/ '//A" ORY OF LUDU r IG. 2 8 7 
transudation of water carrying solids in solution, the solids being taken 
up by the tissues, and bhe pure water which is Left over returned by 
the lymphatics to the blood. We get here the first conception of the 
irrigation theory of tissue nutrition which has played so great a part in 
the speculations of later physiologists. 
With Hunter 1 and Monro a we find a return to the older theory, that 
lymph was produced by a process of suction. This indefinite conception, 
however, allowed a considerable degree of individual licence as to the 
details of the process, and important authors, such as Hunter and 
Mascagni, 3 recognised the possibility of a simple transudation or filtra- 
tion through the blood-vessel walls. This latter view, however, did not 
meet with general recognition, physiologists preferring to believe 
in the existence of the exhalant arteries which no one had yet seen 
or was ever going to see. Thus we find Bichat 4 definitely asserting 
the existence of "vasa exhalantia." Speaking of connective tissues, he 
writes: " Chaque cellule du tissue cellulaire est un reservoir inter- 
medial aux exhalants, qui s'y terminent, et aux absorbants qui en 
naissent." The absorption through the supposed open mouths of the 
lymphatic and lacteal vessels was attributed by most authorities of this 
time to capillary attraction, while the onward How of the fluid in the 
lymphatics could, according to Cruickshank, only be explained as due 
to the vital activity of living cells or tissues. Haller describes the 
movement of the chyle from the intestines in exactly the same manner. 
Particularly ingenious is Hewson's 5 explanation of the absorption and 
movement of chyle in the lacteals. He shows that during life the blood 
vessels of the villi and in the papilke of the skin and mucous mem- 
brane, by their turgescence, keep the orifices of the lacteals or the similar 
openings of the lymphatics patent, so that these are now capable of 
attracting like capillary tubes made of hard substances. The further 
movement of the chyle and lymph he ascribes to the peristaltic con- 
traction of muscular fibres in the walls of the lacteals or lymphatics. 
Views very similar to these were held by some of the most dis- 
tinguished of subsequent physiologists, such as Prochaska, Fohmann, 
Burdach and Henle. In opposition to this mechanical theory of lymph 
formation, Johannes Muller, 6 having regard to the apparent power of 
choice possessed by the lacteals, some substances being absorbed while 
others were left, was inclined to ascribe at any rate the act of absorption 
to the vital activities of the living cells of the body. 
On the discovery of endosmosis by Dutrochet, 7 many physiologists 
believed that at last the riddle of absorption and secretion of lymph was 
solved, and from this time onwards we find an invocation, generally 
more or less vague, of osmotic action to explain the phenomena of 
absorption and secretion. 
Theory of Ludwig.— The beginning of the new 7 era in the history 
of the physiology of lymph formation is marked by the important 
paper of Ludwig and Noll. 8 In consequence of experiments on 
1 Works, edited by Palmer, London, 1835, vol. iv. p. 299. 
2 " De venis lymphaticis valvulosis," 1757. 
3 " Vasorum lyniphatieorum corporis humani historia et iconographia, " 1787. 
4 " Anatomie generate, " 1812. 
5 "A Description of the Lymphatic System, etc.," Collected Works, 8yd. Soc, 1846. 
6 "Elements of Physiology," Baly's trans., 1838, vol. i. p. 248. 
7 Previous article, p. 273. See also "Cyclopaedia of Anat. and Phys.," art. " Endosmose." 
8 Ztschr.f. rat. Med., 1S50, Bd. ix. S. 52. 
