190 THE CIRCULATION OF THE BLOOD AND LYMPH 



from the body, and yet death may occur from haemorrhage into the 

 pleural or the abdominal cavity, into the stomach or intestines. 

 Not only so, but a man may bleed to death into his own blood- 

 vessels ; in surgical shock, it would appear that the blood which ought 

 to be circulating through the brain, heart and lungs may stagnate 

 in the dilated veins. 



SECTION VII. THE LYMPHATIC CIRCULATION. 



As has already been stated, some of the constituents of the blood, 

 instead of passing back to the heart from the capillaries along the veins, 

 find their way by a much more tedious route along the lymphatics. 

 The blood capillaries are everywhere in very intimate relation with 

 lymph capillaries, which, completely lined with epithelioid cells, lie in 

 irregular spaces in the connective -tissue that everywhere accompanies 

 and supports the bloodvessels. The constituents of the blood-plasma 

 are filtered through, or secreted by the capillary walls into these lymph 

 spaces, and mingling there with waste products discharged by the cells 

 of the tissues, form the liquid known as tissue liquid or tissue lymph. 

 From the tissue liquid the lymph capillaries take up the constituents 

 of the ' lymphatic ' lymph, which then passes into larger lymphatic 

 vessels, with lymphatic glands at intervals on their course. These fall 

 into still larger trunks, and finally the greater part of the lymph reaches 

 the blood again by the thoracic duct, which opens into the venous 

 system at the junction of the left subclavian and internal jugular veins. 

 The lymph from the right side of the head and neck, the right extremity, 

 and the right side of the thorax, with its viscera, is collected by the 

 right lymphatic duct, which opens at the junction of the right sub- 

 clavian 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, although differing 

 from ordinary lymph spaces, are connected through small openings, 

 called stomata, with lymphatic vessels. 



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

 pared 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 main- 

 tenance of the lymph flow are : 



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

 the lymph spaces and from the lymph spaces into the lymph capillaries. 

 The pressure in the thoracic duct of a horse may be as high as 12 mm. 

 of mercury; in the dog it may be less than i mm. The difference is 

 probably due, in part at least, to a difference in the experimental con- 

 ditions, dogs being usually anaesthetized for such measurements, horses 

 not. The pressure in the lymph capillaries 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, aid 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 



