IX.--The portal system of veins passing to the liver, and the hepatic veins passing from this organ to join the inferior vena cava, exhibit in respect to the median line of the body an example of a-symmetry, since appearing on the right side, they have no counterparts on the left. As the law of symmetry seems to prevail universally in the development of organized beings, forasmuch as every lateral organ or part has its counterpart, while every central organ is double or complete, in having two similar sides, then the portal system, as being an exception to this law, is as a natural note of interrogation questioning the signification of that fact, and in the following observations, it appears to me, the answer may be found. Every artery in the body has its companion vein or veins. The inferior vena cava passes sidelong with the aorta in the abdomen. Every branch of the aorta which ramifies upon the abdominal parietes has its accompanying vein returning either to the vena cava or the vena azygos, and entering either of these vessels at a point on the same level as that at which itself arises. The renal vessels also have this arrangement. But all the other veins of the abdominal viscera, instead of entering the vena cava opposite their corresponding arteries, unite into a single trunk (vena portae), which enters the liver. The special purpose of this destination of the portal system is obvious, but the function of a part gives no explanation of its form or relative position, whether singular or otherwise. On viewing the vessels in presence of the general law of symmetrical development, it occurs to me that the portal and hepatic veins form one continuous system, which taken in the totality, represents the companion veins of the arteries of the abdominal viscera. The liver under this interpretation appears as a gland developed midway upon these veins, and dismembering them into a mesh of countless capillary vessels, (a condition necessary for all processes of secretion,) for the special purpose of decarbonizing the blood. In this great function the liver is an organ correlative or compensative to the lungs, whose office is similar. The secretion of the liver (bile) is fluidform; that of the lungs is aeriform. The bile being necessary to the digestive process, the liver has a duct to convey that product of its secretion to the intestines. The trachea is as it were the duct of the lungs. In the liver, then, the portal and hepatic veins being continuous as veins, the two systems, notwithstanding their apparent distinctness, caused by the intervention of the hepatic lobules, may be regarded as the veins corresponding with the arteries of the coeliac axis, and the two mesenteric. The hepatic artery and the hepatic veins evidently do not pair in the sense of afferent and efferent, with respect to the liver, both these vessels having destinations as different as those of the bronchial artery and the pulmonary veins in the lungs.