The physiology of digestion considered with relation to the principles of dietetics

by Andrew Combe

To fit the mucous coat for its office of absorption, an immense number of minute vessels, called absorbents, are ramified on its internal surface, the nature and purposes of which are analogous to those mentioned in the former volume when describing the functions of the skin.[34] In both structures the absorbents are small capillary or hair-sized vessels, so infinite in number that at least one goes to every little point or papilla. Those which open upon the inner surface of the smaller intestines, and which suck in or absorb the chyle, are called lacteal absorbents, or simply the lacteals or milk-vessels (marked LLLL in the subjoined wood-cut), from the white colour of the chyle shining through them and giving them the appearance of vessels full of milk. In that part of the gut they are so numerous that every minute point of the villous coat may be seen by the aid of a microscope to contain one with its mouth open to receive the chyle as fast as it is formed. Even in the colon the absorbents are numerous; but, as all traces of chyle have there disappeared, they are much fewer than in the smaller intestines. In the colon they serve chiefly to remove the more watery portions of the intestinal contents, by which means the fæces are rendered more solid and less bulky, and therefore better adapted for being retained for a time without inconvenience. It sometimes happens that when food or medicine cannot be swallowed in the usual way, life is preserved by injecting it into the bowels; in which case the absorbents of the large gut become active, and carry it into the system. Strong soups, milk, opium, laxatives, and other remedies are often administered in the same way, when any reason exists against giving them by the mouth.

There are absorbents in every structure of the body, because there are everywhere waste particles to be taken up and removed; but, except in the case of the lacteals, their contents are limpid or colourless, and hence in other places they are called lymphatics: in almost every other respect, however, the two classes of vessels are analogous to each other.

The peculiar property by which the minute lacteal vessels imbibe or take up the white chyle is not well understood. From the fact in physics that liquids rise in capillary tubes, the inference has been drawn that absorption in living vessels also takes place from capillary attraction. But in the animal body the application of the principle is undoubtedly modified by the properties peculiar to organization, and one of the most remarkable proofs of this is the circumstance of the absorbent vessels in different situations having to some extent a specific adaptation to the qualities of the substances upon which they are severally destined to act. At one time, indeed, it was supposed that the principle of exclusive adaptation was so complete that every absorbent vessel was permanently shut to every thing except its own peculiar object, and that, from amidst many elements, each selected its own with unerring tact. But of late it has been proved that the absorbents are less rigidly discriminating than was previously supposed, and that substances are readily taken up by them which Nature never intended them to receive. In mixing madder with the food of fowls, for example, for the purpose of dyeing their bones, the colouring matter of the root is taken up without difficulty by the absorbents along with the chyle, although madder was certainly never intended to be their natural stimulus. But even admitting this latitude in its fullest extent, there still exist a fitness and peculiarity of relation between the absorbents and their proper objects, which renders the latter more accessible to them than to any foreign body.

The lacteal vessels are most easily seen an hour or two after a meal; because they are then fully distended with chyle, even in their smaller branches. The latter, indeed, may then be distinctly traced proceeding from the different portions of intestine, and gradually coalescing into larger trunks, as seen at LL in the figure on p. [163]. These, again, terminate in the vessel called the thoracic duct (the beginning of which is seen at TD in the same figure), by which the chyle is conveyed almost in a direct course along the spine, and which is represented at DDDD in the annexed cut. On its arrival at the upper part of the chest, the thoracic duct crosses over and opens into the subclavian vein S, just before the latter reaches the right side of the heart, so that the chyle is there poured into the circulating current of the venous blood.

Such is the course of the chyle. But the lacteal absorbents, in their progress from the intestine to the thoracic duct, pass through the small glandular bodies called the mesenteric glands (MG, p. [163]), where some change, the nature of which is not at all understood, is produced upon the chyle, but which seems nevertheless to be of importance to its constitution. Where these glands are hardened and enlarged, as they often are in scrofulous children with large prominent bellies and thin bodies, nutrition is greatly impaired, although the appetite and stomachic digestion remain comparatively unaffected.

The reason why the chyle is carried so far, to be poured into the current of the venous blood just before the latter reaches the right side of the heart, is on consideration not less obvious than cogent. Chyle itself is not fitted to become a constituent part of the animal frame. Before it can become so it must be converted into blood; and this can be effected only by exposing it to the action of the air in the air-cells of the lungs in a state of intimate mixture with the venous blood. This admixture, again, is insured by the gradual way in which the chyle advances along the thoracic duct and falls into the circulating current almost drop by drop; and it takes place just before the dark blood has finished its course, and is again subjected to complete aëration in its passage through the lungs. As explained in the former volume, this aëration is so indispensable to the renovation of the old and the formation of new blood, that whenever it is rendered imperfect, either by obstructions in the lungs themselves or by the absence of a sufficiently pure air without, the result is invariably injurious to health; because the blood, being no longer properly constituted, becomes incapable of furnishing a healthy stimulus and nourishment to all the parts of the body. Hence the rapid “decline” which follows the appearance of pulmonary consumption, and other diseases affecting the structure and interrupting the functions of the lungs.

Every body knows as a fact that bad air is hurtful, and that wasting disease of the lungs is attended with rapid loss of flesh and strength; but the manner in which these effects are produced is not so familiarly known. Yet, in a practical point of view, a knowledge of the principle is highly important. Properly considered, respiration is in reality the completion of digestion. The stomach may convert the food into chyme, the small intestines may convert the chyme into chyle, and the absorbents may take up the latter and duly convey it into the circulating system; but unless it undergo the necessary change in the air-cells of the lungs it will not constitute good blood or afford due nourishment to the body. Hence it is that those among the working classes who are much confined in an impure and insalubrious atmosphere, even when plentifully supplied with food, are generally thin and ill-nourished; and hence those who, along with good digestion, have small narrow chests and very limited respiration, are commonly found to be constitutionally lean,—while those who, along with good digestion, have amply developed lungs and free and powerful respiration, are at the same time remarkable for proportional vigour of nutrition and stoutness of body. It is on this account that in chronic pulmonary disease recovery is always to be distrusted, unless, along with the disappearance of the prominent symptoms, restoration of the lost flesh occurs. If nutrition remains impaired, however great the relief may be in other respects, there is reason to believe that the lungs are still so extensively diseased as to injure their functions, and that, on the application of any fresh exciting cause, the dormant mischief will resume its activity. In such cases, when stomachic digestion is sound, a full diet generally over-stimulates the system, by pouring into the blood more chyle than the lungs are able to assimilate; in consequence of which it is diffused over the whole body in an imperfect state of preparation.

The mucous membrane is, like the skin, well provided with nerves, and has a mode of sensation peculiar to itself. Every villous point indeed has a nervous fibre ramified on it, to give it the necessary sensibility to its own stimuli. It is true we are not conscious in health of the impressions made on the intestinal nerves; but this, as already shewn in describing the stomach, is a privilege and not a defect. They recognise their appropriate stimuli, and cause the necessary actions to follow without requiring aid from the will. But when they meet with substances which ought not to be there, such as pieces of undigested food, or foreign bodies which have no natural relation to their constitution, they immediately indicate uneasiness, and excite the muscular contractions to rid them of the offending cause.