This section is from the book "Health Without Medicine. A Treatise On The Laws Of The Human System", by Larkin B. Coles. Also available from Amazon: Philosophy Of Health.
There is no part of the human system which has such controlling influence over the whole body, as respects health or disease, as the Digestive Organs. Any derangement in these, especially the stomach, calls up a sympathy of action from the whole animal economy. Nearly all the morbid actions found in the general system are produced from causes first operating on the stomach. Hence, keeping the digestive system in a healthy state, secures, as a general rule, a healthy action in every other part of the physical organization. Therefore, to know something of the anatomy and physiology of the digestive organs, together with the laws of digestion, seems indispensable for every individual who would know how to take care of his health.
By the term digestive organs, are intended the Mouth, Stomach, Liver and Bowels, including the whole alimentary canal, commencing with the mouth and terminating with the extremity of the bowels. Extending through the whole length of this canal is a lining membrane, called mucous membrane, continuous throughout, from the lips to the opposite extremity. This membrane is filled, throughout its whole distance, with minute blood-vessels, and in some parts abundantly supplied with fine filaments of nerves. This membrane has important functions to perform in the process of digestion. It is a membrane of much delicacy of structure and sympathy. Its healthy action is easily deranged, and when deranged in one part, becomes, by sympathy, deranged in every part.