SECT. VI.
Of Chylifaction and Nutrition.
In the preceding we have briefly considered the state of the fluid of life, and mechanical instruments of their motion in the human machine. In this we will consider how the whole is supported, and the vital lamp, from time to time, becomes supplied.
Under this consideration we will examine the canal of food, and the various changes the morsel undergoes from the time we take it in our mouths, till the nutriment is reduced to blood, and the remaining dross is expelled the body as useless.
The animal functions towards chylifaction and nutrition, are mastication, or chewing the food; deglutition, or swallowing; digestion; chylifaction; nutrition; and the excretion of the fæces.
In order therefore to understand this clearly, it becomes necessary to describe the organs which nature has formed for the requisite performances thereof.
In the first place then, smelling is that sensation which nature has given to every animal, that has a choice of food, as the first safe-guard to inform it of any thing agreeable or disagreeable, useful or pernicious to its body; its seat is in the nose chiefly, but we find by experience, that it has a friendly connection with our palate and stomach; for the effluvia of any thing will either create a desire, or give us the greatest aversion to every substance that throws out a flavour.
It is performed by means of a subtile spreading of nerves, peculiarly delicate; which continues through the membrane of the nose, the roof of the mouth, gullet, and the very stomach.
Taste is the next sensation, which nature has given us, not only as a distinguisher of proper food, but a sensation from which we receive many luxurious pleasures; and to the indulgence of this sensation, most evils and plagues to mankind, take their origin.
The tongue is the principal instrument of that peculiar quality; but if we examine somewhat closer into this affair, we shall find that the soul of pleasure and pain of that sensation, as well as that of smelling, has its seat in the stomach; for that which will taste pleasing and good at the first approach, will soon lose its relish when the stomach is gratified: and, if any thing tastes disagreeable, the stomach receives it with reluctance, and will ever incline to discharge it again.
The tongue is an instrument (if I may be allowed the term) very curiously constructed; it is moved by a variety of muscles, and serves not only for tasting, but also as a labourer, to shovel and to turn our meat between our grinders; so that nothing may escape being well masticated, and intermixed with that fine digesting balsam, the spittle, in order that it may be easily swallowed.
Besides this, it makes the most requisite instrument for the noble and excellent faculty of speaking; which forms one of the principal characteristics that distinguishes man from the brute creation.
The gullet or oesophagus, is the canal which conveys drink and food from the mouth to the stomach; this canal is a muscular, tendinous, and vascular tunic.
The commencement of it is in the mouth, and is called the pharynx; a curious structure, that receives the food, and by its contractive motion, and the help of the tongue, forces the aliment into the stomach.
The stomach is much like the bag of a Scotch bag-pipe; it lies immediately under the diaphragm or midriff, covered partly on the right side with the liver, and on the left side with the spleen. The left and superior part, is continued with the oesophagus; and the right and inferior part, or orifice, commences the intestines.
The first orifice is called the mouth of the stomach; and the second the pylorus, or porter: At the porter there is a curious valve which lets the aliment out by small parcels into the intestines, where it undergoes its various other changes.
The stomach has three teguments, a muscular, a tendinous, and nervous coat; this nervous coat has another slimy one, but this in reality, is a delicate lining, interwoven with nerves, and the ramifications of fine blood vessels.
The intestines or guts, are a continuation of the stomach, they are a canal which is generally reckoned six times as long as the subject it is taken from; it is distinguished in small tenuia, and wide crassa.
Each again is divided into three parts: the tenuia, or small narrow intestines, are the duodenum, or twelve finger-gut; the jejunum; the ileum: The wide or crassa, is divided into the cæcum; the colon; and the rectum.
Throughout the whole canal of intestines are numbers of little vessels, called lacteals, which lead the chyle, extracted from the aliment, into a receptacle, which is lodged in the mesentery, and from thence, by another duct call’d the thoracic duct, is carried along the back-bone upwards, and joins to the left subclavian vein, where the chyle gradually commences to be blood.
This short description we will let suffice, and now enter upon the action itself.
The morsel now, which is designed for food, is taken into the mouth, masticated with the teeth, turned about with the tongue; and as the mouth is at work, the saliva or spittle is squeezed from the salival glands, and thus intermixed with the aliment; when enough chewed and moistened with this saliva, it is conveyed to the pharynx, or swallow, which receives it, and, by its contraction, forces it into the oesophagus; and by a repeated contraction, is carried down into the stomach.
There it is again moistened with saponaceous liquid, or pancreatic juice; by which and by a perpetual motion of the stomach, it is brought into a state of digestion; then by small degrees entered through the pylorus or porter, into the first division of the gut, the duodenum.
This gut is about twelve fingers long; and whilst the aliment is there, it is intermixed with the gall, which is a liquor separated by the liver, and contained in the gall bladder; this liquor, the gall, is carried into the duodenum, by a small duct, called the ductus cysticus; where also enters another kind of liquor called the pancreatic juice.
When the aliment is thus prepared, and fit for a particular state of dissolution, it is carried into, and through the jejunum. This gut is in length about twelve or thirteen hands breadth, and its motion somewhat brisk; through which the aliment passes pretty quick, and hence, generally is somewhat empty.
As it passes through this part, the chyle is separated from it by the lacteals, which are small vessels that separate the chyle from the aliment, and abound there more than in any other part of the gut.
From thence it comes into the ileum; that is the longest of all the divisions of the guts, and is in length about twenty-one hands breadth; it has a great many circumvolutions, and next to the jejunum, has many lacteals to separate the chyle.
Now the aliment comes into the wide gut, and gradually becomes fæces; and first, the cæcum: This part is rather an appendix only, and hangs from the main part like a finger to a glove. The use of this gut has been much controverted by anatomists; it seems however, very probable, that this appendix is designed to keep the aliment in for further digestion, as it now begins to putrefy, and becomes fæces or excrement.
From thence it enters the colon, which is a long, and very winding intestine; it runs up along and about the liver, touches the gall bladder, and the spleen; from thence it descends again to the os sacrum. It has but few lacteals, and is, as it were, the last drainer of the fæces: It is this intestine which is the seat of the Colic, and of most other complaints of the belly.
Next to this comes the last and straightest, the rectum; this gut is closely adherent to the sacrum, and ends in the fundament; which is provided with muscles to open and shut the anus, in order to contain the fæces, and discharge it.
The mechanism of chylifaction in the human body differs from the brute creation in general, except that most contemptible of the whole, the hog; to which it bears a very near resemblance, insomuch that there is very little distinction.
Both have that advantage over the generality of terrestrial animals, that they are confined to no particular food; which favours greatly the luxury of the one, and the beastiality of the other.
The brute creation are generally distinguished into carnivorous and granivorous: The first is that kind which feeds upon flesh; and the latter upon grain and vegetables. Upon examination however, we find, that the stomach and guts are peculiarly adapted to their food; and that grass agrees no more with the dog, than mutton does with the horse.
But man is so happily made, that any thing which is food, is proper for him, and he may become used to it; and thus is either carnivorous or granivorous.
The whole canals, from the stomach to the anus, is in a continual vermicular motion, which is called the peristaltic motion; by this the aliments are dissolved, and disunited; and as they pass along, are drained by the lacteals, of their nutriment or chyle.
These lacteals are, by means of a membrane (with which they are surprisingly interwoven, and connected to the whole canal) called the mesentery, lead regularly into one common cistern, lodged almost in the middle of the intestines, in that membrane, called the receptacle of chyle; and from thence the chyle is carried by a duct up along the back-bone, called the ductus thoracicus, into the left subclavian vein, where it gradually commences to be blood.
By this mechanism we are nourished, and the substance of our food converted into blood, and transported through the whole animal machine, for the support of every part of its wonderful composition.
As all animals which feed upon flesh, are more subject to diseases, nature has provided them with these advantages: that when any thing is obnoxious to their nature, and received into their stomach, or their being over loaded, it can discharge itself of so troublesome a burthen, by vomiting, which is effected thus: when the inner coat of the stomach, which is irritable and nervous, is stimulated by whatever is obnoxious, it will cause in the whole stomach, a contraction; and by that, force its contents to the shortest direction of evacuation, namely, by the canal of the oesophagus, through the mouth. This expulsion is peculiar to carnivorous animals only.
Purging or discharging by the fundament is common to all animals of whatever kind; and is performed by an irritation in the intestines, by which the peristaltic motion is increased; to this I must add a reversion of the secretion of the lacteal vessels, by which the humours are increased, the motion accelerated, and the fæces discharged, without giving any nourishment to the body, and consequently the system diminished.
Digestion is that act by which the aliment or food is prepared, so as to produce a good chyle, and consequently good blood, for the nourishment of the body. Though no animal has a more delicate stomach than man, yet it must be observed, that none has a stomach better adapted for all kinds of food.
Whence therefore in the common course of life, temperance and gentle exercise is what nature requires to maintain health. But nothing becomes more obnoxious to that blessing, than gluttony, voluptuousness, and idleness.