With the exception of articles that are in solution, the condition in which all foods should enter the digestive organs is in finely divided, yet distinct particles, and not in pasty or gummy masses. In this latter form "bolting" is encouraged, and mastication greatly discouraged. THE MUSCULAR MOVEMENT OF DIGESTIVE ORGANS [Sidenote: Peristaltic movement in alimentary canal] Another point to be considered in digestion, and which may well be classed under the mechanics of digestion, is the muscular action or peristalsis of the alimentary tract. The best example is the swallowing action observed in the throat of a horse, or of a cow, when drinking. At each swallow, what appears to be a lump goes down the throat. This is a wave-like relaxation of the muscular walls of the esophagus, followed closely by a muscular contraction. This is the action that takes place in the intestinal tract, and that which Nature employs to move the contents along toward the final point of excretion. [Sidenote: X-ray examination of peristalsis] A very fascinating and scientific demonstration may be performed in the following manner: A cat may be given food mixed with some such substance as bismuth subnitrate, which is opaque to X-rays. Upon placing the animal under an X-ray during digestion, this peculiar peristaltic motion can be observed, one "swallow" passing rapidly after another down the alimentary tract. This method of investigation has also shown that peristaltic action stops immediately in the case of fright, or anger, but is shown to proceed with regularity during sleep, contrary to the antiquated idea that digestion ceases when sleep begins. [Sidenote: Milk for relieving constipation] Peristaltic action in the lower parts of the alimentary canal is stimulated by taking food into the stomach. This explains the laxative action of such foods as fruits, or, sometimes, milk, taken at frequent intervals. When all other methods fail, constipation can oftentimes be relieved by taking a glass of milk every thirty minutes until four glasses have been consumed. [Sidenote: Moisture and peristaltic action] The longer food remains in the intestines, the more completely is the water absorbed from the residue. The object to be obtained in relieving constipation is to increase the moisture and the peristaltic action. Whatever will accomplish these things will relieve and perhaps cure intestinal congestion. The subject of intestinal congestion and purgative medicines will be discussed at length in Lessons IX and XI, Vol. II, p. 375 and p. 436, respectively. LESSON VI CHEMISTRY OF METABOLISM [Sidenote: Meaning of metabolism]