CHAPTER IX.

OF SECRETION.

We have now gone through the functions which are more directly essential to the mere continuance of vitality, the circulation and the respiration, functions which cannot be suspended, even for a very short interval of time, without the immediate extinction of life, or a serious derangement of its more important actions. We have now to consider a second order of functions, which are absolutely necessary for our continued existence, but which would appear to be exercised only at certain periods, either when circumstances admit of their action, or, if we regard their final cause, when there is a demand for them in order to supply the wants of the system.1. These are the three functions of secretion, digestion, and absorption. The first affords the means by which certain parts of the blood are separated from the mass, either to serve some useful purpose after their separation, or to remove some substance which is superfluous or injurious. By the function of digestion the aliment taken into the stomach experiences a series of changes in its constitution and properties, probably by the intervention of certain

1 The old division of the functions into vital and natural, essentially depends upon this principle; but there was some inaccuracy in the mode of applying it, while the nomenclature is decidedly inappropriate.

secreted fluids, which converts it into the substance that seems to be the immediate source of nutrition, while, by means of absorption, the substance thus elaborated is carried from the digestive organs into the blood, where it becomes assimilated to this fluid, and is then transferred to every part of the system, dispensing life and heat, and affording materials for the formation of all the solids and fluids which compose the great machine.

It is obvious that the two former of these functions are so connected together, that it is impossible to give an account of one of them, without presuming upon a certain acquaintance with the other. The secretions cannot be formed until the blood has been already elaborated by the digestive and assimilating processes, while digestion, in its turn, cannot be effected until the stomach has secreted the gastric juice, which is the immediate agent in converting aliment into the materials of the blood. Upon the whole, however, it appears more convenient to begin by considering the function of secretion, as we shall, by this means, be better enabled to judge of the merits of the different hypotheses that have been formed to account for the operations of the digestive organs.

In considering the subject of secretion, I shall begin with a few preliminary observations on the nature of the function and on the structure of the secretory organs; I shall next give a brief account of some of the more important of the secreted substances, and shall endeavour to form an arrangement of them.

I shall, at the same time, inquire into the mode of their formation considered individually, and shall conclude with the various hypotheses that have been formed to explain the operation.

§1. Description of the Organs of Secretion.

The term secretion, according to its original and primary meaning, is equivalent to separation, and it would appear that this was likewise the technical sense in which it was used by the ancient physiologists, and by the earlier of the moderns, who, for the most part, conceived that the secretions previously existed in the blood, and were merely separated from it, either by mechanical means, or by certain chemical operations, somewhat analogous to precipitation. At present we generally attach a different meaning to the term, and conceive of it as essentially consisting in the production of some change in the secreted substance, either of a physical or chemical nature, proceeding upon the supposition that it did not previously exist in the blood. Perhaps, however, it

2 The late experiments of MM. Prevost and Dumas, where urea was detected in the blood, after the extirpation of the kidney, may, indeed, lead us to the former opinion; they will be more fully considered in a subsequent page. M. Magendie's definition may appear to favour the idea of mere separation. "On donne le nom generique de secretion à ce phenomène par lequel une partie du sang s'echappe des organs de la circulation pour se répandre au dehors ou au dedans; soit en conservant ses propriétés chimiques, soit après que ses élémens ont éprouvés un autre ordre des combinaisons;" Physiol. t. ii. p. 343. Haller

would be more correct to combine both these ideas in

our conception of the process of secretion, and to define it, that function by which a substance is separated from the blood, either with or without experiencing any change during its separation.

But although it may be found convenient, or even technically correct, to extend the term secretion to both these classes of substances, it may still be proper to employ this difference, as the foundation for a subdivision of the secretions, into such as are simply separated, and such as are actually formed, as it may be presumed, that the separation of the former and the production of the latter will depend upon essentially different operations.

Secretion, in the same manner with all the other operations of the animal œconomy, may be considered as consisting in a vital process, operating through the intervention of certain physical powers; those which we suppose to be concerned in secretion are both mechanical and chemical; the mechanical means em

simply defines secretion to be "ea corporis animati functio, qua de communi sanguinis massa; alii, et a sanguine diversi, et a se ipsis varii, humores ea lege parantur, ut in qualibet ejus corporis particula idem constanter humor generetur." El. Phys. vii. 1. 1. It may be objected to this definition, that the latter clause prevents our admitting those varieties of action, which not unfrequently take place in the same organ, in consequence of different circumstances, both external and internal. Cullen's section on secretion in his "Institutions," § 275.. 285. besides its other merits, is valuable as showing into how small a compass what was really known respecting this function, at that period, might be comprised..

ployed are usually conceived to be something analogous to filtration or transudation, where a portion of a compound is separated from the remainder, in consequence of the minuteness of its particles enabling it to pass through orifices or pores, which will not allow of the larger particles being transmitted. It is not, however, impossible, that a substance which previously existed in the blood, may be separated by a chemical operation, although, in this case, we may generally suspect that the substance separated will have its constitution altered. In the case of those substances which are actually formed, not having previously existed in the blood, it is extremely probable, although not absolutely necessary, that something more than a mere mechanical operation must have taken place; and when a new chemical compound is formed, we may reasonably infer, that it must have been produced by the intervention of a chemical agent. These agents may be of two kinds; they may be either extraneous bodies introduced into the blood, and acting upon some of its elements, or they may consist of some of the constituents of the blood acting upon the other parts of this fluid. We shall find, in our examination of the different secretions, that it is often very difficult to ascertain, in which of these two operations the substance in question may originate, or what is the exact nature of the action, even where we have reason to conceive that we know to what class of operations it should be referred. It It may be assumed as a general principle, that those secretions