If we take the case of a muscle, one end of which ter minates in a tendon, while the other is immediately attach ed to a bone, this, in the infant, is of a certain length, and afterwards attains to two or three times its former size, while its general form and the relation of its different parts remain the same. Now it is obvious that this increase of bulk is not produced by the distention of its parts, because the muscle, when at its full size, exhibits no appearance of this kind. Nor can the growth be effected by accretion, because the situation of the different parts with respect to each other is obviously ioconsistent with this supposition. We must, therefore, conclude, that the growth of the muscle and tendon can be accomplished in one way only —the gradual removal of all the particles of which they originally consisted by the lymphatics, and the deposition of new particles by the secreting arteries. The new mat ter which is deposited does not occupy precisely the same place with the former, but its situation is adapted to that of the neighbouring parts. The fibres of the muscle, and the cells and plates of the membrane, are thus continually growing more numerous, and, perhaps, each of them like wise becoming larger, while, at the same time, all the sur rounding parts change their position, so that the new mat ter is exactly adapted to the place which it is intended to occupy.
\Ve shall be still more convinced of the correctness of this mode of viewing the subject, when we apply it to the growth of the hard parts. The bone of a young animal has a certain figure, is furnished with a certain number of eminences and depressions, projecting spines and cavities, and, as a general rule, the shape of the bone in the adult is the same with that of the infant, but it is much larger. Now, it is obvious that bone is incapable of distention, and that, by the mere addition of more matter to the original bone, we should have a body of a totally different form. Nothing, in fact, can produce the effect in question, ex cept the process we have described above—the removal of the old bone, particle by particle, and the substitution of new particles in a situation different from the former. This reciprocal change appears to be connected with a principle in the animal economy to which we have already alluded, that the matter of which the body is composed gradually undergoes some alteration, by which it becomes unfit for the performance of its proper functions, and re quires, from time to time, to be replaced by fresh matter. We are ignorant in what this change consists, and what are the ultimate purposes which it serves, but it seems to be intimately connected with the process by which the body is moulded into its proper form. The conclusion to which we arrive is, that both the lacteals and the lympha tics are employed in the formation of the body, but that they contribute to this end in different ways; the lacteals convey the nutritive matter to the blood, while the princi pal office of the lymphatics is to dispose of this matter, so as to give the body its proper form, and enable each indi vidual part to increase in size without affecting the rela tion which they bear to each other.
We arc now to inquire, in the third place, into the mode in which the absorbents act ; and here several interesting questions present themselves to us. We must endeavour to ascertain, first, how the substances enter the mouths of the vessels ; and second, how they are carried along them. We should examine into the nature of the substances that arc adapted for the action of the absorbents, and into the changes which they undergo before they arc in a state proper for absorption. We have already spoken of the minute vessels termed villi, which are described as con stituting the mouth of the lacteals ; a structure which would seem to favour the opinion, that the operation is merely that of capillary attraction, were it not that the thinness and flexibility of the vessels appear inconsistent with this effect, so as to lead to the idea that the vessels must be endued with some specific power, different from that of mere inorganic tubes. But before we can decide upon this point, it is necessary to determine the exact sense in which we employ the term capillary attraction. If we regard it as not a mere mechanical action between the tube and the contained fluid, but as farther partaking of an elective attraction, which varies according to the na ture of the tube and the fluid, we may perhaps be able to explain the effect which is produced by the absorbents upon the matter that is presented to their orifices. And there are, indeed, several circumstances which seem to render it probable that this kind of selection actually takes place, and that the absorbents receive certain particles and reject others, not from any relation to their physical pro perties, but from something specific in their chemical na ture. But it would still appear, that the mouths of the absorbents could not permit the particles to enter without exercising some kind of vital action, by which they may be alternately contracted and relaxed, such as we have reason to believe the vessels themselves possess, and which it is therefore natural to attribute to their extremities.
The conclusion, therefore, to which we are brought is, that the mouths of the absorbents possess contractility, and likewise a power something analogous to elective at traction, by the continued operation of which the appro priate particles are enabled to enter the vessels. When they have once entered, the vessels carry them forwards by their contractile force, assisted by external pressure, and by the valves with which they are furnished; the con tractility and external pressure forcing the matter from one part of the vessel to another, and the valves determin ing this motion to be always in one direction.