The Spleen is a viscus of a deep blackish-red colour, situated on the left hypochondrium, immediately under the diaphragm, and above the kidney. Its figure may be said to be that of a depressed oval, nearly twice as long as it is broad, and almost twice as broad as it is thick. However ingeniously we may speculate upon the uses of this organ, nothing satisfactory has been hitherto obtained upon the subject. It certainly does not appear to be essential to life, for Mr. John Hunter removed it from a wounded man, and the patient did well. Various other instances of a similar kind stand on record. Mr. Herbert Mayo, in whose accuracy I place the highest reliance, says, "A dog, from which I removed the spleen twenty-eight months ago, became upon recovering from the wound fatter than before; at present there is not any essential difference in its appearance or habits from those of other dogs1." Hoffman also relates that, when the spleen is removed from dogs, they rapidly increase in fatness. It cannot supply any fluid for the digestive process, since it has no excretory duct. Some have supposed, from the peculiarly dark livid colour of its blood, and the difficulty with which it coagulates, that its use is to produce some change upon the blood, in order to adapt it for the secretion of bile.

I think it very probable, from the relations which its blood-vessels bear to those of the liver, that it administers, in some way or other, to the latter viscus; but it cannot be instrumental in the formation of bile, as we have seen that this fluid can be properly elaborated without it. Is it not an organ of compensation, - a waste pipe, for the removal of any redundant blood which may be thrown into the liver? or a reservoir, to supply any deficiency which circumstances may create? and that, in this respect, it is to its sanguiferous, what the gall-bladder is to its biliary system?

56. Having completed the history of the glands destined for the secretion of the several fluids which are essential to the digestive process, I shall pause a short time, in order to offer a few observations upon the nature of those wonderful phenomena, which arise, as it were, on the doubtful confines of chemistry and vitality. What is secretion? How are we to explain the fact of blood being successively converted into saliva, gastric juice, bile, and a variety of other equally dissimilar fluids, by its mere transmission through a series of minute tubes? If we direct our attention to the most simple form of secretion, termed serous exhalation, we shall hastily arrive at the conclusion, that a separation of the thinner from the thicker parts of the blood is all that has been effected by the operation, and that the organs by which the several membranes are thus supplied, are consequently mere sieves; for if we examine the composition of the fluid secreted by the serous membranes, it will appear to be the serum of the blood, deprived of a certain quantity of albumen. The following very interesting observations by M. Majendie prove that the physical disposition of the small vessels has an influence upon the exhalation.

When, in the dead body, tepid water is injected into an artery that goes into a serous membrane, as soon as the current is established from the artery to the vein, a great number of small drops pass out of the membrane, and quickly evaporate. This phenomenon has certainly a close analogy to exhalation. If we employ a solution of gelatine, coloured with vermilion, to inject a whole body, it frequently happens that the gelatine is deposited round the circumvolutions, and in the cerebral anfrac-tuosities, without the colouring matter having escaped from the vessels; on the contrary, the whole injection spreads at the external and internal surface of the choroid. If linseed oil be used, coloured also by vermilion, the oil, deprived of the colouring matter, is often seen deposited in the great synovial capsule of articulations; whilst there is no transudation at the surface of the brain, nor in the interior of the eye. With such results before us, and observing, at the same time, that the structure of all glands agrees in this fact, that they are composed of vessels of infinitely small diameter, we are bound to conclude, that one part, at least, of the process of secretion is mechanical, and that, whatever other office a gland may perform, it undoubtedly acts as a sieve or filter.

In reflecting upon all the circumstances and bearings of this important and interesting subject, I have often been struck with the wisdom and extreme simplicity of the process by which the blood is separated, by the structure of the eye, into those parts which are of such striking utility in its economy. It was essential that the crystalline humor should be perfectly transparent, and that the interior surface of the choroid coat should be impregnated with a dark pigment, in order to absorb the light immediately after it has traversed the retina: now, if we submit the matter of the lens to analysis, we shall find that it coagulates by boiling, and has all the chemical properties of the colouring matter of blood, except colour, which is entirely absent. What then has become of this colouring ingredient? we shall find that it has been appropriated by the vessels of the choroid, for the important purpose above stated. On examining this pigment, its composition will be found to confirm such a theory; for, when dried and ignited, it will burn as easily as a vegetable substance, and the ash will contain much iron.

For these results we are indebted to the labours of Berzelius; and they certainly show that the circulating blood is decomposed on the interior surface of the choroid, leaving there its colouring matter, and conveying the remainder to the inner part of the eye perfectly limpid and colourless.

1 "Outlines of Human Physiology".

57. But filtration cannot explain the developement of those secreted fluids which contain proximate principles that do not exist in the blood; - no foreign ingredient is added, no chemical re-agent is interposed, and yet the fluid which flows from the organized laboratory has acquired chemical properties, which render it decidedly different from the common circulating mass. The agent in this case can only be the nervous influence, or, in other words, the influence of life - and thus is it ever with our physiological researches; sooner or later we are obliged to abandon the light of chemistry, and to seek some unknown agent for the explication of the phenomena.

58. Every attempt to understand the manner in which the nervous fluid produces the phenomena of secretion has hitherto completely failed; the changes to which it gives origin no art can imitate, nor any philosophy explain; but although we are thus unable to trace the steps of nature, we may venture to inquire into the general direction of the path which she follows. It must be allowed that a considerable analogy subsists between the operations of the nervous fluid, as an agent of secretion, and that of galvanic electricity; they both suspend the natural affinities of bodies, dissever elements between which the strongest attractions exist, and determine them to unite in different forms and proportions. In illustration of this truth, the following ingenious experiment of Dr. Wollaston may be introduced: - he took a glass tube, two inches long, and three-quarters of an inch in diameter, and closed one of its extremities with a piece of bladder: he then poured a little water into the tube, with 1/240 part of its weight of common salt: he wetted the bladder on the outside, and placed it on a piece of silver: he then bent a zinc wire, so that one of its ends touched the silver, and the other entered the tube the length of an inch: in the same instant, the external face of the bladder gave indications of the presence of pure soda; so that, under the influence of this very weak electricity, there was a decomposition of muriate of soda, and a passage of the soda, separated from the acid, through the bladder.

It seems rational to believe that something analogous may happen in the act of secretion. Dr. Young has developed this idea still further; and has observed, that we may easily imagine that, at the subdivision of a minute artery, a nervous filament may pierce it on one side, and afford a pole positively electrical, and another opposite filament a negative one; then the particles of oxygen and nitrogen contained in the blood being most attracted by the positive pole, will tend towards the branch which is nearest to it, while those of the hydrogen and carbon will take the opposite channel; and that both these portions may be again subdivided, if it be required: and the fluid thus analyzed may be re-combined into new forms by a re-union of a certain number of each of the kinds of minute ramifications.