Those who labor much with the hands would do well to take some time each day for mental recreation, and those who work in mental channels should get joy and benefit from physical efforts. A few hobbies, depending upon circumstances, may be: Photography, music, a foreign language, the drama, literature, history, philosophy, painting, gardening, raising chickens, dogs or bees, floriculture, and botany. Some people have become famous through their hobbies. They are excellent for keeping the mind fluid, which helps to retain physical youth. There is something peculiarly beneficial about tending and watching growing and unfolding things. It is well known that women remain young longer than men. We have good reason to believe that one of the causes is their intimate relation with children. Growing flowers, vegetables, chickens and pups have the same influence in lesser degree. Tender, helpless things bring out the best qualities in our natures. We can not be on too intimate terms with nature, so, if possible, select a hobby that brings you closely in contact with her and her products. CHAPTER XXIII. BREATHING AND VENTILATION. The respiratory apparatus is truly marvelous in beauty and efficiency. Medical men complain about nature's way of constructing the alimentary canal, saying that it is partly superfluous, but no such complaint is lodged against the lungs and their accessories. The respiratory system may be likened in form to a well branched tree, with hollow trunk, limbs and leaves: The trachea is the trunk; the two bronchi, one going to the right side and the other to the left side, are the main branches; the bronchioles and their subdivisions are the smaller branches and twigs; the air cells are the leaves. The trachea and bronchi are tubes, furnished with cartilaginous rings to keep them from collapsing. They are lined with mucous membrane. The bronchi give off branches, which in turn divide and subdivide, until they become very fine. Upon the last subdivisions are clustered many cells or vesicles. These are the air cells and here the exchange takes place, the blood giving up carbonic acid gas and receiving from the inspired air a supply of oxygen. This exchange takes place through a very thin layer of mucous membrane, the air being on one side and the blood capillaries on the other side. The whole respiratory tract is lined with mucous membrane. This membrane is ciliated, that is, it is studded with tiny hairlike projections, extending into the air passages. These are constantly in motion, much like the grain in a field when the wind is gently blowing. Their function is to prevent the entry of foreign particles into the air cells, for their propulsive motion is away from the lungs, toward the external air passages.