An enlarged view of an absorbent vessel.—1. External surface, with the jointed appearance produced by the valves.—2. The same vessel laid open, showing the arrangement of the valves.
822. The special apparatus of absorption, commonly termed the proper absorbent system, consists of the lacteal and lymphatic vessels and of the conglobate glands. The lacteals arise only from the intestines; the lymphatics, it is presumed, from every organ, tissue, and surface of the body. Both sets of vessels possess a structure strikingly analogous to that of veins, the common agents of absorption. The coats of the lacteals and lymphatics are somewhat thinner and a good deal more transparent than those of veins; yet thin and delicate as they are, they possess considerable strength, for they are capable of bearing, without rupture, injections which distend them far beyond their natural magnitude.
823. When fully distended, these vessels present a jointed appearance somewhat resembling a string of beads (fig. [CXCII]. 1). Each joint indicates the situation of a pair of valves (fig. [CXCII]. 2). These valves are of a semilunar form, and are composed of a fold of the inner coat of the vessel (fig. [CXCII]. 2). The convex side of the valve, in the lacteals, is towards the intestines; in the lymphatics towards the surfaces; in both towards the origins of the vessels. The valves allow the contents of the vessels to pass freely towards the main trunk of the system, but prevent any retrograde motion towards the origins of the vessels.
824. By continued pressure the resistance of the valves may be overcome, so that mercury may be made to pass from the trunk into the branches. When this is done in an absorbent trunk proceeding from certain organs, such as the liver, it is seen that the absorbents are distributed, arborescently, in such vast numbers that the surface of the viscus appears as if it were covered with a reticular sheet of quicksilver.
825. The internal coat of the small intestines has been shown to present a fleecy surface, crowded with minute elevations called villi, which give this surface an appearance closely resembling the pile of velvet. Each villus consists of an artery, a vein, a nerve, and a lacteal, united and sustained by delicate cellular tissue. After a meal the lacteals become so turgid with chyle that they completely conceal the blood-vessels and nerves, so that the surface of the intestine presents to the eye only a white mass, or a surface thickly crowded with white spots (fig. [CXCIII].)
Appearance of the lacteals turgid with chyle, as seen in the jejunum some time after a meal.