Section 47. Between the final branches of the arteries and the first fine factors of the veins, and joining them, come the systemic capillaries. These smallest and ultimate ramifications of the circulation penetrate every living part of the animal, so that if we could isolate the vascular system we should have the complete form of the rabbit in a closely-meshed network. It is in the capillaries that the exchange of gases occurs and that nutritive material passes out to the tissues and katastases in from them; they are the essential factor in the circulatory system of the mammal-- veins, arteries, and heart simply exist to remove and replace their contents. The details of the branching of the pulmonary artery and the pulmonary veins need not detain us now. Section 48. Summarising the course of the circulation, starting from the right ventricle, we have-- pulmonary artery, pulmonary capillaries, pulmonary vein, left auricle, left ventricle, aorta, arteries, and systemic capillaries. After this, from all parts except the spleen and alimentary canal, the blood returns to systemic veins, superior or inferior cavae, right auricle, and right ventricle. The blood from the stomach spleen, and intestines however, passes via {through} the portal vein to the liver capillaries and then through the hepatic vein to inferior cava, and so on. Material leaves the blood to be excreted in lungs, kidneys, by the skin (as perspiration), and elsewhere. New material enters most conspicuously; (a) by the portal veins portal veins and (b) by the thoracic duct and left superior cava. Section 49. The following table summarises what we have learnt up to the present of the physiology of the Rabbit, considered as a mechanism using up food and oxygen and disengaging energy:-- -Air_ {Nitrogen... returned unchanged.} {Oxygen... through Pulmonary Vein to--} {see 3.} -Food_ {Carbo-Hydrates (Starch, Sugar, Cellulose.)} Sugar. {Protein.} {Peptones.} {Fat (little in Rabbit.)} {Glycerine, and fatty acids in soups.} {Rejected matter got rid of in Defaecation.} 1a. {Chyle in Lacteals going via {through} Thoracic Duct and Left Superior Cava to--} {see 2.} 1b. {Veins of Villi--} {Portal Vein--} {Liver--} {Hepatic Vein and Inferior Cava to--} {see 2.} 2. {Right side of heart; then to lungs, and then to--} {see 3.} 3. {Left side of heart; whence to Systemic Arteries and Capillaries.} 4. {The tissues and -Kataboly_.} 5. {Urea (?Liver) Kidney and Sweat Glands} {CO2} {Lungs} {H2O} {Lungs, Kidney, Sweat Glands} {Other Substances} {Mainly by [Kidney,] Liver and Alimentary Canal} 4. _The Amoeba. Cells, and Tissue_