VASOMOTOR SUPPLY OF THE ORGANS. 585 



hind limbs. The whole subject, however, of venomotor nerves 

 has been but little investigated, and at present little or no use is 

 made of this possible system in explaining the facts of the circu- 

 lation. 



THE CIRCULATION OF THE LYMPH. 



The direction of flow of the lymph is from the tissues toward the large 

 lymphatic trunks, the thoracic and the right lymphatic duct. The flow is 

 maintained in this direction mainly by a difference in pressure at the two ends. 

 At the opening of the large trunks into the veins the pressure is very low; 

 in the vein, in fact, it may be zero or even negative. The opening between the 

 lymph vessel and the vein is protected by a valve which opens toward the 

 vein, and the lymph, therefore, will flow into the vein as long as the pres- 

 sure in the latter is lower than that in the lymphatic duct. At the other ex- 

 tremity of the system, in the tissue spaces to which the lymphatic capillaries 

 are distributed, the pressure, on the contrary, is high. Its exact amount is 

 not known, but, since the pressure in the blood capillaries is equal to 40-60 

 mms. Hg., the pressure in the liquid of the surrounding tissues must also be 

 considerable. The tissues are, in fact, in a condition of turgidity owing to 

 the pressure of the lymph in the tissue-spaces. This difference in pressure 

 at the two ends of the lymphatic system is the main constant factor in mov- 

 ing the lymph. It is obvious that in the long run it is dependent upon the 

 pressure within the blood-vessels and therefore upon the force of the heart 

 beat. The contractions of the heart supply the energy, not only for the move- 

 ment of the blood, but also for the much slower movement of the lymph. The- 

 circulation of the lymph is aided, however, by many accessory factors. In 

 some animals there are genuine lymph hearts upon the course of the vessels, 

 that is, pulsatile expansions of the lymph vessels whose force of beat, con- 

 trolled by valves, is directly applied to moving the lymph. No such structures 

 are found in the mammalia, but according to some observers the large re- 

 ceptacle at the beginning of the thoracic duct, receptaculum chyli may 

 undergo contractions, and is, besides, under the influence of motor and 

 inhibitory nerves. Such movements, if they occur, must be equivalent to the 

 action of a lymph heart in their influence upon the flow of lymph. The 

 flow of lymph or chyle in the intestinal area is also, without doubt, greatly 

 assisted by the peristaltic and especially by the rhythmic contractions of the- 

 musculature of the intestines. The volume of the lymph in this region is 

 especially large and the lymph capillaries and veins are provided with valves. 

 Rhythmical contractions of the musculature of the intestine must squeeze 

 the lymph toward the thoracic duct, acting like a local pump to accelerate 

 the flow of lymph. A similar influence is exerted by the contractions of the 

 skeletal muscles. The compression exerted by the shortened fibers squeezes 

 the lymph vessels and, on account of the valves present, forces the lymph 

 onward toward the larger ducts. The flow of lymph from the resting muscles 

 the arms and legs, for instance ^is normally small in quantity, but during 

 muscular exercise and massage it is obviously increased. This increase may 

 be observed in experimental work by placing a cannula in the thoracic duct. 

 Active or passive movements of the limbs under these conditions will cause a 

 noticeable increase in the outflow from the duct. Still another factor which 

 exercises an influence upon the flow of lymph is found in the respiratory move- 

 ments of the thorax. At each inspiration the pressure within the thorax is 

 diminished (increase of negative pressure), and this factor influences the lymph 

 flow in several ways : By increasing the flow of blood through the large veins 

 at the edge of the thorax, jugulars and subclavians, it doubtless aspirates 

 lymph from the thoracic and right lymphatic ducts into these veins. More- 

 over, by lowering the pressure upon the intrathoracic portion of the thoracic 

 duct it also aspirates the lymph from the abdominal portion of this vessel. 



When we place a cannula in the thoracic duct and measure the outflow 

 directly it is found to be exceedingly slow and variable. Older measure- 



