Our remarks have hitherto referred principally to the lacteals, as concerned in the absorption of the chyle, but a more difficult point still remains to be considered, how the absorption of the solids is effected. We have every reason to suppose that this absorption actually takes place, so that there is not a single particle of the hardest bone, which is not, in due course of time, taken up by the lym phatics, and, in short, that after a certain period not a sin gle atom remains of the former individual. Yet this effect is brought about so slowly, and the particles are removed in so imperceptible a manner, that the identity is not de stroyed, and it even requires a considerable degree of ac curate observation to become sensible of the effect. Many particular cases of the absorption of solids daily present themselves to our observation; constant pressure upon a part causes it to waste away, even although the part press ed upon be harder than the part producing the pressure. The pulsation of an artery will even cause the removal of bone, and by the gradual operation of certain medical agents, large solid tumours are occasionally dispersed. In all these cases the lymphatics are the immediate agents ; and it is a curious speculation to inquire in what manner these vessels act, how the texture of the part is broken down, and reduced to a state proper for absorption. Most writers, when treating upon this subject, have employed language which can only be regarded as metaphorical, al though they have not expressly stated this to be the case, as where they speak of the lymphatics eating away or cor roding the hard body. When we come to consider the nature of the operation, it must, we conceive, be admit ted as a necessary step in the process, that the solid, be fore it can enter the vessel, must either be reduced to a soluble state, or to a state of very minute comminution. We are indeed ignorant how this is brought about, whether by the intervention of any solvent, or, in short, by what other means, yet it seems scarcely possible to dispense with this operation.
There is one fact that has been ascertained on this sub ject, that dead matter is absorbed more readily than the same matter while it is possessed of life, although agree ing with it in every other property, and indeed a moment's reflection must convince us that no part can be absorbed until it is first deprived of life. This has been spoken of as depending upon some mysterious power of the vital principle, by which it enables a part to resist the action of the absorbents, but perhaps a little reflection may ena ble us to view this subject in a more comprehensible man ner. It is obvious, that while a solid retains its state of aggregation it cannot enter the absorbents, and therefore, whatever process adapts it for entering these vessels mast previously decompose it. If we suppose a part to be sim ply deprived of life, and, at the same time, to retain its physical structure and chemical properties unaltered, the absorbents could have no effect upon it. It would there fore appear that the death of the part, although the first step in the process, is not properly the effect of the ab sorbents, and that they cannot act until the decomposition has commenced. How far we are able to trace a connexion
between these two effects remains to be considered; in the case of absorption, as resulting from external pressure, we probably have it in our power to do so. We may con jecture, that in consequence of this pressure, the secret ing arteries are prevented from performing their functions, while the same action may not affect, or may even promote, that of the absorbents; the consequence will be, that the old particles of the solid are removed more rapidly than ordinary, while no fresh ones are provided to supply the deficiency. The balance is therefore destroyed between the powers of accumulation and of expenditure, and when the latter prevail, the necessary consequence must be ab sorption. The difficulty which attends the investigation is to explain the manner in which the old materials are reduced to a state proper for absorption, what share the absorbents have in this part of the process, or rather, what arc the powers by which it is effected.
There is one part of the subject of absorption which re mains to be considered, the power which the external sur face of the body possesses of taking up extraneous sub stances that are presented to it. We learn from the researches of the anatomists, that lymphatics arc very co piously distributed to all parts of the skin, and that they appear to terminate under the cuticle ; we likewise know that various medical substances applied to the skin, parti cularly if we use the aid of friction, will enter the circula tion, exhibiting the same action upon the system, as if they had been received into the stomach. In these cases there is sufficient proof of the existence of cutaneous absorp tion, but it is a question that has given rise to much con troversy, whether, when water is applied to the surface, as in the warm bath, or the aqueous vapour which exists in the atmosphere, it can be absorbed by the skin, simply in consequence of the body being immersed in it. A num ber of circumstances led us to believe in the reality of this absorption, particularly those that were brought forwards by Sanctorius, who was conceived to have proved, in the most decisive manner, that, under certain circumstances, the weight of the body is sensibly augmented by the wa ter which it imbibes by the skin. Until very lately, the doctrine of cutaneous absorption was universally assented to, and many important pathological speculations were founded upon it, but the progress of modern discovery has thrown a doubt upon our former conclusions ; and the opinion, perhaps at present the most generally adopted is, that when the surface of the body is in its sound state, and where no external force is employed, the skin is imper vious to moisture, but that various substances may he forced into it by friction, and even by long immersion in warm water, by which the epidermis becomes softened, and perhaps partially destroyed, at the same time that the mouths of the lymphatics are relaxed, and rendered more disposed to receive what is presented to them.