In addition to simple imbibition, Dr. Dutro chet has shewn that fluids situated in contact with animal membranes permeate them in obedience to certain laws. When two fluids of different densities are in contact with the opposite sides of a membranous septum, they both permeate it, hut with different degrees of rapidity. The more rapid current takes place from the rarer to the denser fluid ; to this he applies the term of Endosmosis : the slower current from the denser to the rarer fluid he calls Exosmosis. These remarkable powers must be continually in action in the animal machine, composed as it is of solids and fluids, and cannot for the future be lost sight of in considering the sub ject of the absorption and deposition of fluids in a living animal, or the arrangement of the structures by which these important functions are accomplished. Taking these facts into con sideration, and bearing in mind the experi ments above detailed, we are led to the con clusion, that the capillary bloodvessels and even other tissues imbibe indiscriminately fluids brought in contact with them, and appa rently in obedience to the physical or ineclia nical laws regulating imbibition, rather than in virtue of any new and essential agency which they may be endowed as living struc tures ; while the lymphatic system is left it possession of a higher grade of absorption ac-, companied with an elective power (especially manifest in the lacteals) existing only with life and if not entirely independent of mechanical or physical laws, at any rate frequently a variance with them ; by this elective powei they are enabled, to a great extent, to refusl materials injurious to the economy of th animal, and to select those alone which may be made subservient to the nutrition of the system.
These physiological considerations will pre pare us better for the examination of the mainhig theories on the mode of commence ment of the lymphatic vessels. We shall next enter upon that which ascribes to them art origin from the cellular tissue. Fohmann has injected the lymphatic vessels from the cellular tissue, and most anatomists have remarked that when an injection thrown into the bloodvessels has extravasated into the cellular membrane, it has occasionally entered the lymphatic vessels. I have several times injected Fohmann's so called lymphatic cells of the umbilical cord, and have preserved a specimen shewing them, but cannot acquiesce in the opinion that they form a part of the lymphatic system. These cells, which readily receive the quicksilver in troduced into a puncture made in the cord, vary in diameter from 1-100th to 1-250th of an inch, and communicate freely with each other ; they have a very regular and organized appearance, but can only be injected after putrefaction has commenced. Fohmann describes them as situated between the lym phatics of the placenta, which terminate in them, and those of the foetus, which commence from them. I have never succeeded in pressing the injection from the cells of the cord into the lymphatics of the fmtus or of the placenta. Treviranus conceives that the lymphatics every where commence by elementary cylinders of cellular tissue, and that in the villi of the intes tines these elementary cylinders are so arranged as to have one of their extremities terminating in a lacteal vessel situated in the centre of each villus, while the other reaches the periphery of the villus, on the surface of which they present little vesicular projections, in whose centre he thinks he perceives a minute orifice. Arnold also observed a similar arrangement of the cel lular tissue of the orbit into minute cylinders, which he supposed to be an incipient net-work of lymphatic vessels. Cruveilhier considers it probable that the cellular tissue and the serous membranes are formed of lymphatic vessels ; and Mascagni makes a more sweeping assertion that all the white textures and the whole cel lular web of the body is composed of these vessels.
The opinion that the lymphatic system com mences by a plexus or net-work of vessels larger than the capillary bloodvessels, and which can always be seen by the unassisted eye when injected, appears to be the best supported by evidence, and to be that more generally received by modern investigators. These incipient plexuses are considered to be destitute of orifices either on the villi or elsewhere, and in this respect to resemble the peripheral branches of the arteries, veins, and excretory ducts, which, according to the more recent views in minute anatomy, no where present open mouths. The splendid injections of Mascagni, Fohmann, Lauth, and Panizza, shewing these vessels in various parts of the body, on the interior of mucous mem branes, on the surface of particular portions of the skin, on the serous membranes, especially that part covering the solid viscera, on the lining membrane of the heart and blood vessels, and of the excretory ducts of the glandular viscera, offer a body of evidence which can scarcely be resisted. In none of these situations can open orifices be discovered by the aid of the microscope or by means of urging on the injection. On the surface of the liver, where the lymphatics may be injected with great facility, by pressing in a retrograde direction the injection may be forced beyond the valves, and by continuing the foN some extremely minute globules of mercury may be made apparently to pass through the coats of the vessels. IIaase has also, by the same pro cedure, forced the mercury through a net-work of lymphatics on the surface of the skin ; but these circumstances can hardly warrant the supposition of lateral organized pores. These primary networks of lymphatic vessels are said to be deficient in valves in some situations. The meshes of the networks are of various forms and sizes : sometimes they are nearly equal-sided, at others oblong or irregular ; iu some places the vessels are so closely set that the spaces between them can scarcely be seen ; in others they are larger and very distinct. The plan adopted by Fohmann to display these vessels, though liable to some objections, ap pears to be the most successful. He pierces the part to be injected with a sharp-pointed lancet, held nearly horizontally, so as to pro duce a very superficial wound, the lymphatic net-work being generally nearer the surface than the capillary bloodvessels ; into the wound thus effected the pipe of the mercurial injecting tube is inserted, and the quicksilver is made to enter some of the vessels opened in the in cision, either by the weight of the column of the mercury or by urging it on with the handle of a scalpel. On the glans penis this method scarcely
ever fails to fill the lymphatics, which are of large size. In the skin of the scrotum, and in the neighbourhood of the nipple, success will occasionally attend the attempt to shew these vessels ; but in other parts of the integument the endeavour has with me always been fruit less, so much so that I cannot help doubting their universal existence on the surface of the true skin. Ilreschet's description of a net work of lymphatics brought into view by piercing the cuticle only, with a capillary tube of glass connected with a column of mercury, I am convinced is deceptive: his words are 11 consiste (ce procede) it percer superficielle ment le tissu cutane avec l'extremite d'un tube capillaire en verre ou en acier, de faron n'interesser quo l'epiderme, pour arnver au reseau vasculaire situe entre cot Epiderme et le chorion. On obtient ainsi l'injection de reseaux admirables de vaisscaux lymphatiques." 1 have frequently produced the appearance here alluded to in all parts of the body : a foetus answers best for the purpose, but the proper lymphatics are never filled from these supposed net-works of lymphatic vessels. They are clearly nothing more than the spaces around the bases of the papilla of the skin, from which, as putrefaction commences, the cuticle sepa rates more readily than from their apices, con sequently little canals are left around the pa pilla, which communicate with each other and form a pretty exact resemblance to vessels fill ing rapidly as the mercury runs around the bases of the papilla. The appearancecan only be produced at a certain stage of putrefaction when the cuticle is about to separate. On removing the cuticle, the pretended vessels im mediately disappear; but on the glans penis, on the scrotum, and on the skin of the nipple the removal of the cuticle will not disturb the net-works of vessels which may he there in jected; moreover from these the lymphatic trunks can always be filled. I am also dis posed to think, contrary to the received opinion, that the serous membranes do not universally present this superficial network of lymphatics ; there are at any rate parts of these membranes where I have never seen these vessels injected, while there are others in which anatomists in variably succeed in shewing them • and for the mere purpose of absorbing the fluid secreted by the serous sacs, there appears to me nothing extraordinary in the supposition, that those por tions of the membrane only which are most conveniently situated for the purpose should be endowed with the proper organization to effect it. The mode of procedure, however, adopted by Fohmann and others to display the incipient lymphatic net-works is open to serious objec tions, and calculated without great circumspec tion to lead into error. The capillary blood vessels will often be implicated in the wound required to pierce the lymphatic net-work, con sequently the injection may be found in the arterial and venous as well as in the larger lymphatic branches leading from the part. To succeed to any extent many punctures may be required, and in all probability some of these will conduct the injection into the three sets of vessels; but I have several times by the first puncture succeeded in injecting a net-work of vessels on the glans penis, which has conveyed the injection at once into the lymphatic branches on the body of the penis, and into these vessels only. The cellular tissue will also readily receive the injection, and where the cells are very small and uniform, as is the case with the umbilical cord, they resemble very much a net-work of vessels distended with quicksilver ; and although Folimann ad mits these to be cells, yet from their regularity he has been led to consider them a part of the lymphatic system. In the same category may be classed the supposed lymphatic cells of the cornea observed by Arnold and by Midler. The submucous cellular tissue also is fre quently arranged in little cylindrical cells which communicate with each other, and these cells on receiving the mercury put on the appearance pretty exactly of a net-work of vessels, but lymphatic vessels are not found conveying the injection away from them to the nearest lym phatic glands, which I imagine should be the proof required before we admit any vessels or cells to belong to the lymphatic system, how ever beautifully displayed by our injections. The subserous tissue is open to the same re mark, and I can hardly offer a better instance of what appears to me to be an error arising from this source, than by quoting Fohmann's own words in reference to what he describes as the lymphatics of the brain. " Les vaisseaux lympbatiques des enveloppes des masses cen trales du systeme nerveux sont taus faciles demontrer, surtout au cerveau et au cervelet. Lorsqu'on enfonce une lancette entre la pie mere et l'arachnoide, et qu'on insuffie le canal que l'on vient de pratiquer, on volt paraitre un reseau lymphatique interpose entre ses deux tuniques, reseau forme de rameaux d'un calibre plus considerable que dans les autres tissus du corps; cependant leurs parois sont si foibles qu'elles se dechirent presque aussitbt qu'on y introduit le mercure." With respect to the universal net-work of lymphatics attributed to the lining membrane of the heart, and to that of the arteries and veins, I cannot admit, that the injections of a few minute canals with quicksilver on the lining membrane of the heart in the horse, by Lauth, and similar in jections by Cruveilhier and Bonamy, can be received as demonstrative : the injection was not traced from them to a distinct lymphatic vessel, armed with valves and pursuing its course towards a lymphatic gland ; these mi mite canals might have been capillary blood vessels, or, as 13reschet observes in his expla nation of the plate which he gives from Lauth Of these supposed vessels, " Nous pensons qu'ils sont uniquement constitues par des la cunes du tissu cellulaire." In concluding what I had to say of the origin of the lymphatic vessels, a subject so inextricably mixed up with our preconceived physiological notions, I ought, perhaps, to offer some apology for advancing in an article of this nature any opinion peculiar to myself; I mean in reference to curtailing the extent to which the lymphatic system will be found to exist in the organism. My own mind has been forced to this conclusion after some years of attention to the subject, both from anatomical and physiological considerations.