The other theory concerning the origin of the lymphatics has been maintained by some very eminent physiologists later than Boerhaave; and supposes, that these vessels receive their lymph from the blood-vessels, or from the excretories of the larger glands, by the intermediation of only one small vessel, which these authors term a lymphatic artery, invisible in its natural state, nor yet rendered subject to the senses by experiments. But to this it may be answered, that the lymphatics are traced into many parts of the body, and lost there; and therefore most probably have their origin there, where no large gland nor blood-vessel is to be found in their neighbourhood: that it contradicts the whole analogy of nature, to suppose the motion of an animal fluid more discernible in the veins than in the arteries: and, finally, that it seems rather an instance of want of thought, and of being imposed upon by words, to call the lymphatic vessels veins, because they are furnished with valves; and then, because they are called veins, to take for granted, that of course they must be the continuation of arteries.
In attempting to investigate matters too subtile for the cognizance of our senses, the only method, in which we can reasonably proceed, is by inferring from what we know in subjects of the same nature: and our conclusion thus inferred, concerning the subject sought, will be firmer and more unquestionable, in proportion as it resembles the subject known. But if the subjects be really of the same kind; if no difference can be shewn between them, in any respect material to the inquiry, in which we are engaged; in this case our inference from analogy becomes the very next thing to a physical certainty: and this I apprehend to be true in relation to the problem before us, concerning the origin of the lymphatic vessels. Tho' in general we cannot, by experiments, arrive at the extremities of those tubes, nor satisfy ourselves, by inspection, in what manner they receive their fluid; yet in a very considerable number of them we can do both. There is a certain part of the human body very abundantly provided with lymphatics; in which part we can actually force injections thro' those vessels into a cavity, where their extremities open: and from this cavity, on the other hand, we can at pleasure introduce a coloured liquor into their extremities, and trace it from smaller into wider canals; from capillary tubes, without valves, into large lymphatic trunks copiously furnished with them. We know likewise, that into this cavity are continually exhaling an infinite number of watery and mucous vessels, both arterial tubes and excretory ducts: that these keep it moist with a perpetual vapour, which the extremities of those lymphatics are, in the mean time, perpetually imbibing. Does it not seem strange, while these particulars are known and acknowledged by all the world, that the great authors of anatomy and physiology should never have reasoned from them; but should run into complex and obscure suppositions, in order to explain a process, which they may at any time examine with their own eyes? But perhaps this inadvertency may be accounted for, if we recollect, that at the time when these vessels, and the structure of this part, were discovered, the lymph, and every thing belonging to it, was utterly unknown; and that the vessels in question were first seen and considered as performing another and more remarkable office: which circumstance, it should seem, has prevented succeeding authors from being duly attentive to them in the capacity of lymphatics. However this be, it is certain, that the lymphatics of the mesentery, commonly called the lacteals, differ from those of the other parts in no one particular, save that occasionally they carry chyle instead of lymph; or rather carry lymph mixed, at stated times (that is, for two or three hours after the creature has taken food) with an emulsion of vegetable and animal substances, and coloured white by that mixture. At other times, (that is, during sixteen of eighteen hours out of the twenty-four) they contain nothing but lymph; and are, in every respect, mere lymphatic vessels, not to be distinguished from those in any other part of the body. Their structure is the same; the membrane of which they are formed, their valves, the lymph which they contain, the glands thro' which they pass, their direction from smaller tubes to larger, and from these to the blood, differ in nothing from what we observe of the other lymphatics. Their lymph, in the mean time, is without doubt or controversy supplied from the cavity of the intestines; being the watery moisture continually exhaled there for the purposes of digestion, and for the preservation of the alimentary canal, and as continually taken up by the roots or extremities of these vessels, in order to be carried back to the blood, after it has performed its office in the bowels. Let it also be remembered, that these vessels, in other places of the body, are generally, when we trace them, lost in muscular, tendinous, or membranous parts: and then, I should presume, it may fairly, and with a good degree of evidence, be concluded, that the lymphatics of the body, in general, have their origin among the little cavities of the cellular substance of the muscles, among the mucous folliculi of the tendons, or the membranous receptacles and ducts of the larger glands: that their extremities or roots do, from these cavities, imbibe the moisture exhaled there from the ultimate arterial tubes, just as the lacteals (the lymphatics of the mesentery) do on the concave surface of the intestines: and that the minute imbibing vessels, by gradually opening one into another, form at length a lymphatic trunk, furnished with valves to prevent the return of its fluid, and tending uniformly, from the extremities and from the viscera, to reconvey to the blood that lymph, or that fine steam, with which they are kept in perpetual moisture; a circumstance indispensibly necessary to life and motion: while, at the same time, the continual re-absorption of that moisture by the lymphatics is no less necessary, in order to preserve the blood properly fluid, and to prevent the putrefaction, which would inevitably follow, if this animal vapour were suffered to stagnate in the cavities where it is discharged.
XLI. A Letter to the Right Honourable the Earl of Macclesfield, President, the Council, and Fellows, of the Royal Society, concerning the Variation of the Magnetic Needle; with a Sett of Tables annexed, which exhibit the Result of upwards of Fifty Thousand Observations, in Six periodic Reviews, from the year 1700 to the year 1756, both inclusive; and are adapted to every Five Degrees of Latitude and Longitude in the more frequented Oceans. By William Mountaine and James Dodson, Fellows of the Royal Society.
Dated London, Nov. 9th. 1757.
SIRS,
Read Nov. 10, 1757.
ON the 20th of March 1755, we presented an address to this illustrious Body, intituled, “An Attempt to point out, in a concise manner, the Advantages which would accrue from a periodic Review of the Variation of the Magnetic Needle, throughout the known World; requesting contributions thereto, by communicating such observations concerning it, as had then been lately made, or could be procured from correspondents in foreign parts.”
This address was read at the same time, and afterwards honoured with a place in the Transactions, vol. xlviii. part ii. for 1754: which favour we now acknowledge in the most grateful manner; and, pursuant to our engagements, beg leave to lay before you some account of the communications received, with a specimen of the uses and applications which we have been enabled to make of those, and other assistances with which we have been indulged.
On application to the Honourable the Commissioners of the Navy, we were obliged with an order of free access to all their masters log-books and journals.
The Directors of the Honourable East India Company granted the like privilege.