280 ABSORPTION. 



is no essential distinction, however, between lacteals and lym- 

 phatics. 



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 membranes 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 different organs 

 are composed. The former is the rule of origin with those 

 lymphatics which are placed most superficially, 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 mem- 

 brane, 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. 



The lacteals apear to offer an illustration of another mode 

 of origin, namely, in blind dilated extremities (Figs. 81, 82) ; 

 but there is no essential difference in structure between these 

 and the lymphatic capillaries of other parts. 



Recent discoveries seem likely to put an end soon to the 

 long-standing discussion whether any direct communications 

 exist between the lymph-capillaries and blood-capillaries ; the 

 need for any special intercommunicating channels seeming to 

 disappear in the light of more accurate knowledge of the struc- 

 ture and endowments of the parts concerned. For while, on 

 the one hand, the fluid part of the blood constantly exudes or 

 is strained through the walls of the blood-capillaries, so as to 

 moisten all the surround ing tissues, and occupy the interspaces 

 which exist among their different elements, these same inter- 

 spaces have been shown, as just stated, to form the beginnings 

 of the lymph-capillaries. And while, for many years, the no- 

 tion of the existence of any such channels between the blood- 

 vessels and lymphvessels, as would admit blood-corpuscles, has 

 been given up, recent observations have proved that, for the 

 passage of such corpuscles, it is not necessary to assume the 



