FORCES CONCERNED IN MOl 'EMENT OF L YMPH. 299 
The Physical Foki es concerned in the Movement of Lymph. 
We may now consider briefly the forces which bring about the flow 
of the lymph and chyle from the origin of the lymphatics towards the 
termination of the thoracic duct in the subclavian vein. 
In the living animal the lymphatics, like the blood vessels, are in a 
condition of moderate distension. The lateral pressure in the lymphatic 
duct of the neck was measured in 1849 by Ludwig and Noll. 1 In the 
dog they found that this pressure varied from 8 to 18 mm. sodium car- 
bonate solution. A little later, Weiss 2 measured the pressure in the 
same vessel in the dog and horse. In the dog he found that it varied 
from 5 to 20 mm., and in the horse from 10 to 20 nun. soda solution. 
The latter observer also estimated the velocity of the lymph flow in the 
cervical lymphatic by means of Volkmann's heemodromometer. He 
found that the average velocity was ahout 4 mm. in the secern 1, a 
velocity which is exceedingly small as compared with the velocity of 
blood in arteries or veins of the same calibre, and is only a few times 
greater' than the velocity in the capillaries. Since there is a constant 
flew of lymph from the periphery to the thoracic duct, it is evident that, 
as we trace the lymphatics towards their- radicles, the pressure of the 
lymph must increase. This increased pressure in the peripheral parts 
of the lymphatic system is shown by the fact, to which Paidbeck 3 first 
called attention, that if a lymphatic lie emptied by pressure, it always 
fills from the periphery, and if a ligature be placed round it, the vessel 
swells upon the peripheral, and sin inks on the central side of the 
ligature. 
We see then that the first and chief factor in the onward flow 
of lymph is the pressure under which this is formed in the radicles 
of the lymphatics and in the tissue spaces. As the blood flows through 
the capillaries at a given pressure, a certain proportion of its fluid con- 
stituents filteis through the vessel wall, forming a transudation which 
is still under a certain amount of pressure, and it is this remaining 
pressure which causes the onward flow of the lymph. Hence the 
ultimate cause of the lymph flow must be looked for in the energy of 
the heart's contraction. 
When this hypothesis was first put forward by Ludwig and Xoll (in 
opposition to the suction theories mentioned previously), it was objected 
to by Bonders 4 en anatomical grounds. At that time it was thought 
that the lymphatics formed a closed system of capillaries, ramifying in 
the tissues ; and Donders pointed out that if the pressure in the tissue 
juices were higher than that of the contents of the lymphatic capillaries, 
the effect would be, not a flow from spaces into capillaries, but a collapse 
of the latter with obliteration of then lumen. Further anatomical 
investigations have shown us, however, that, in the first place, the 
lymphatics are probably not a closed system of tubes, but are in com- 
munication with the tissue spaces (Eecklinghausen, 5 Ludwig) ; and 
secondly, that the walls of the lymphatics, at any rate in certain situa- 
tions, are so connected by strands of elastic fibres with the surrounding 
1 Loc. czt. 
2 " Experiraentelle Untersuch. ueber die Lymphstrom," Diss., Dorpat, lS60(cpiotedby 
Gruenhagen, Bd. i. S. 282). 
3 Loc. cit. 4 Ztschr.f. rat. Med., 1853, N. F., Bd. iv. S. 238. 
5 Strieker's " Histology," Syd. tioc. Trans., 1869, vol. i. p. 297. 
