2233. The ramification of the trachea into two bronchi or branches is constantly progressing, and at length these tubes divide into a great number of vesicles, which together form the lungs. The lung, which was in the commencement a simple saccular inversion of the integument, has now become a self-substantial organ, to which the respiratory vessels have been subordinated. The lung also divides upon each side into five lobes.
b. VASCULAR ORGANS OF THE INTESTINE.
Liver.
2234. The self-substantial development of the vascular, and its separation from the general, system, is most perfectly attained in the Liver.
2235. In the liver, as being the vascular system, which combines with the intestinal canal, the venous system has become independent. The portal vein arises from the intestinal canal, collects into one trunk, and again ramifies in order to unite with the biliary ducts, which are only a ramified saccular eversion of the intestine. This conjunction is represented by the liver.
2236. The liver as being a venous organ stands therefore in opposition to the lung, and produces, instead of oxydes, a basic body, or the bile. The liver, as being the venous system which has become free, is to be regarded as the highest development or blossom of the vascular system.
2237. It is for the vegetative body, what the brain is for the animal; and hence, therefore, the similarity of structure and the sympathy between both organs.
Spleen.
2238. As opposed to the liver, the arterial system also develops itself upon the intestine as a respiratory or branchial organ. These intestinal branchiæ are found in several of the lower organized animals, especially in the Holothuriæ and Aphroditæ.
2239. In the higher animals they are aggregated into a special organ, through which the gastric juice obtains oxygen; this is the spleen. The spleen is the branchia of the stomach; it has therefore no excretory duct and requires none.