1980. These arterio-lymphatic vessels (veins) necessarily convey their arterial mucus or blood into the stem of the original lymphatic system, or the thoracic duct. For every Indifferent must be brought toward the respiratory organ.
1981. The tegumentary lymphatic vessels (absorbents) consequently unite with the arterio-lymphatic vessels (veins), before arriving at the respiratory organ, and pursue their course thither in common, where they pass over into the air-vessels. The usual notion or idea is, that the lymphatic vessels, from conveying their fluid into the veins, should be subordinated to the latter. But the true philosophical view, is of reverse import, although the veins are larger than the thoracic duct. In the investigation of such relations, recourse can by no means be had to quantity or size, but to the importance of the quality or contents.
1982. The veins are, properly speaking, subordinated to the lymphatic vessels, just as the arteries are to the tracheæ, or air-tubes, and the former therefore pass over into the lymphatic vessels.
c. Circulation.
1983. Through the veins, as arterio-lymphatic ducts, the vascular system has become a closed system in itself, because, on account of the polarity, the vein unites at both extremities directly with the artery. It is a continuation of the artery, like the air-duct is of the skin, and the lymphatic vessel of the intestine. Thereby a Circulation of the arteriose mucus or blood originates.
1984. In its essence the circulation is a combination of the intestinal with the branchial system into one anatomical system.
1985. The circulation is therefore a higher formation, since through it the vascular system repeats in itself the totality of the vegetable organism.
1986. On that account the circulation is the vital process proper.
1987. But for that reason also, the circulation is impossible in the plant, since it is devoid of arteries and veins.
1988. It makes its appearance in the aquatic animals, for they have, generally speaking, vessels. Molluscs, snails, worms, and crabs already possess a circulation; it is wanting, on the contrary, in those animals that are without intestine, and ceases in insects, when, or in whom, the air-vessels obtain the preponderance.