MO VEMEXT OF THE LYMPH. 331 



ferent vessels arise in its centre. Hence, the lymph in its 

 flow traverses the cellular gland substance, and in its 

 course picks up extra corpuscles which it carries on to the 

 blood. In the gland there is a close network of blood capil- 

 laries. It is clear that these organs are not glands at all, in 

 the proper sense of the word. They are sometimes called 

 lymphatic ganglia, but that suggests a connection with 

 nerve-centres; a good name^for them is lymphatic nodes. 



The Movement of the Lymph. This is no doubt some- 

 what irregular in the commencing vessels, but, on the whole, 

 sets on to the larger trunks and through them to the veins. 

 In many animals (as the frog) at points where the lymphatics 

 communicate with the veins, there are found regularly 

 contractile "lymph-hearts" which beat with a rhythm inde- 

 pendent of that of the blood-heart, and pump the lymph 

 into a vein. In the human body, however, there are no 

 such hearts, and the flow of the lymph is dependent on less 

 definite arrangements. It seems to be maintained mainly 

 by three things. (1) The pressure on the blood plasma in 

 the capillaries is greater than that in the great veins of the 

 neck; hence any plasma filtered through the capillary walls 

 will be under a pressure which will tend to make it flow to 

 the venous termination of the thoracic or the right lym- 

 phatic duct. (2) On account of the numerous valves in 

 the lymphatic vessels (which all only allow the lymph to 

 flow past them to larger vessels) any movement compress- 

 ing a lymph- vessel will cause an onward flow of its contents. 

 The influence thus exerted is very important. If a tube be 

 put in a large lymph-vessel, say at the top of the leg of an 

 animal, it will be seen that the lymph only flows out very 

 slowly when the animal is quiet; but as soon as it moves 

 its leg the flow is greatly accelerated. (3) During each 

 inspiration the pressure on the thoracic duct is less than 

 that in the lymphatics in parts of the Body outside the 

 thorax (see Chap. XXIV. ). Accordingly, at that time, lymph 

 is pressed, or, in common phrase, is " sucked," into the 

 thoracic duct. During the succeeding expiration the pres- 

 sure on the thoracic duct becomes greater again, and some 

 of its contents are pressed out; but on account of the valves 



