Having been acted upon by the mesenteric glands, and passed through them, the chyle flows onward until it is poured into a dilated reservoir for the chyle, known as the receptaculum chyli.  This is a sac-like expansion of the lower end of the thoracic duct.  Into this receptacle, situated at the level of the upper lumbar vertebrae, in front of the spinal column, are poured, not only the contents of the lacteals, but also of the lymphatic vessels of the lower limbs.

158.  The Thoracic Duct.  This duct is a tube from fifteen to eighteen inches long, which passes upwards in front of the spine to reach the base of the neck, where it opens at the junction of the great veins of the left side of the head with those of the left arm.  Thus the thoracic duct acts as a kind of feeding pipe to carry along the nutritive material obtained from the food and to pour it into the blood current.  It is to be remembered that the lacteals are in reality lymphatics—­the lymphatics of the intestines.

[Illustration:  Fig. 61.—­Section of a Lymphatic Gland.

  A, strong fibrous capsule sending partitions into the gland;
  B, partitions between the follicles or pouches of the cortical or
     outer portion;
  C, partitions of the medullary or central portion;
  D, E, masses of protoplasmic matter in the pouches of the gland;
  F, lymph-vessels which bring lymph to the gland, passing into its
     center;
  G, confluence of those leading to the efferent vessel;
  H, vessel which carries the lymph away from the gland.
]

159.  The Lymphatics.  In nearly every tissue and organ of the body there is a marvelous network of vessels, precisely like the lacteals, called the lymphatics.  These are busily at work taking up and making over anew waste fluids or surplus materials derived from the blood and tissues generally.  It is estimated that the quantity of fluid picked up from the tissues by the lymphatics and restored daily to the circulation is equal to the bulk of the blood in the body.  The lymphatics seem to start out from the part in which they are found, like the rootlets of a plant in the soil.  They carry a turbid, slightly yellowish fluid, called lymph, very much like blood without the red corpuscles.

Now, just as the chyle was not fit to be immediately taken up by the blood, but was passed through the mesenteric glands to be properly worked over, so the lymph is carried to the lymphatic glands, where it undergoes certain changes to fit it for being poured into the blood.  Nature, like a careful housekeeper, allows nothing to be wasted that can be of any further service in the animal economy (Figs. 63 and 64).

The lymphatics unite to form larger and larger vessels, and at last join the thoracic duct, except the lymphatics of the right side of the head and chest and right arm.  These open by the right lymphatic duct into the venous system on the right side of the neck.