Chap. X] 



LYMPH VASCULAR SYSTEM 



175 



The lymphatics from the right side of the head, neck, the right 

 arm, and the upper part of the trunk enter the right lymphatic 

 duct. The parts drained by each are suggested by Fig. 109. 



Structure of the lymph vessels. — The lymphatics resemble the 

 veins in their structure as well as in their arrangement. The 

 smallest have but a single coat 

 of endothelioid cells, having a 

 peculiar dentated outline. The 

 larger vessels have three coats, 

 similar to veins, except that 

 they are so thin as to be trans- 

 parent, and they are more abun- 

 dantly supplied with valves. 

 The valves are constructed and 

 arranged in the same fashion as 

 those of the veins, but follow 

 one another at such short inter- 

 vals, that, when distended, they 

 give the vessel a beaded or 

 jointed appearance. They are 

 usually wanting in the smaller 

 networks. The valves allow the 

 passage of material from the 

 smaller to the larger lymphatics, 

 and from these into the veins, 

 and obstruct the flow of any- 

 thing in the opposite direction. 



Classification of lymphatics. 

 — The lymph, like the blood in 

 the veins, is returned from the 

 limbs and viscera by a deep and 

 by a superficial set of vessels. 

 The deep lymphatics accom- 

 pany the large blood-vessels, 

 and the superficial are distributed through the subcutaneous areo- 

 lar tissue. There is no communication between these two sets of 

 vessels, but each set forms frequent anastomoses. 



Lacteals. — The lymphatics that have their origin in the villi of 

 the small intestine are called lacteals. During the period of in- 



FiG. 111. — Lacteals a.nd Lymphatics, 

 DURING Digestion. (Dalton.) 



