438 AN AMERICAN TEX1-BOOK OF PHYSIOLOGY. 



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 

 dealt with in dealing with the origin of the leucocytes and the nature of the 

 lymph (p. 345). 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. 362), 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. 



Absence of Lymph-hearts. It is a striking fact that, in man and the 

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

 lymphatic flow. Unstriped muscular fibres, indeed, exist in the walls of the 

 lymphatics ; and rhythmical variations in the calibre of some of these have 

 been described. It remains doubtful, however, whether these variations, when 

 present, are produced by muscular contractions in the walls of the lymphatics, 

 or whether the muscular fibres exist in these, as in the blood-vessels, rather 

 for the regulation of their calibre than for the propulsion of their contents. 

 It is not improbable that the muscular fibres of the walls of the lymphatics 

 further resemble those of the blood-vessels in being under the control of the 

 nervous system ; and it has been shown that, in the splanchnic nerve of the 

 dog, there exist centrifugal fibres, stimulation of which produces dilatation of 

 the receptaculum chyli. 1 



Differences of Pressure. 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 against 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, 2 which latter has been stated (p. 376) to be from 24 to 54 millimeters 



1 L. Camus et E. Gley : " Kecherches expe'rimentales sur les nerfs des vaisseaux lymph- 

 atiques," Archives de physiologie normale et pathologique, 1894, p. 454. 



2 A. Landerer : Die Gewebsspannung in ihrem Einfluss auf die ortliche Slut- und Lymphbewegung, 

 Leipzig, 1884, p. 103. 



