Note (46) — CANTO FIFTH.

It may be necessary to explain briefly the passage of the food through the alimentary canal, and the manner in which the chyle is converted into blood.

The food is received into the mouth, masticated between the teeth, imbued with saliva, and forced through the constrictors of the pharynx down the œsophagus into the stomach. It is then mixed with the gastric juice, which is secreted from the inner surface of the stomach; and this is found to be the immediate agent for effecting the change that the food undergoes. A knowledge of no part of physiology is more useful, than of the digestive organs.

“The peculiar properties of gastric juice render it one of the most powerful productions of the animal body. It is not a simple dilutent, but a solvent; and has the power of breaking down the food, and converting it into a soft, homogeneous paste, known by the name of chyme. So powerful is its faculty of solution, that the hard bones which dogs devour cannot resist its action. It not only unites with and dissolves the food, but changes its nature and composition. It is found to be of an antiseptic property, and corrects putrescency instead of inducing it. After the food has been properly acted upon by the gastric juice, it passes through a muscular contraction of the stomach, called the pylorus, into the duodenum. Here the food undergoes other changes, equally as important as those already produced on it in the stomach:—it mixes with the bile brought by the ducts from the liver, and with the pancreatic fluid from the pancreas. Having remained some time exposed to the action of these fluids, it is separated into two parts, an excrementitious, and a nutritious.”—(Richerand’s Physiology, p. 119.)

“After the food has remained a certain time within the duodenum, and this separation taken place, it proceeds along the other smaller intestines, the jejunum and ilium. By means of the peristaltic contractions of these intestines, the nutritious part of the food is said to be pressed out, and this is taken up by the inhalent mouths of the lacteals. The alimentary mass parts gradually with its nutritive particles, and passes from the small into the large intestines, the cœcum, the colon, and rectum.

“The nutritious part of the food taken up by the lacteals, is conveyed by different branches into the thoracic duct, and thence into the left subclavian vein, where it mixes with the blood brought back from the upper extremities. The subclavian vein terminates in the vena cava, and this in the right auricle of the heart.” The reason for the chyle entering the blood in the subclavian vein is explained as follows, by Dr. Paley:—“The chyle enters the blood in an odd place, but perhaps the most commodious place possible, viz., at a large vein in the neck, so situated with respect to the circulation, as speedily to bring the mixture to the heart. And this seems to be of great moment; for had the chyle entered the blood at an artery, or at a distant vein, the fluid composed of the old and new materials must have performed a considerable part of the circulation, before it received that churning in the lungs which is probably necessary for the intimate and perfect union of the old blood with the recent chyle.” (Paley’s Natural Theology.)

“It is now necessary that this new venous blood, which is of a dark color, should undergo changes indispensable to life. The right auricle contracting, the blood is propelled into the right ventricle, and from the ventricle it is farther forced through the pulmonary artery into the lungs. While circulating through them, it is exposed in the air cells to the atmospheric air taken in during respiration:—a change takes place; the blood becomes of a florid red color, subservient to the principles of life, and is returned by the pulmonary veins into the left auricle of the heart; and passing from them into the left ventricle, this ventricle contracts and propels the blood, by means of the aorta, to all parts of the body.

“Different opinions are entertained of the change which the blood undergoes in the lungs. Some physiologists imagine that it receives oxygen from the air; others that it gives off its superabundant carbon, and that the air is expired as carbonic acid gas. It is here sufficient to state, that the change is necessary to life, and that the blood sent to the lungs is of a dark color, while that which returns from them is a florid red, subservient to the functions of the animal economy.” (Waite’s Manual, 1826, p. 70.)