ABSORPTION. 153 



easy osmosis of plasma through, them. In fact, the lymphatic ves- 

 sel-walls are similar in structure to those of the veins, differing 

 mainly in the fact that the former are thinner. Like the larger 

 veins, the larger lymphatics consist of three coats. The inner con- 

 sists of endothelium (tunica intima), the middle coat contains some 

 muscular fibers (tunica media), while the external coat is connective 

 tissue (tunica adventitia). 



Lymphatic Capillaries. The walls of the lymphatic capillaries 

 simply consist of a layer of endothelial cells applied directly to a 

 connective-tissue framework. In section, the endothelial cells are 

 more prominent and more turgid than those of the blood-vessels. 

 Their nuclei project into the vascular cavity, which appears as 

 though lined by a row of little pearls. Their nuclei are oval. The 

 lymphatic capillaries are more easily stained than the blood-capil- 

 laries by silver nitrate. When so stained they appear marked out 

 by black lines, which, like the sutures of bone, are sinuous. It is 

 usual to compare the borders of these cells to an oak-leaf. The 

 diameter of the lymphatic capillaries is much larger than that of the 

 blood-capillaries. 



So thin and translucent are the walls of the capillaries, that the 

 clear lymph contained in them can be clearly defined. 



Like some veins, the larger lymphatics contain valves of a 

 fibrous nature lined with endothelium. In form, structure, and 

 attachments they are identical with those of the veins. Usually 

 two valves of equal size are found opposite one another; these, by 

 their functions, prevent reflux of the lymph when pressure or other 

 disturbance is brought to bear upon their course. 



Where Nature has vessels with thin walls and which vessels con- 

 tain fluids propelled by very weak vis a tergo, she must needs resort 

 to numerous valves. So numerous are these little safeguards that 

 when the lymphatics are injected they present the appearance of a 

 string of beads. 



While dealing with lymphatics, mention must be made of those 

 modified lymphatics known from ancient times as the lacteals. 

 These vessels take their origin from the intestines to empty their 

 contents via the thoracic duct into the left subclavian vein for 

 admixture with the systemic blood. The lacteals were so named 

 from their white color at certain times; that is, during active diges- 

 tion, when the lymph-stream is overwhelmed by the absorbed fatty 

 granules, which give to it its milky hue. The milky-colored fluid has 

 been termed chyle. 



