276 A MANUAL OF VETERINARY PHYSIOLOGY 



vein, the mixed contents of chyle and lymph will rise in the tube, 

 and in about five minutes reach a maximum. The amount of 

 this is variable ; Colin has registered during full digestion a 3-foot 

 pressure (say 75 mm. mercury) in the thoracic duct, which is 

 about one-third of the aortic pressure. This is very high — indeed, 

 is higher than that of the blood in the capillaries or of lymph in 

 the lymph-vessels. It would appear that this marked degree of 

 pressure can only be due to contraction of the walls of the recep- 

 taculum chyli and contraction of skeletal muscles, the abdominal 

 muscles doubtless taking the largest share. The fluid in the 

 tube rises and falls, and if the tube be removed lymph issues 

 from the vessel in jets. The rise and fall of level in the tube 

 is associated with respiratory movements, but the effect produced 

 is not the same as the general effect caused by inspiration and 

 expiration on blood pressure. Inspiration does not, for instance, 

 draw the lymph from the duct towards the vein — its effect is the 

 reverse ; while expiration, on the other hand, raises the pressure 

 in the duct, and discharges its contents. Colin, who first de- 

 scribed this singular fact, explains it by saying that the intra- 

 thoracic pressure being reduced during inspiration, the vessel 

 dilates, and, in consequence, its capacity is increased. During 

 inspiration it fills up, while at expiration the pressure exercised 

 upon it, no doubt particularly by the abdominal muscles during 

 expiration, causes the now distended duct to discharge its con- 

 tents. Colin, in fact, observed that the oscillations of the fluid 

 in the manometer tube are greater during laboured respiration. 

 In the horse the pressure in the thoracic duct must be much less 

 than in the duct of the ox ; in fact, for blood to pass into the duct 

 the pressure must be at times below that of the pressure in the 

 anterior vena cava. There are great experimental difficulties in 

 getting at the duct in the horse, and exact information is 

 wanting. 



As fast as the lymph finds its way from the bloodvessels into 

 the spaces it is normally passed on to the lymphatic capillaries, 

 so that the rate of output is equivalent to the rate of removal ; 

 when, however, the output is greater than the rate of removal 

 the lymph accumulates in the tissues, and CEdema results. It 

 is conceivable that the rate of removal need not necessarily always, 

 be at fault, but that the rate of secretion may be so greatly 

 increased that the outgoing channels are unequal to the demands 

 made upon them. Such an increased secretion of lymph lies on 

 the shoulders of the vascular system, and experience shows that 

 in the majority of cases increased formation of lymph is a more 

 common cause of oedema than defective drainage. It is well 

 known that interference with the venous circulation is productive 

 of oedema, the explanation being that there is not only an 



