146 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



received by the thoracic duct, which runs the whole length of the chest. 

 Both of the main ducts have walls which, relatively, are very thin ; and, like 

 the smaller lymphatics, the ducts are abundantly provided with valves so 

 disposed as to prevent any regurgitation of lymph from either duct into its 

 branches. Each duct terminates on one side of the root of the neck, where, 

 in man, the cavity of the duet joins by an open mouth the confluence of the 

 internal jugular and subclavian veins where they form the innominate vein. 

 At the opening of each duet into the vein a valve exists, which permits the 

 free entrance of lymph into the vein, but forbids the entrance of blood into 

 the duet. 



It is a peculiarity of the lymphatic system that some of its vessels end and 

 begin by open mouths in the so-called serous cavities of the body — those vast 

 irregular interstices between organs the membranous walls of which interstices 

 are known as the peritoneum, the pleura?, and the like. For present purposes, 

 therefore, these serous cavities may be regarded as vast local expansions of 

 portions of the lymph-path. Another peculiarity of the lymphatic system de- 

 pends upon the presence of the lymphatic glands or ganglia, which also are 

 intercalated here and there between the mouths of lymphatic vessels which 

 enter and leave them. The nature and importance of these bodies have been 

 referred to in dealing with the origin of the leucocytes and the nature of the 

 lymph (p. 47). For the present purposes the ganglia are of interest in this, 

 that the lymph which traverses their texture meets, in so doing, with much 

 resistance from friction. Physiologically, therefore, the lymph-path as a whole, 

 extending from the tissue-gaps to the veins at the root of the neck, both differs 

 from, and in some respects resembles, the blood-path from the capillaries to the 

 same point. 



The origin of the lymph has been discussed already (p. 71), and has been 

 found to be partly from the blood in the capillaries, and partly from the tis- 

 sues, to say nothing of the products directly absorbed from the alimentary 

 canal during digestion. The quantity of material which leaves the lymph-path 

 and enters the blood during twenty-four hours is undoubtedly large, amount- 

 ing, in the dog, to about sixty cubic centimeters for each kilogram of body- 

 weight. The movement of the lymph is, therefore, of physiological import- 

 ance ; and the causes of this movement must now be considered. 



Differences of Pressure. — It is a striking fact that in man and the 

 other mammals there exist no " lymph-hearts" for the maintenance of the 

 lymphatic How. The fundamental causes of the movement of the lymph 

 are that at the beginning of its path in the gaps of the tissues it is under 

 considerable pressure; that at the end of its path at the veins of the neck 

 it is under very low pressure, which often, if not usually, is negative; and 

 that throughout the lymph-path the valves are so numerous as to work 

 effectively againsl regurgitation. The pressure of the lymph in the gaps of 

 the tissues has been estimated at one half, or more, of the capillary blood- 

 pressure, ] which latter has been stated (p. 84) to be from 24 to 54 millimeters 



1 A. Landerer: Die Qewebsspannung in ihrem Einfluss auf die SrUieheBlut- und Lymphbewegung, 

 Leipzig, 1884, S. 103. 



