The Spleen (figs. 57 and 78) is an organ engaged beyond all question in the elaboration of the blood, and however obscure the particulars of its function may be, there is at least more known about it than about the other organs which have been briefly described. It is the largest of the ductless glands, very variable in size, but usually from 5 to 7 oz. in weight. It increases largely some hours after eating, then gradually diminishes while fasting is continued. It is a flattened oval body about 4 or 5 inches long, and 3 inches or more in breadth, and lies against the left end of the stomach. It has a tough capsule, and consists of a deep purple pulp imbedded in the meshes of a network of fibrous trabeculæ, which is highly elastic, and probably also contains some muscular tissue. The pulp consists of granular bodies of deep colour and about the size of blood corpuscles, and nucleated corpuscles of very variable size, the larger of which have several nuclei. In sections of spleens of the domestic animals, and in spleens of young subjects, but not so easily detected in the healthy adult human spleen, are less deeply coloured spots like sago grains, called Malpighian corpuscles of the spleen. They are collections of small nucleated corpuscles in the sheaths of the arterioles. The splenic artery and vein are very large for the size of the organ. The venous blood is conveyed into the portal vein to be sent through the liver.

The very fact that the large supply of arterial blood sent to the spleen is, after passing through that organ, transmitted to the liver, seems to point to its having undergone, meanwhile, some great change which renders necessary the action of the liver, as well as of respiration, before it is fit again to traverse the tissues, and this idea is supported by examination of the blood. The blood in the splenic vein has the serum of a reddish colour unlike that of any other blood, and it contains less solid matter than other venous blood, a circumstance easily explained on the supposition that there is in the spleen a greater amount of chemical action, involving the formation of carbonic acid and water at the expense of solid matter, than occurs in the tissues throughout the body. It contains also a smaller proportion of red corpuscles than other blood, which perhaps results from a portion of their contents having transuded into the serum, as shown by its colour, and by the corpuscles being firmer, smaller, and more nearly spherical. Lastly, the colourless corpuscles are exceedingly plentiful; and it is to be noted that in a diseased condition called leucocythæmia, in which the colourless corpuscles of the blood are remarkably increased in number, there is likewise great enlargement of the spleen. These are the principal facts on which rest the two theories generally held as to the function of the spleen, namely, that it is a manufacturer of white corpuscles, and a destroyer of the red.

That the spleen is a source of white corpuscles can scarcely be doubted; but that it is its special function to destroy red corpuscles, is not so clear. No doubt heaps of withered red corpuscles have been seen in the spleen as an exceptional occurrence; but it is plain that there is an action exercised on all the red corpuscles, and it seems very possible that the object of that action is restoration rather than destruction. A very curious circumstance, which by no means makes the function of the spleen more comprehensible, is that the whole organ may be extirpated, not only without death ensuing, but without any inconvenience resulting. It is obvious, however, that this is more explicable on the supposition that the spleen is one of many structures which produce blood corpuscles, than if we consider it as the sole agent of their destruction.