DEFINI racter; whilst the thoracic duct acts as a great reservoir preparatory to the CHAP. VII. new materials of blood passing into the subclavian vein and being there SECT. I. mixed with the mass of blood. In other words, absorption is principally the process by which the new materials, arising from food and nourish- TION, &c. ment, and converted in the stomach and smaller intestines into chyle, and serving for the nourishment, growth, and support of the body, are separated and in a degree more assimilated, and ultimately carried into the blood. (e) This is the primary and most essential office of the absorbent function, and is principally performed by one description of the absorbent vessels, which thus receive the recent new matter, the products of aliment and digestion, and convey it principally through the thoracic duct. to the blood. But there is also a further and distinct object effected by another and totally different set of absorbents; for it is at least supposed. if not established, that all the particles of which the body is composed from time to time lose the power of performing their appropriate functions, and that, therefore, to continue the animal economy perfect, they must be removed and new matter deposited in lieu, and which object is effected by this other branch of the system of absorption (though not perceptible to our faculties) so that a continual exchange of particles takes place, the former constituent being taken up by certain absorbent vessels, namely, the lymphatics, and returned into the general circulation to be either discharged or employed under some new form;(f) and the vessel employed in the latter functions are termed the lymphatics, from the circumstance of the fluid which they contain being lymph. SECT. II. THE OR GANS AND PARTS OF THE FUNC TION OF AB SECTION II.-THE ORGANS AND PARTS OF THE FUNCTION OF ABSORPTION. The absorbent vessels, which imbibe the fresh materials for new blood from the digested food, are principally found in the small intestines, where they receive into one common reservoir (the thoracic duct) all the new and recruiting materials for new blood, and which are from thence conveyed gradually into the subclavian vein; and from thence into the right auricle of the heart: and from thence, as we have seen, in considering the course of circulation of the blood, (g) into the lungs, and there exposed to atmospheric air, and returned through the pulmonary veins In general. into the left auricle and ventricle of the heart, and thence passed into and through the aorta in circulation through the arteries, &c. (h) It is obvious, that were it not for such contrivance the nutritious qualities of the chyle would pass off uselessly from the frame through the small, and then through the large, intestines, the same as the fæces. The materials thus absorbed are principally the chyle, which is found in the intestines from the nutriment received into the stomach and separated from the fæces in the small intestines, as explained in the last chapter. (i) But there are various other materials from other parts of the body, many of which are particles of the refuse matter separated by the secreting vessels, which are not wholly wasted, but much of it absorbed and passed into the thoracic duct and there mixed with the chyle, and also becomes part of the blood passed into circulation.(k) By this arrangement there is great saving of animalized fluids, which, however they may differ from SORPTION. CHAP. VII. each other in several properties, are yet far more easily reducible to geSECT. II. nuine blood than mere new and unassimilated matter obtained from withORGANS AND PARTS. Enumera absorbent vessels. out. (1) For the purpose of this absorption, there are also the halitus of the cavities, the gall bladder, and not a small portion of the stillatitious fluids which are applied to the common integuments. (m) It is curious to observe and consider the winding course which nature has provided for the purpose of changing and assimilating the chyle and other absorbed fluids, before their admixture with the blood, and which, if introduced immediately with the blood, would, probably, occasion instant death. (n) These several numerous absorbent vessels are divided into four, nametion of the ly, first, the lacteal vessels; secondly, the lymphatic vessels; thirdly, the conglobate glands; and, fourthly, the "thoracic duct."(o) The exact apertures or orifices of these cannot be traced, but they may be followed up either singly or in conglobate glands, until (with the exception of a few, which enter directly into the right subclavian vein) (p) they, in general, terminate in the thoracic duct, which finally pours the whole of this mixed fluid, steadily and slowly, by means of a valve placed at its opening, into the subclavian vein of the left side. (9) The use of that valve is, probably, not so much to prevent the influx of the blood, as to modify the entrance of the chyle, or to secure the gradual intermixture in the thoracic duct, and thence into the subclavian vein, and to cause it to enter by drops rather than in a stream. (r) The lymphatic system consists of an elaborate set of vessels, being more numerous than either the arterial or venous, and a number of glands distributed in various parts in the course of these vessels.(s) First, the The Lacteals are so termed from their general milky or white appearance: but it is said they ought more properly to be called the chyliferous vessels; and they are sometimes termed the nutrient absorbents.(u) These originate amongst the villi of the internal coats of the small intestines, and partly in the duodenum, but principally in the jejunum and ileum; by them the chyle is conveyed from those intestines into the thoracic duct, and thence into and through the subclavian vein into the right auricle; and although so much of this supposition as respects the villi has been disputed, the residue appears to be universally assented to,(x) and it is agreed that the orifices in the small intestines have a peculiar absorbent or attractive quality, like capillary tubes, so as to attract and imbibe the chyle:(y) and it is further agreed, that after the chyle has been thus once absorbed through such orifices, then by the contractile power of the lacteal vessels, assisted by the mechanical action of their valves, and, probably, by other causes, the chyle is necessarily propelled along these vessels, and prevented by the valves from retrograding until it has finally entered the thoracic duct. (2) One remarkable peculiarity of these lacteal vessels is agreed to be, that they select and absorb only chyle, and never, or very rarely, imbibe any other fluid. (a) The lacteals are vessels in part of the same description in construction as lymphatics; but the former, arising between the commencement of the duodenum and CHAP. VII. the termination of the ileum, are confined to the small intestines, the SECT. II. jejunum and ileum.(b) The lacteal vessels, which take up the chyle are ORGANS therefore only one branch of the system of absorbents.(c) Within the AND PARTS. last few years, however, there have been disputes respecting the termination of both lacteals and lymphatics, viz., whether they belong ultimately and solely to the principal trunks, the thoracic duct and subclavian vein, or whether they are not in a great measure inserted into and connected immediately and directly with neighbouring veins, and also whether absorption is a function peculiar to them, it being also ascribed by some to the mesenteric as well as to other veins. (d) The non-absorption, or imperfect absorption of chyle, must proceed from some mischief in the lacteals or mesenteric glands, and which may either labour under an inertness in carrying on their proper functions, or may be so obstructed in their course as to be prevented from exercising their functions, notwithstanding their being in a state of health, and this will occasion diarrahcea alba, or loose or fluid alvine evacuations. (e) The Lymphatic vessels are so called from their usually containing Secondly, lymph. They are nearly similar in their structure and composition to the the lym lacteals; but in consequence of their contents being transparent and phatics. colourless, they are the less easily discerned. They are provided with numerous valves, and form frequent anastomoses, and possess a degree of contractility similar to the lacteals; and it is said that the main branches of the lymphatics are finally reduced into three or four great trunks, which probably, like the lacteals, terminate in the thoracic duct. (f) They differ, however, materially from the lacteals, as well in their situation as in their contents and offices; for whilst the lacteals are confined to the mesentery and small intestines, and imbibe and convey only chyle, the lymphatics are found in the larger intestines and in almost every part of the body; and with respect to their contents, they not only take up fluids, containing particles adapted for the nourishment of the body, other than chyle, but also absorb all worn out or decomposed particles of the frame of every description separated from it, and which we have seen is an incessant operation, whether it be a fluid or the densest substance, such even as the particles of decayed bone, and reduce and mix the whole into the fluid called lymph, and carry the same into the thoracic duct, to be again carried into circulation, some part to serve useful purposes, and the rest to be carried off as effete and useless. (g) This lymph is a transparent and colourless fluid, which, as its name imports, was supposed to consist principally of water:(h) but it is said that it bears a very strong analogy to chyle, especially in the characteristic property of separation by rest into parts; one more solid and fibrous, and the other, which remains fluid, and more resembles albumen.(i) But nutrition is not the sole use or even the primary function of the lymph or of the lymphatic vessels, although it may be a secondary office.(k) The exact use of the lymphatics is still a very questionable point in physiology; the common supposition has been that they absorb (b) 2 Horn. Anat. 298. (c) 3 Bell, 238. (d) El. Blum. 363, note (u;) 2 Bost. 440 to 450, where Dr. Bostock, although he does not entirely assent to venous absorption, yet appears to acknowledge that it may exist; see, in particular, id. See 2 Dungl. Phy. 35 to 58. (e) 1 Good, 195; Copland's Dict. tit. (g) 2 Bost. 437. (h) Id. 431. (i) 2 Dungl. Phy. 29, 30. AND PARTS. CHAP. VII. all the old and redundant materials of the body, and also various kinds of SECT. II. superfluous fluids formed within its textures and cavities, (1) as well from ORGANS the external common integuments as from the fauces and oesophagus, the pleura and peritonæum, and from the thoracic and abdominal viscera themselves.(m) They accompany every part of the frame closely and with great minuteness of structure, and exist very numerously even in the coats of small arteries and veins.(n) They have numerous valves, so disposed in pairs opposite each other, that the whole cavity is accurately closed.(o) Of the solu tion and absorption of even solid parts of bone. (r) Magendie having ascertained, by a train of experiments, the degree of effect which certain narcotic substances produce upon the system, so as to be able to refer to these as a standard of comparison, was induced to examine how far the absorbing power of the vessels was promoted or retarded by the states of plethora or depletion. The result seemed to be that plethora uniformly retarded, and depletion as constantly promoted, absorption; and hence he concluded that it must consist in a mere mechanical action, independent of any principle connected with vitality, and he therefore attributed absorption to the capillary action, exercised by the sides of the vessels upon the substances to which they are exposed, and that absorption is merely a filtration through the sides of the vessels;(p) but subsequent physiologists deny this doctrine of lateral filtration, and limit the passage of the lymph through the orifices of the vessels.(g) It is admitted that there is much difficulty as well in ascertaining the nature of the lymph to be found in the lymphatics as in accounting for its production. (8) It is said to be uniform in its nature and appearance as previously described,(t) and yet to be an aggregate of various particles, fluids, and substances, materially differing from each other. Nor is there any satisfactory explanation how or by what means the densest solids as well as more fluid components of the body, are absorbed and taken up by these vessels. (u) All modern physiologists appear to agree in the supposition that every part of the human frame is perpetually changing, and old parts removed and carried off, and new parts deposited in lieu; but how in the removal of parts of bone and other solids, the same are solved before they can be absorbed, and how reduced to and blended with and assimilated to the rest of the fluid lymph, seems to be undiscovered.(x) We know only of the gastric juice in the stomach as an adequate solvent for such solids, but that has not been discovered in any other part of the frame. Some have supposed that the parts taken up by the lymphatics must have been previously decayed and absolutely dead, and that then they are more readily and rapidly decomposed; but still, as we know bone will retain its solidity and resist decomposition for many years after death, it remains unexplained how parts of that substance are rapidly broken up and solved into liquid, so as to pass the minute orifices of the lymphatics and become undistinguishable from the residue of the lymph.(y) (1) Lizars, part xii. 13; 5 Good, 176 to 178; El. Blum. 325; 2 Bost. 438. (m) El. Blum. 364; 5 Good, 178; 2 (n) 5 Good, 178, 179; Quain, El. 280, (q) 2 Bost. 467. (u) Id. 465; as to this operation, see Cruickshank on the Absorbing System, p. 108, &c. (x) Ante, 42, 43; 2 Bost. 465; 2 Dungl. (p) M. Magendie, Journ. Phy. 6; 2 Phy. 58, 59. Bost. 466; 2 Dungl. Phy. 53. (y) 2 Bost. 464, 465. AND PARTS. in mould There is another most important office or function ascribed by modern CHAP. VII. physiologists to the lymphatics, viz. that their primary use is to mould SECT. II. and fashion the body so as to give it its proper form, and to enable it to ORGANS increase in bulk, while its individual parts retain their appropriate figure and proportionate size. (z) It has been observed, that when we reflect upon the mode in which an organized part is enlarged, we perceive that Utility of it does not grow by accretion, like a crystal, nor by simple distention, absorption but by a numerical as well as a specific increase of each individual part. ing and With respect to the muscles, for example, the number of fibres are aug- preserving mented at the same time that each individual fibre is increased in size; the form of and the same thing takes place with respect to its tendinous extremities, all parts of and the other membranous parts that are dispersed through the body, the body. This extraordinary operation is supposed to be affected by absorption; and hence we conclude, that the lymphatics alone, or, at least, in conjunction with the veins, are the agent employed for this purpose. (a) By these small particles, even of bones, are supposed to be gradually removed, and a deposition made of other materials in their proper situations, so as to create or continue the perfect bone. (a) This property in the absorbents is obviously connected with that universally admitted principle in the animal economy so often noticed, that the matter of which the organs of the body are composed, during the performance of their various functions, undergoes some change, which renders it no longer fit for its original purpose, and that it, therefore, becomes necessary that it should be removed, and new materials deposited in its place:(b) but, like most parts of the absorbent system, we are ignorant as well of the nature of this change as of the means by which it is effected.(b) Upon the whole, we may conclude, that, the lacteals and the lymphatics are essential to the growth of the body, although in different ways. The lacteals procure the materials, and convey them into the blood, whence they are abstracted by the secreting arteries, while the lymphatics regulate the mode of their deposition, and contribute to reduce the parts into their proper form and dimensions. (c) It seems also to be established, that there are absorbent vessels upon of cutanethe surface of the body, and, that, even through the external skin there ous absorpare nutritious liquids as well as poisons, that may, especially by rubbing tion. and friction, be absorbed and carried into the circulating system; (d) and instances are stated, by high authority, where, from a stricture in the œsophagus, or from the suspension of the power of deglutition, no nourishment could be taken into the stomach, yet the patient was supported for a time by means of absorption from the surface, aided by the aliment of nutritive clysters.(e) Dr. Barry has from his experiments inferred, that, the whole function of external absorption is a physical effect of atmospheric pressure; and he even contends, that the circulation in the absorbing vessels and in the great veins depends on the same cause, and that vital action is not concerned in absorption; (f) but this doctrine has been questioned, at least, as regards the lacteals. (g) Dr. Bostock observes, that, under certain circumstances, the absorbents are able to take up substances applied to the skin, especially when aided by friction, which is sufficiently proved by the effect of various medical agents which (z) 2 Bost. 454. (a) Allen's Lectures on the Animal Economy, and other authorities; 2 Bost. 454, 455. But some physiologists doubt this conclusion. (b) 2 Bost. 456. (c) Id. (d) Amer. Cyclop. Prac. Med. Absorp- (f) 2 Bost. 471, note. |