684 ANGIOLOGY 



derived are six in number; two paired, the jugular and the posterior lymph-sacs; 

 and two unpaired, the retroperitoneal and the cisterna chyli. In lower mammals 

 an additional pair, subclavian, is present, but in the human embryo these are 

 merely extensions of the jugular sacs. 



The position of the sacs is as follows: (1) jugular sac, the first to appear, at 

 the junction of the subclavian vein with the primitive jugular; (2) posterior sac, 

 at the junction of the iliac vein with the cardinal; (3) retroperitoneal, in the root 

 of the mesentery near the suprarenal glands; (4) cisterna chyli, opposite the .third 

 and fourth lumbar vertebra? (Fig. 592). From the lymph-sacs the lymphatic 

 vessels bud out along fixed lines corresponding more or less closely to the course 

 of the embryonic bloodvessels. Both in the body-wall and in the wall of the 

 intestine, the deeper plexuses are the first to be developed; by continued growth 

 of these the vessels in the superficial layers are gradually formed. The thoracic 

 duct is probably formed from anastomosing outgrowths from the jugular sac and 

 cisterna chyli. At its connection with the cisterna chyli it is at first double, but 

 the two vessels soon join. 



All the lymph-sacs except the cisterna chyli are, at a later stage, divided up by 

 slender connective tissue bridges and transformed into groups of lymph glands. 

 The lower portion of the cisterna chyli is similarly converted, but its upper portion 

 remains as the adult cisterna. 



Lymphatic Capillaries. The complex capillary plexuses which consist of a 

 single layer of thin flat endothelial cells lie in the connective-tissue spaces in the 

 various regions of the body to which the}' are distributed and are bathed by the 

 intercellular tissue fluids. Two views are at present held as to the mode in which 

 the lymph is formed : one being by the physical processes of filtration, diffusion, and 

 osmosis, and the other, that in addition to these physical processes the endothelial 

 cells have an active secretory function. The colorless liquid lymph has about the 

 same composition as the blood plasma. It contains many lymphocytes and fre- 

 quently red blood corpuscles. Granules and bacteria are also taken up by the lymph 

 from the connective-tissue space's, partly by the action of lymphocytes which pass 

 into the lymph between the endothelial cells and partly by the direct passage of the 

 granules through the endothelial cells. 



The lyftiphatic capillary plexuses vary greatly in form; the anastomoses are 

 usually numerous; blind ends or cul-de-sacs are especially common in the intestinal 

 villi, the dermal papillae and the filiform papillae of the tongue. The plexuses are 

 often in two layers : a superficial and a deep, the superficial being of smaller caliber 

 than the deep. The caliber, however, varies greatly in a given plexus from a few 

 micromillimeters to one millimeter. The capillaries are without valves. 



Distribution. The Skin. Lymphatic capillaries are abundant in the dermis 

 where they form superficial and deep plexuses, the former sending blind ends into 

 the dermal papillae. The plexuses are especially rich over the palmar surface of the 

 hands and fingers and over the plantar surface of the feet and toes. The epidermis 

 is without capillaries. The conjunctiva has an especially rich plexus. 



The subcutaneous tissue is without capillaries. 



The tendons of striated muscle and muscle sheaths are richly supplied. In muscle, 

 how r ever, their existence is still disputed. 



The periosteum of bone is richly supplied and they have been described in the 

 Haversian canals. They are absent in cartilage and probably in bone marrow. 



The joint capsules are richly supplied with lymphatic capillaries, they do not, 

 however, open into the joint cavities. 



Beneath the mesothelium lining of the pleural, peritoneal and pericardial cavities 

 are rich plexuses; they do not open into these cavities. 



The alimentary canal is supplied with rich plexuses beneath the epithelium, often 

 as a superficial plexus in the mucosa and a deeper submucosal plexus. Cul-de-sacs 



