COURSE OF THE LYMPHATICS. 349 



through a comparatively large trunk called the thoracic 

 duct, which finally empties its contents into the blood-stream 

 at the junction of the internal jugular and subclavian veins 

 of the left side. There is a smaller duct on the r'ujlit side. 

 The lymphatic vessels of the intestinal canal are called 

 lacteals, because, during digestion, the fluid contained in 

 them resembles milk in appearance ; and the lymph in the 

 lacteals during the period of digestion is called chyle. 

 There is no essential distinction, however, between lacteals 

 and lymphatics. 



In some part of their course all lymphatic vessels pass 

 through certain bodies called lymphatic glands. 



Lymphatic vessels are distributed in nearly all parts of 

 the body. Their existence, however, has not yet been 

 determined in the placenta, the umbilical cord, the mem- 

 branes of the ovum, or in any of the non- vascular parts, 

 as the nails, cuticle, hair, and the like. 



The lymphatic capillaries commence most commonly 

 either in closely-meshed networks, or in irregular lacunar 

 spaces between the various structures of which the dif- 

 ferent organs are composed. The former is the rule of 

 origin with those lymphatics which are placed most super- 

 ficially, as, for instance, immediately beneath the skin, or 

 under the mucous and serous membranes ; while the latter 

 is most common with those which arise in the substance of 

 organs. In the former instance, their walls are composed 

 of but little more than homogeneous membrane, lined by 

 a single layer of epithelial cells, very similar to those 

 which line the blood-capillaries (fig. 49). In the latter 

 instance the small irregular channels and spaces from 

 which the lymphatics take their origin, although they are 

 formed mostly by the chinks and crannies between the 

 blood-vessels, secreting ducts, and other parts which may 

 happen to form the framework of the organ in which they 

 exist, yet have also a layer of epithelial cells to define and 

 bound them. 



