The body cannot properly be compared to any single machine, but must be looked upon as a complex organization which employs a number of different kinds of machines in carrying on its work. The majority of these machines are found in the skeleton. The bones are the parts that are moved, and the joints serve as bearings. Connected with the bones are the muscles that supply energy, and attached to the muscles are the nerves that control the motion. Other parts also are required for rendering the machines of the body effective in doing work. These are supplied by the tissues connected with the bones and the muscles. HYGIENE OF THE SKELETON Of chief concern in the hygiene of the skeleton is the proper _adjustment_ of its parts. The efficiency of any of the body machines is impaired by lack of proper adjustment. Not only this, but because of the fact that the skeleton forms the groundwork of the whole body—muscles, blood vessels, nerves, everything in fact, being arranged with reference to it—any lack of proper adjustment of the bones interferes generally with the arrangement and work of tissues and organs. The displaced bones may even compress blood vessels and nerves and interfere, in this way, with the nourishment and control of organs remote from the places where the displacements occur. For these reasons the proper adjustment of the different parts of the skeleton supplies one of the essential conditions for preserving the health. *Hygienic Importance of the Spinal Column.*—What has been said about the adjustment of the skeleton in general applies with particular force to the spinal column. The spinal column serves both as the central axis of the body and as the container of the spinal cord. Thirty-one pairs of nerves pass between the vertebræ to connect the spinal cord with different parts of the body, and two important arteries (the vertebral) pass through a series of small openings in the bones of the neck to reach the brain. Unnatural curves of the spine throw different parts of the body out of their natural positions, diminish the thoracic and abdominal cavities, and, according to the belief of certain physicians, compress the nerves that pass from the cord to other parts of the body. Slightly misplaced vertebræ in the neck, by compressing the vertebral arteries, may also interfere with the supply of blood [Fig. 104] Fig. 104—A tendency toward spinal curvature (after Mosher) [Fig. 105] Fig. 105—Effect on spinal column of improper position in writing. (From Pyle’s _Personal Hygiene._)