Page:Popular Science Monthly Volume 2.djvu/80

68 whatever but mucus. As soon as articles of food made their way into the stomach, and touched the mucous membrane, its circulation grew rapid and its color lively, while peristaltic movements became evident. The mucous papillae then poured out their gastric juice, a clear and transparent fluid, designed to dissolve the food. On wiping away the mucus that covered the villous membrane, with a sponge or fine cloth, the gastric juice was soon seen reappearing and gathering in little drops that ran along the walls of the stomach like perspiration on the face. What we have just seen as to the mucous membrane is known to occur alike throughout the intestine, and in all the glandular organs connected with the digestive apparatus. The salivary glands and the pancreas, in the intervals of the act of digestion, present a pale and bloodless tissue, the secretions of which are wholly suspended. During the period of digestion, on the contrary, these same glands are swollen with blood, as if inflamed and erectile, while their vessels pour out the secreted fluids abundantly.

Two orders of circulation, then, must be recognized in the organs: one, the general circulation, known since Harvey's day; and the other, local circulation, only discovered and studied in recent times. In the phenomena of general circulation the blood may be said to do nothing more than traverse the parts, to pass from the arteries into the veins; in the phenomena of local, which is the true functional circulation, the blood penetrates all the folds of the organ, and gathers closely about its anatomical elements, to arouse and excite their special mode of activity. The nervous system, sensitive in its action through the vessels, governs all those phenomena of local circulation which attend organic activity; thus, the saliva flows copiously when a sapid substance makes an impression on the nerves of the mucous membrane of the mouth, and the gastric juice forms under the influence of contract between food and the sensitive surface of the stomach. But, for this mechanical excitement of the peripheral nerves of sensation, influencing the organ by reflex action, a purely psychic or cerebral excitement can be substituted. A simple experiment proves this: If a horse is taken while fasting, and the excretory duct of the parotid gland upon the side of the jaw is exposed and divided, nothing flows from it; the gland is at rest. If, now, oats are shown to the animal, or, still better, if, without any thing being shown, a movement is made which leads him to think he is about to have food given him, immediately a continuous flow of saliva issues from the duct of the parotid, and at the same time the tissue of the gland is injected, and becomes the seat of a more active circulation. Dr. Beaumont remarked similar phenomena in his Canadian. The idea of savory food not only solicited a secretion from the salivary glands, but provoked, besides, an immediate flow of blood to the mucous membrane of the stomach.

What we have just said as to the local or functional circulations, applies not only to those secreting organs in which there takes place