686. In this oval sac or receptacle of the chyle (fig. CLXXVII. 5), which rests upon the second or the first lumbar vertebra, also terminate the trunks of the general absorbent vessels of the system (fig. clxxvii. 6), called from the _lymph_ or the pellucid fluid which they contain, lymphatics, as the lacteals are named from the lactitious or milky appearance of their contents. 687. The receptacle of the chyle produced forms the thoracic duct (fig. CLXXVII. 7), a canal about three lines in diameter. This tube rests upon the spinal column, ascends on the right side of the aorta, passes through the aortic opening in the diaphragm (fig. CXXXIV. 9, 10), and enters into the chest. Here it forms a transparent tube about the size of a crow-quill; it rests upon the bodies of the dorsal vertebræ; it continues to ascend still on the right side of the aorta, until it reaches the sixth or fifth dorsal vertebra, when changing its direction, it passes obliquely over to the left side (fig. CLXXVII. 7). From this point it continues its course upwards, on the left side of the neck, as high as the sixth cervical vertebra; when suddenly turning forwards and a little downwards, it terminates its course in the angle formed by the union of the internal jugular with the subclavian vein (fig. CLXXVII. 8, 9). At its termination in these great venous trunks are placed two valves, which prevent alike the return of the chyle, and the entrance of the blood into the duct (fig. CLXXVIII.). [Illustration: Fig. CLXXVIII.—_Valve at the termination of the Thoracic Duct._ 1. The Thoracic Duct. 2. Lymphatics entering the duct. 3. The vein laid open, showing the valve at the termination of the duct. 4. The left internal jugular vein. 5. The left subclavian vein. 6. The vein called innominata. formed by the union of the internal jugular and subclavian veins. 7. The right jugular vein. 8. The right subclavian vein. 9. The superior cava formed by the union of the veins above. 10. The inferior cava formed by the union of the veins below. 11. The two venæ cavæ passing to the right auricle of the heart. 12. The heart. 13. The pulmonary artery dividing into right and left branches. 14. The aorta.] 688. This account of the course of the thoracic duct is a description of the course of the chyle. Performing a double, circuitous, and slow circulation through the minute convoluted tubes of which the double series of mesenteric glands are composed, the chyle, in its receptaculum, is mixed with the contents of the lymphatic vessels, lymph (fig. CLXXVII. 6, 5), that is, organic matter brought from every surface and tissue of the body. Both fluids, chyle and lymph, mixed and mingled, flow together into the thoracic duct, by which in the course traced (687) they are poured into the blood, just as the venous torrent is rushing to the heart (fig. CLXXVIII. 6, 9, 11).