602 SPECIAL PHYSIOLOGY. 



a little to the right of the aorta, it passes, opposite the third dorsal 

 vertebra, behind the arch of that vessel, crosses over the oesophagus, 

 and ascends on its left side to the root of the neck, c, where it curves 

 downwards and outwards, behind the great bloodvessels, and finally 

 opens into the angle of junction of the left internal jugular and sub- 

 clavian veins, the entrance being guarded by a strong double valve. 

 The thoracic duct measures from eighteen to twenty inches in length, 

 and from two to three lines in width ; it is somewhat varicose, or con- 

 stricted at intervals, owing to the presence within it of numerous 

 double semilunar valves, which have their free margins directed up- 

 wards, so that they are closed by downward pressure, and support the 

 weight of the column of fluid contained in the duct. At the root of 

 the neck, the contents of the absorbent system are poured into the 

 venous system, and are mixed with the venous blood flowing towards 

 the heart, regurgitation from the veins to the absorbent trunks being 

 prevented by the valves placed at the opening of the latter into the 

 veins. 



The coats of the lymphatics, as elsewhere explained, are remarkably 

 thin, and therefore highly permeable to fluids. The trunks themselves 

 are very difficult to find, and even the thoracic duct eludes an ordinary 

 dissection. 



Lymphatic glands are found (Fig. 100) in the arm-pits and groins, 

 and a few at the bend of the elbow, and in the ham, where they are 

 named respectively axillary, inguinal, anti-brachial, and popliteal 

 glands ; chains of glands on each side of the neck, are named the cer- 

 vical or concatenated glands ; in the thorax, numerous glands, placed 

 around the great air-tubes or bronchi, and usually containing a black 

 deposit, are named bronchial glands ; lastly, in the pelvis and abdomen, 

 are the iliac, lumbar, and mesenteric glands. 



Like general absorption, the absorption of food from the alimentary 

 canal is performed by the agency not only of the bloodvessels but also 

 of the absorbents proper; those of the small intestines, which occasion- 

 ally that is, during digestion convey the milky white fluid, chyle, 

 are named the lacteals, or chyliferous vessels. 



The arteries of the intestine, chiefly derived from the mesenteric 

 arteries, subdivide and inosculate in the mesentery, forming numerous 

 vascular arches before they reach the attached border of the intestine ; 

 entering and ramifying in the submucous coat, their branches pene- 

 trate, and further subdivide in the mucous membrane, in which they 

 end in close networks of capillaries, near the mucous surface, around 

 the intestinal tubuli and glands, and within the countless villi. From 

 the capillary networks, the minutest venules proceed, and soon join to 

 form larger veins, running to the attached border of the intestine ; 

 beyond this, the veins unite in the mesentery into still larger trunks ; 

 these, with the veins of the stomach, pancreas, and spleen, ultimately 

 form the portal vein, which enters, and subdivides in the liver. The 

 veins from the lower part of the large intestine, however, do not enter 

 this portal system, but join the veins from the lower half of the body; 

 so, too, the veins proceeding from the mouth, pharynx, and gullet, 

 enter the general venous system. 



