122 LYMPHATIC GLANDS. 



and in having more numerous and perfect valves. These 

 valves are generally semilunar, and arranged in pairs, 

 though some are circular and do not close the canal 

 entirely. 



The origin of the lymphatics is still involved in doubt, 

 whether in the various tissues they commence by open 

 mouths, or are continuous with one set of the arteries, and 

 carry the serous portion of the blood, or whether they 

 begin by a fine net-work of vessels. Let their microscopi- 

 cal origin be what it may, it is well ascertained that the 

 lymphatics come from nearly every portion of the body, 

 while the lacteals spring exclusively from the interior of 

 the intestinal tube, and especially its upper part. 



The former carry lymph, the worn-out material of the 

 body; the latter, the lacteals, convey chyle, the fresh mate- 

 rial formed by the process of digestion. Both sets of vessels, 

 after passing through the various lymphatic glands, ulti- 

 mately meet and converge to a point upon the second lum- 

 bar vertebras, behind the aorta, and below the diaphragm, 

 called the receptaculum chylL 



This receptacle of the chyle forms the commencement of, 

 and is continuous with the thoracic duct, (Fig. 18,) a tube 

 extending up the thorax between the aorta and vena azygos 

 to the fourth dorsal vertebras, where it inclines obliquely 

 to the left, behind the oesophagus and aorta, and inside of 

 left subclavian artery, to the seventh cervical vertebra, 

 from which it arches downward and outwards to the junc- 

 tion of the internal jugular and left subclavian veins, at 

 which angle it enters, protected by a pair of valves which 

 prevent regurgitation. This tube receives the lacteals and 

 the lymphatics belonging to the lower extremities, abdo- 

 men, left half of the chest, left side of the head and neck, 

 and the left upper extremity. The lymphatics of the right 

 side of the head, right neck, right upper extremity, and 

 right lung, meet, and enter the venous system on the right 

 side by a second tube, at the junction of the right subcla- 

 vian and internal jugular. 



The lymphatics are endowed with the properties of con- 



