i/6 A MANUAL OF PHYSIOLOGY - 



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 accom- 

 panies 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, fcrm 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 

 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 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 conditions, 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, are an 

 important 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- 



