Lymph hearts.—I must not here omit to al lude to the pulsatile sacs or hearts belonging to the lymphatic system, discovered by Miller in frogs, toads, salamanders, and lizards, and by Panizza in serpents. In frogs, Miiller describes two pairs, one situated just under the skin in the ischiadic region, the other more deeps;' seated over the third cervical vertebra. Their pulsations he describes as about 60 in a minute and not synchronous with those of the heart. The lower pair propel the lymph into the ischiadic vein ; the upper, into the internal jugular.* I have seen these transparent pul sating bodies in the frog, where they may be easily exposed by removing the skin from either side of the rudimentary tail, but have not examined them sufficiently to pass any opi nion upon them.
External to the fibrous tunic is situated a delicate and loose cellular tissue, which per forms the same offices for the lymphatic, which the cellular tunic does for the artery and vein, viz. it conveys to it the vasa vasorum for its nu trition, and connects it to the surrounding tis sues. The supply of nerve to the lymphatic has not hitherto been detected ; there can how ever be no doubt but that it possesses tts proper degree of sensibility, and its contractile power is in all probability regulated by nerve.
The valves of the lymphatics resemble in their mode of formation and in their appear ance the same structures in the veins ; they are however more frequent and more universal in the lymphatic system ; indeed, in the more per fect animals they are found every where except in the incipient networks of this system. It has already been stated that in fishes and in some amphibious animals the lymphaticsystem is either entirely deficient in valves, or is only supplied with them in a partially developed state. The valve is composed of two semi lunar flaps, so arranged that the reflux of the fluid within, forces them away from the sides of the vessel towards its centre, where the two flaps meet and completely close it, while the fluid passing in its proper direction simply presses the flaps back against the sides of the vessel, and thus no obstruction is offered to its onward course. The flap of a valve con sists of a fold of the inner coat of the vessel, which, where a valve is to be formed, ceases to line the vessel, and is reflected towards its inte rior; having reached half-way across, it is doubled upon itself, and returns to the side of the vessel, which it continues to line as if it had never been interrupted. The two layers of this fold adhere very firmly together so as to form a very delicate transparent semilunar flap. It presents a convex attached, and a straight or slightly concave unattached edge ; the former corresponds to a semilunar line on the interior of the vessel, the horns of which look towards the trunks of the system, where the lining mem brane was reflected from and returned to the side of the vessel ; the latter to the line of doubling of the membrane upon itself; thus a little pouch is formed between the flap and the side of the vessel, which can only be filled by the fluid passing in one direction ; and as a valve is constituted of two such pouches, when they are filled the vessel is completely closed. Some
anatomists conceive that a lamina of fibrous tissue intervenes between the layers of the fold. Breschet, in his "Systeme Lymphatique," adopts Lauth's view of the structure of the valve. Ile describes the flap of a valve as composed of two parts, one thicker and situated at the base of the fold, the other forming the rest of the flap more thin and delicate. It is this latter pert which he conceives is formed by a doubling of the lining membrane only, while the thicker part, near the base, lie has assured himself is produced by a prolongation which the fibrous coat sends inwards between the folds of the inner tunic. I have not been able to verify this description of the structure of the valve, but I have distinctly observed circular constrictions in the more bead-like lymphatics seen in the neighbourhood of some of the lymphatic glands, into the formation of which the fibrous coat does appear to enter. On laying open one of these vessels previously distended with quicksilver and dried, opposite the external constrictions, which were numerous, and not more than a line apart, valvular folds, differing from those hitherto described, were seen to project into the interior of the vessel ; they did not completely close its cavity, hut left a circular or oval opening, through which the contents of the vessel might pass in either direction. These valvular constrictions re sembled much the dried pyloric valve (fig. 50, B); and I am inclined to believe, from their thickness, that they contain circular fibres de rived from the middle coat, by which during life they may be able to close their vessels as perfectly as the pyloric valve closes the com munication between the stomach and duode num. In very many places there occur two semilunar folds (fig. 50, A), apparently formed of the lining membrane only, like the flaps of the ordinary valves, from which they differ, how ever, in having their attached and unattached edges, as well as the flaps themselves, on the same plane, consequently not forming pouches, but a transverse though incomplete septum across the vessel. Each of these flaps extends only one-third across the vessel, and terminates by a crescentic edge, by which arrangement an elliptical opening is left in the central third of the vessel, between the two folds. This form of valve would appear to offer a partial obstruc tion to the passage of the lymph in either d irec tion, as no provision is manifest by which these flaps would be made to fall against the sides of the vessel, either by the onward or retrograde course of its contents. I have frequently no ticed a combination of the circular constriction with the semilunar flaps here described (fig.50, C), by which mechanism, supposing the former to be endowed with a vital contractility, the latter might he brought in contact, so as com pletely to close the elliptical opening that would otherwise be left in the centre of the vessel.