I should be disposed to think that the insensible contraction of which the small capillary system of each gland is capable, would aid the motion of the lymph, by diminishing the course that this fluid must take, without a new impulse, from the origin of the absorbents to the black blood, if these organs were wanting. In fact, we know that in the extremities where there are much fewer glands, there are more frequent effusions than in the trunk where the absorbents pass through them at every instant; I speak of those effusions which ought evidently to be attributed to the want of circulation of the lymph, as those arising from compression, too long standing, &c. and not those that depend on an increased exhalation, like those after organic affections.
We see, from what I have said thus far, that we have only a few disconnected views upon the motion of the lymph; that of the veins, though still requiring much research, is yet more known; but in order to give a perfect knowledge of these subjects, the first especially, many experiments and much further labour are necessary.
IV. Of Absorption in the different Ages.
In the fœtus and in childhood, absorption relative to nutrition is not in proportion to exhalation. Many substances remain in the organs, but few go out, hence there is growth.
But little is known of the differences which the internal absorptions of synovia, serum, fat, marrow, &c. then present.
The external absorptions appear to be more active, for we know that contagions are taken with much more facility in the first age. We know not however whether the skin and mucous surfaces then constantly introduce more foreign substances into the body, or if they are only more disposed to introduce them.
We are deficient in positive data as to the state in which absorption is found in childhood. To judge of it however by that of the lymphatic glands, it would appear that it was very energetic. In fact these glands are in proportion much developed; they appear to be the seat of very active functions; they have a peculiar life more developed than afterwards, hence a greater disposition to diseases. We know that until puberty, or rather until the end of growth, they are the seat of many affections which entirely disappear after that age, and lessen the numerous series of those to which we are exposed.
This double circumstance, 1st, the precocious and proportionably great development of the lymphatic glands in childhood; 2d, their very great disposition to diseases, indicates certainly remarkable activity in their functions; for it supposes a great development of the vital forces; now these vital forces being more developed ought necessarily to preside over more energetic functions. See in fact the organs whose functions we know, which are on the one hand much developed in childhood, and on the other much disposed to diseases; the functions of these organs are more active. Thus the brain and nerves being more developed, have more sensibility; thus the vessels with red blood have an activity of nutrition, in proportion to their size, &c. In youth, it is when the genital organs are more developed and they become more exposed to diseases, that their functions are greater. Examine all the organs and their functions, you will see that a general law of the economy is, that these three things, 1st, great development; 2d, greater disposition to diseases; 3d, greater activity of functions, are constantly united. Now when these two first exist in the glands of the absorbents, we ought to conclude that the third is there also, though we are not certain of it, since, from what I have said, we are ignorant of the uses of these small organs. Grimaud has considered them, it is true, as essential to nutrition; he even calls the nutritive system the combination of these glands and the cellular texture, a gratuitous supposition, which is not proved. All that we know upon this point, is that on the one hand nutrition, and the development of these glands on the other, are very great in the fœtus. But does it follow from this, that the first phenomenon proceeds from the second? Undoubtedly not; no more than because the brain, the liver, &c. are early developed in the fœtus, and nutrition is very active, they should be considered as the agents of this function. Besides, nutrition is a function that has no particular organ for its centre and agent. Each organ is itself the machine which separates from the blood or the fluids that enter it, the nutritive materials that are suitable for it, and afterwards appropriate them to itself. The muscle separates its fibrin, the bone its phosphate of lime, &c. But one common and central organ does not elaborate these nutritive materials, as one common viscus moves the blood, as one central organ presides over sensibility, &c.
As to the anatomical state of the absorbents in the fœtus and childhood, we can know but little; I do not know that any author has injected them comparatively in this age and in the adult. I have but one fact upon this point, it is that the lacteals, examined in an experiment upon two young dogs, who had only left off sucking eight days, appeared to me larger in proportion than at a more advanced age. I will make one remark that has often struck me; it is that the size of the animal has much less influence than would be thought upon the diameter of these vessels. For example, an adult dog, twice as large as another, has not by a great deal, vessels of double size. Accident led me to examine them the same day, three years since, upon two large grey-hounds, that were among the dogs brought to me, and upon one of those vulgarly called cur dogs; they were nearly equal in all three; this struck me.
We know but little of the different revolutions that absorption undergoes in the ages that succeed infancy. Only there is no doubt that puberty is the limit of this kind of predominance which the lymphatic glands enjoyed in the economy. The age of their diseases is then passed; often even these diseases, heretofore beyond the reach of art, are spontaneously cured. The predominance of the genital organs which succeeds to this and some others, those of the sensitive-organs, &c. seems to destroy the germ which this first supported.