefore to be spread 
out, as it were, in the thinnest possible stream, or sheet, 
and the air gets to both sides of it. In order that so vast 
a volume may be so spread out into a stream sufficiently ° 
thin the area of the stream is enormous. Lieberkiihn 
has estimated the respiratory surface in the human lung 
at fourteen hundred square feet, and if we regard the 
human lungs as one-third the size of those of the horse, 
we then have the vast area of four thousand two hundred 
square feet. It is clear, then, that if this interchange 
between the air and the blood is to go on with ease and 
rapidity, the air must find easy access 
1. To the lungs. 
2. Over the respiratory area. 
3- Away from the lungs. 
And at the same time the blood must find easy access 
1. To its airing ground. 
2. Through its airing ground. 
3. Away from its airing ground. 
The air gets to the lungs and into the numerous recesses 
by virtue of its own pressure, which, as you are aware, is 
fifteen pounds to the square inch. With such enormous 
pressure and gifted with great elasticity (compressibility), 
it is ever ready to rush in and fill a vacuum. A vacuum 
