in its chemical and its physical properties, exhibits a considerable resemblance to the blood, the question then for us to determine is, whether, after it is poured into the veins in the imperfect state, in which the process of sanguification appears to be only in part accomplished, it is, in the first instance, mixed with, or diffused through the whole mass of blood, and that all the remaining changes are effected by the ordinary operations of secretion and excretion, or whether there be any specific process for the more complete assimilation of the chyle, by which it may be, in the first instance, converted into one or more of the constituents of the blood.

That the function of respiration, in some measure, contributes to assimilation is at least a probable opinion, and we may also suppose, that the consequence

of the transudation which is carried on from all the surfaces of the body may have its effect in discharging any redundant quantity of water; but, except these, we are not acquainted with any processes which can affect the constitution of the blood, while it remains in the larger vessels. We know, however, that it is principally in these vessels that it acquires its characteristic properties, more especially that the fibrin obtains its peculiar texture and its power of spontaneous coagulation, which existed in a less perfect degree at least in the chyle, and this is still more remarkably the case with the colouring matter. It appears to be pretty well established, that the substance which gives the red colour to the blood is the vesicle that surrounds the globule; it

may also be laid down as a point which is generally admitted by physiologists, that although this substance contains iron, yet that the iron is not the immediate cause of the red colour. We have likewise reason to conjecture that it is upon this colouring matter that the oxygen of the atmosphere more particularly acts, when the blood is transmitted through the lungs, and is converted from the venous to the arterial state; yet this seems to afford us no insight into the mode in which the red colour is, in the first instance, produced, why it exists in a slight degree in the chyle, and why it is found so much more copiously in the blood. We may conclude, that it is not the consequence of mere concentration, nor are we able to refer it to any of those operations which are known to be going forwards in the animal œconomy, with the nature of which we are so well acquainted, as to allow us to speculate upon their ultimate effects.

When we consider the nature of the relation which exists between the sanguiferous and the absorbent systems, we appear to be warranted in supposing that the fluid which is contained in the arteries should be regarded as constituting the blood in its most perfect state; that it is then carried by the capillary vessels into all parts of the body, where it loses a certain quantity of its constituents, and is thus reduced to the state of venous blood, that the thoracic duct pours into the great veins the materials necessary to supply the loss which it has experienced, and that some change, either chemical or mechanical, is effected in the lungs, which again converts it into the state

of arterial blood. According to this view of the subject, the process of the conversion of venous into arterial blood, will consist in the addition of a quantity of carbon, hydrogen, and oxygen, with an undue proportion of nitrogen; in passing through the lungs a portion of carbon, oxygen, and hydrogen, are separated, under the form of carbonic acid and water,* while in the course of the circulation, we may presume, that the excess of nitrogen which must be thus produced, will be separated from the blood, in the form of muscular fibre or of membrane, or still more, of urea, of which nitrogen composes so large a proportion.

It only remains to inquire into the relation which subsists between the absorbent and the nervous systems. And this, we have every reason to suppose, is of that indirect nature only, which I have had occasion to describe when treating of the action of the heart, where the function of the part is not necessarily dependent upon the power of the nerves, although there may be many cases in which it is materially influenced by this power. It appears that the examinations of anatomists indicate that there are but few nerves sent to the absorbent system, and that even these few seem rather to pass by them, in order to be transmitted to more distant organs, than to be ultimately destined for the lymphatic vessels or glands

It is scarcely necessary to remark, that the water which is exhaled from the lungs is not supposed to be generated in this organ, but merely abstracted from the pulmonary blood by a species of secretion.

themselves. The mode of action of the absorbents, (at least if we except that of their mouths, which is altogether involved in obscurity) is of that kind which may be explained without the aid of nervous sensibility, and, indeed, every circumstance connected with them, seems to show, that, like the sanguiferous system, their ordinary operations are to be referred to contractility alone.

ADDITIONAL REFERENCES.

A.

in Phil. Trans. for 1793 and 1796.

Lond. 1793

Acad. del Cimento, Exper. fatte nell' (2a. ed.) .... Fir. 1691

Abernethy's Essays..

Abildgaard, in Ann. de Chim. t. 36.

Tabulæ Vasis Chyliferi &c......... Lugd. Bat. 1757

Allen in Phil. Trans. for 1808 and 1809.

Anatomy, Edinburgh System of

Edin. 1791

Barry, Recherches sur le Mouvement du Sang. ....Par. 1825