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in a morbid state, found tinged with a mixture of the 'red globules or crassamentum of the blood. Upon this foundation 2 different theories have been raised, concerning the connection of the lymphatics with the arteries.
Of these, we shall first consider that of the famous professor Boerhaave. He observed, that every artery of the body is greater, in its diameter, than any of its branches; and this observation being found true, as far as our eye and the microscope can inform us, he inferred, by analogy, that it held good even through the most minute subdivisions of the arterial system. But, says he, proportionable to the diameter of the canal is the size of the particles moving through it: therefore, if an ultimate capillary artery, admitting only one red globule at once to pass through it, send off lateral branches, these brancbes will be capable of receiving such particles only as are smaller than a red globule. But the particles next in magnitude below the red globules are the yellow serous ones; and the lateral vessel, thus receiving them, is a serous artery, and the trunk of a 2d order of vessels. In like manner, this trunk, being continued on through many lessening branches, will at last grow so minute, as to admit only 1 serous globule: its lateral branches, therefore, will receive only such particles as are smaller than the serous ones: but these are the particles of the lymph; and this lateral branch is a lymphatic artery, and the trunk of a 3d order of vessels. Thus, in the red arteries are contained all the circulated fluids of the body; in the serous arteries, all except the red blood; in the lymphatics, all except the red blood and
Presbyterian sect, not being in a situation to afford the money necessary to procure a liberal education for his son, who shewed a taste for intellectual pursuits, some assistance was offered and received from the dissenters' fund, for defraying the expences of his further education; and he was sent to Edinburgh, to qualify himself for the office of a dissenting minister. He had not been long at this university before he determined to relinquish the study of divinity for that of physic; repaying that contribution (says Dr. Johnson) which being received for a different purpose, he justly thought it dishonourable to retain. In 1741 he went to Leyden, and 3 years after took his degree of M.D. there. On his return from Holland, he first settled at Northampton, but not getting into ninch practice, he removed from thence to Hampstead, and afterwards to London.
At London he was known as a poet, but had still (as we are also informed by Johnson) to make his way as a physician; and would perhaps have been reduced to great exigencies, but that Mr. Dyson, with an ardour of friendship that has not many examples, allowed him £300 per annum. Thus supported, he advanced gradually in medical reputation. He was appointed physician to St. Thomas's hospital, became F. R. S. was admitted by mandamus to the degree of M. D. in the university of Cambridge, and was afterwards elected a Fellow of the Royal College of Physicians. He had further the honour of being appointed physician to the queen. The last of these honours he did not long enjoy, being cut off by a putrid fever in the summer of 1770, when he was in the 49th year of his age.
The medical writings of Dr. Akenside consist of various papers, published in the Medical Transactions of the College of Physicians, of an Harveian oration, and of his Dissertatio de Dysenteria, the elegant latinity of which has entitled him to the same height (as an able critic has remarked) among the scholars, as he possessed before, in consequence of his poetical writings, among the wits. Of his poems, that on the Pleasures of Imagination, the merits and defects of which have been fully painted out by Johnson, is the most celebrated.
serum; and this subordination is, according to the same laws, continued down through fluids more subtle than the lymph, to the smallest vessel, which is propagated from the aorta. Such was Boerhaave's doctrine concerning the vascular system of animal bodies; like many of his other notions, ingenious, plausible,, and recommending itself, at first sight, by an appearance of geometrical and mechanical accuracy, but founded on insufficient data, and by no means to be reconciled to appearances.
For, in the first place, should we admit his hypothesis, it is certain, that the conical or converging form of the aorta, and the change of direction in its branches, must, in the distant blood-vessels, occasion a great resistance to the moving blood, and a great diminution of its velocity. Suppose that this resistance be, in any capillary red artery, to the resistance in the trunk of the aorta, as any larger assignable number is to unit: the resistance, then, in a capillary serous artery will, to that in the aorta, be as the square of that number is to unit; in the capillary lymphatic, as the cube; and so in progression: that is, the velocity of the fluids, in the remoter series of vessels, will be, physically, nothing. But we know, on the contrary, that some very remote series of vessels have their contents moved with a very considerable velocity; particularly the vessels of the insensible perspiration: and in anatomical injections, the liquor thrown into an artery scarcely returns more easily or speedily by the corresponding vein than by the most subtle excretory ducts. Moreover, there are an infinite number of observations of morbid cases, in which the red blood itself has been evacuated through some of the most remote series of vessels, merely from an occasional temporary obstruction in one part, or a preternatural laxity in another; and without any lasting detriment to the structure and subordination of the vessels; which yet, upon this hypothesis, must have been utterly destroyed before such an irregularity could have happened.
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 bloodvessel 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 shown 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 he apprehends to be true in relation to the problem before us, concerning the origin of the lymphatic vessels. Though 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 through 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 2 or 3 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 16 or 18 hours out of the 24, 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 saine: the membrane of which they are formed, their valves, the lymph which they contain, the glands through 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, it is presumed, 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 stream, with which they are kept in perpetual moisture; a circumstance indispensably necessary to life and motion: while, at the same time, the continual reabsorption 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 was suffered to stagnate in the cavities where it is discharged.
XLI. On the Variation of the Magnetic Needle; with a Set of Tables exhi-
From the first instant, say these gentlemen, that we made this affair the object of our particular consideration, we have attended to the mode of increase and decrease in the variation; and as a considerable number of observations made at periodic times, and duly registered, seem to be the most essential toward determining the laws of its mutation, or proving its irregularity, we have therefore formed a set of tables, from actual observations collected for the years 1710, 1720, 1730, and 1744, the date of our last chart; which, together with Dr. Halley's for the year 1700, and the present chart now publishing, complete 6 reviews: These are tabulated, and show the quantity of the variation at those several periods, to every 5 degrees of latitude and longitude in the more frequented oceans. Under the equator, in longitude 40° E. from London, the highest variation during the whole 56 years appears to be 17° w. and the least 16° w. and in latitude 15° N. longitude 60° w. from London, the variation has been constantly 5° E. but in other places the case has been widely different; for in the latitude 10° s. longitude 60° E. from London, the variation has decreased from 17° w. to 7 w.; and in latitude 10° s. longitude 5° w. from London, it has increased from 2° 4 w. to 12° w.; and in latitude 15° N. longitude 20° w. it has increased from 1° to 9° w.
But there is still a more extraordinary appearance in the Indian seas: for instance, under the equator. Where the west variation in the longitude 40° E. is the same in both the above years; and in 1700 the west variation seemed to be regularly decreasing from longitude 50° E. to the longitude 100° E.; but in 1756 we find the west
Deg. Deg. Deg.
Deg. Deg. Deg. 75E 93 w 1 W
w 14 w
SO E 7 W OE
w 1 E
55 E 161 W
90 E 4
60 E 15 W
95 E 3
W 0 w
65 E 13
variation decreasing so fast, that we have east variation in the longitude 80°, 85o, and 90° E; and yet, in the longitude 95° and 100° E. we have west variation again.
Such are the irregularities that experience has shewn us in the variation of the magnetic needle; which appear so considerable, that we cannot think it wholly under the direction of one general and uniform law; but rather conclude with Dr. Gowen Knight,' in the 87th prop. of his treatise on attraction and repulsion, that it is influenced by various and different magnetic attractions, in all probability occasioned by the heterogeneous compositions in the great magnet, the earth. Notwithstanding all which, should the sagacity of some eminent. philosopher be able to exhibit rules, whereby the quantity of the variation may be computed for future times, yet then such a review as we have now made, will be necessary at a proper interval to prove the truth of them and should no such rules appear, then will a continued succession of such reviews be necessary. so long as commerce and navigation subsist among us. The tables follow.