Walking among and studying various Native American tribes in the 19th century, the author noticed that many of the elders possessed a serene and well-preserved appearance. The young members of the true seemed especially healthy, with an innate resistance to certain illnesses and congenital conditions. Seeing the tribe's members sleeping, he noted that they all did so with closed mouths.

Catlin pondered whether this habit contributed to the physical vigor of the people, and investigated further. After venturing back to the towns of the Midwest, he attests to witnessing how terrible many people who had practiced mouth breathing throughout life appeared, and became deeply opposed to its practice. This book details how children and young people can be encouraged against mouth breathing, and notes how different the facial countenance appears between mouth breathing people and nose breathers.

Today, the notion that mouth breathing promotes physical ugliness or decrepitude is wholly disavowed as an eccentric idea with no basis in fact. However, sleep researchers have demonstrated that breathing with the mouth open while asleep can result in more snoring and thus a lower quality of sleep and therefore health. Overall, one could venture that Catlin's ideas possess a certain merit, even if his book is an exaggeration.

Although primarily known today as a painter and traveler who became an emissary of sorts to the Plains tribes, George Catlin was also an enthusiastic if occasional writer. He admired the Native American peoples for their traditions and distinctive appearance, and took to painting them - his marked talent led to their respect for his gifts, and they duly welcomed him with friendship.