128 THE FUNCTION 



" The motion of the chyle throughout its course is to be 

 ascribed to the contractility of its containing vessels, to their 

 valves, and to the vis-a-tergo." 



The force of their contraction is shown by the rupture of the 

 thoracic duct from over-distention when a ligature is passed 

 around it. l Tiedemann and Gmelin saw the thoracic duct con- 

 tract from exposure to air. 



" The use of the valve placed at the opening of the thoracic 

 duct is probably not so much to prevent the influx of blood, as 

 to modify the entrance of the chyle into the vein, to cause 

 it to enter by drops. 



" By this contrivance, fresh chyle is prevented from having ac- 

 cess to the blood so rapidly as to stimulate the cavities of the 

 heart too violently and be imperfectly and difficultly assimilated. 



" These lymphatics", which constitute the third part of the ab- 

 sorbent system, and resemble the lacteals in their structure and 

 function, are much more, and perhaps, indeed, universally, dif- 

 fused. 31 They arise principally from the cellular membrane, which 

 we may call the grand bond of connection between the sanguifer- 

 ous and absorbing system ; but in great numbers likewise from 

 the external common integuments y, from the fauces, oesophagus," 

 and all mucous membranes, " the pleura, peritonaeum," and all 

 serous membranes, from all excretory ducts, from arteries 35 , and 



* Sir Astley Cooper, Med. Pecords and Researches. A ligature of the thoracic 

 duct does not necessarily deprive the body of nourishment, because there are 

 sometimes two ducts, and sometimes one or more small trunks which unite with 

 it or have a different termination in the venous system. 



Dr. Magendie observed in the dog, that the contents of the thoracic duct flow 

 but slowly ; though more quickly during compression of the abdominal viscera. 



On wounding it after a meal, he obtained half an ounce in five minutes, and 

 they flowed for some time. 



u " Consult, among others already and hereafter quoted, J. F. Meckel, De vasis 

 lymphaticis glandulisque conglobatis- Berol. 1757. 4to. 



And Al. Monro, filius, De venis lymphaticis valvulosis. Ib. same year. 8vo." 



* Dr. W. Hunter, Medical Commentaries^ P. i. p. 5. sq. 



y " J.Elliotson has adduced new arguments showing that cutaneous absorption 

 has been doubted of late without good reason, in his notes to the English trans- 

 lation of these Institutions, p. 129. 3d edit. 1820." 



z Lymphatics may be injected from arteries. Lippi says that he finds many 

 arise from arteries; and they probably originate from them as they do from every 

 other part of the body. But to imagine they arise from arteries only is incorrect ; 

 and to imagine their sole function to be absorption from arteries, absurd. Dr. 

 Magendie, however, attempts to revive the old opinion of lymphatics arising from 

 arteries only and being destined to convey lymph from them. 



