THE TRANSPORTATION SYSTEM 



517 



tern and resembles the latter in many re- 

 spects. It merely is an alternate route by 

 which water and wastes from the cells can 

 reach the general circulation. In other 

 words, it resembles a "sludge pump" in its 

 action. 



The lymph glands which produce the 

 lymphocytes of the blood lie along the 

 lymph channels, through which they are 

 dumped into the blood. The lymph glands 

 have another function, namely, as filters. 

 Foreign particles such as dust, debris, and 

 bacteria float in the lymph and finally make 

 their way to the lymph glands where con- 

 siderable phagocytic activity goes on. This 

 can be so active as to produce noticeable 

 swelling of these glands, particularly under 

 the arms if the source of infection is in the 

 hand, or in the groin if the difficulty arises 

 from some portion of the leg or foot. They 

 function in stopping the infection before it 

 gets into the general circulation where it 

 may do a great deal of damage. Carbon 

 particles are inert, so that those entering 

 the body from air laden with coal smoke 

 will lodge in the lymph glands, contributing 

 a dark color to them if the person has lived 

 in cities where smoke is abundant. 



There is no pumping station in the lym- 

 phatic system of man, although such mech- 

 anisms are present in some of the lower 

 animals. The movement of the lymph is due 

 to the continual massaging of the lymph 

 channels by the contracting muscles, both 

 visceral and skeletal. The contraction of the 

 villi in the intestinal walls, as well as the 

 negative pressure created at the point of 

 entry into the large neck veins, aid in 

 bringing about lymph movement. Any stop- 

 page of this movement results in swelling, 

 as in the case of certain roundworms that 

 invade the lymph glands, thereby clogging 

 them so that the lymph cannot pass through. 

 In such cases a leg may weigh as much as 

 100 pounds (p. 183). 



We have seen how the circulatory system 

 provides for the needs of the individual 

 cells in a complex animal. Let us now deter- 

 mine what happens to tlie food elements 

 that have been carried to the cell. Obvi- 

 ously, they must be utilized in construction, 

 repair, and the releasing of energy through 

 oxidation. These will be considered in the 

 next chapter. 



