Surfactant prevents this collapse by reducing the surface tension of the fluids that line the lungs and helping to equalize the pressures between large and small air spaces. Surface tension is a measure of the attraction molecules of a fluid have for each other.
Buffalo Bill Cody, who later became famous for his Wild West Show, was a rider for the Pony Express and wrote of his experiences. We join Bill's story as he is hired - at the age of 15 - to ride a section of the trail that lies in modern-day Wyoming:
The animals used were chosen for their hardiness and speed, qualities natural to certain breeds. The term Pony Express is actually, a misnomer, for while the animals were generally small and compact, they were, in fact, horses. During the 1800s, America had several varieties of work horses available to them that proved useful in the mail operation.