Over the nature of assimilation the thickest darkness still hangs; all that we know for certain is, that there are some conditions necessary to its action, without which it cannot take place. These are, 1. A sound and uninterrupted state of the nerves. 2. A sound state of the blood vessels. 3. A certain degree of tone or vigour in the vessels of the part.
There remains yet to be noticed another set of vessels, connected with the circulating and nutritive systems, called lymphatics. These vessels are very minute, and filled with a transparent fluid: they rise by open mouths in every cavity of the body, as well as from every part of the surface, and the course of those from the lower extremities, and indeed from most of the lower parts of the body, is towards the thoracic duct, which they enter at the same time with the lacteal vessels already described. They are furnished, like the lacteals, with numerous valves, which prevent their contents from returning towards their extremities.
The minute arteries in every part of the body exhale a colourless fluid, for lubricating the different parts, and other important purposes: and the lymphatic vessels absorb the superfluous quantity of this fluid, and convey it back to the blood.
It must be evident therefore, that, if the lymphatics in any cavity become debilitated, or by any other means be prevented from absorbing this exhaled fluid, an accumulation of it will take place: the same will happen, if the exhaling arteries be debilitated, so as to allow a greater quantity of fluid to escape than the absorbents can take up. When the balance between exhalation and absorption is destroyed, by either or both of these means, a dropsy will be the consequence.
Before we finish the subject of digestion, I shall take a short view of some of the morbid affections, attending this important function of the animal economy.
A deficiency of appetite may arise, either from an affection of the stomach, or a morbid state of the body: for there is such a sympathy between the stomach and the rest of the system, that the first is very seldom disordered, without communicating more or less disorder to the system: nor can the system become deranged and the stomach remain sound.
A want of appetite may arise from overloading the stomach, whereby its digestive powers will be weakened. And this may be occasioned in two ways. First, by taking food of the common quality in too great quantity, which will certainly weaken the powers of the stomach. An excellent rule, and one which if more attended to, would prevent the dreadful consequences of indigestion, is always to rise from the table with some remains of appetite. This is a rule applicable to every constitution, but particularly to the sedentary and debilitated.
The second way in which the stomach may be debilitated, is by taking food too highly stimulating or seasoned; and this even produces much worse effects than an over dose with respect to quantity. The tone of the stomach is destroyed, and a crude unassimilated chyle is absorbed by the lacteals, and carried into the blood, contaminating its whole mass. Made dishes, enriched with hot sauces, stimulate infinitely more than plain food, and bring on diseases of the worst kind: such as gout, apoplexy, and paralysis. "For my part," says an elegant writer, "when I behold a fashionable table set out in all its magnificence, I fancy I see gouts, and dropsies, fevers, and lethargies, with other innumerable distempers, lying in ambuscade among the dishes."
All times of the day are not equally fitted for the reception of nourishment. That digestion may be well performed, the functions of the stomach and of the body must be in full vigour. The early part of the day therefore is the proper time for taking nutriment; and, in my opinion, the principal meal should not be taken after two or three o'clock, and there should always be a sufficient time between each meal to enable the stomach to digest its contents. I need not remark how very different this is from the common practice of jumbling two or three meals together, and at a time of the day likewise when the system is overloaded. The breakfast at sunrise, the noontide repast and the twilight pillow, which distinguished the days of Elizabeth, are now changed for the evening breakfast, and the midnight dinner. The evening is by no means the proper time to take much nourishment: for the powers of the system, and particularly of the stomach, are then almost exhausted, and the food will be but half digested. Besides, the addition of fresh chyle to the blood, together with the stimulus of food acting on the stomach, always prevents sleep, or renders it confused and disturbed, and instead of having our worn out spirits recruited, by what is emphatically called by Shakespeare, "the chief nourisher in life's feast," and rising in the morning fresh and vigorous, we become heavy and stupid, and feel the whole system relaxed.
It is by no means uncommon, for a physician to have patients, chiefly among people of fashion and fortune, who complain of being hot and restless all night, and having a bad taste in the mouth in the morning. On examination, I have found that, at least in nineteen cases out of twenty, this has arisen from their having overloaded their stomachs, and particularly from eating hot suppers; nor do I recollect a single instance of a complaint of this kind in any person not in the habit of eating such suppers.