THE CIRCULATION OF THE BLOOD AND LYMPH 167 



lymphatic duct, which opens at the junction of the right subclavian 

 and internal jugular veins. The openings of both ducts are guarded 

 by semilunar valves, which prevent the reflux of blood from the 

 veins. Serous cavities like the pleural sacs are really large lymph 

 spaces, and they are connected through small openings, called 

 stomata, with lymphatic vessels. 



The rate of flow of the lymph in the thoracic duct is very small 

 compared with that of the blood in the arteries only about 4 mm. 

 per second, according to one observer. Nevertheless, a substance 

 injected into the blood can be detected in the lymph of the duct in 

 four to seven minutes (Tschirwinsky). The factors which contribute 

 to the maintenance of the lymph flow are : 



(1) The pressure under which it passes from the capillaries into 

 the lymph spaces. The pressure in the thoracic duct of a horse may 

 be as high as 1 2 mm. of mercury ; in the dog it may be less than 

 i mm. The difference is probably due, in part at least, to a differ- 

 ence in the experimental conditions, dogs being usually anaesthetized 

 for such measurements, horses not. The pressure in the lymph 

 spaces must, of course, be higher than in the thoracic duct, how 

 much higher we do not know. 



(2) The contraction of muscles increases the pressure of the 

 lymph by compressing the channels in which it is contained, and 

 the valves, with which the lymphatics are even more richly provided 

 than the veins, hinder a backward and favour an onward flow. The 

 contractions of the intestines, and especially of the villi, are an im- 

 portant aid to the movement of the chyle. By the contraction of 

 the diaphragm, substances may be sucked from the peritoneal cavity 

 into the lymphatics of its central tendon, through the stomata in the 

 serous layer with which its lower surface is clad. It is even possible 

 by passive movements of the diaphragm in a dead rabbit to inject 

 its lymphatics with a coloured liquid placed on its peritoneal surface. 

 Passive movements of the limbs and massage of the muscles are also 

 known to hasten the sluggish current of the lymph, and are some- 

 times employed with this object in the treatment of disease. 



(3) The movements of respiration aid the flow. At every inspira- 

 tion the pressure in the great veins near the heart becomes negative, 

 and lymph is sucked into them. 



(4) In some animals rhythmically-contracting muscular sacs or 

 hearts exist on the course of the lymphatic circulation. The frog 

 has two pairs, an anterior and a posterior, of these lymph hearts, which 

 pulsate, although not with any great regularity, at an average rate of 

 sixty to seventy beats a minute, and appear to be governed by motor 

 and inhibitory centres situated in the spinal cord. Such hearts are 

 also found in reptiles. It is possible that in animals without localized 

 lymph hearts the smooth muscle, which is so conspicuous an element 

 in the walls of the lymphatic vessels, may aid the flow by rhythmical 

 contractions. 





