' 



310 THE CIRCULATION 



neck, where it becomes again part and parcel of the blood. The quick- 

 ness with which the plasma osmoses and filters outward and inward 

 again as lymph is surprising. The time varies with the nature of the 

 liquid (colloids not passing out at all), but readily diffusible crystaloids 

 injected into the blood-vessels appear in the lymph without any appreci- 

 able interval, as Colenstein showed and as is demonstrated frequently 

 in the surgical procedure. A minute or two is perhaps an average time 

 for the passage outward of salines from the capillaries. This shows 

 in a striking manner how unified in composition at all times and under 

 most circumstances are the tissue-fluids and the blood. 



Edema is a pathological condition, but one which well illustrates 

 certain principles in the physiology of the lymph. It consists essentially 

 of a collection of lymph in the soft parts or in the great serous sacs of the 

 body. It then has special names, such as ascites when the fluid is 

 collected in the peritoneal cavity. The causes of edema are various and, 

 first or last, mechanical. Thus anything which obstructs the flow of 

 lymph out of the tissues into the veins occasions its collection among 

 the cells, swelling the part and making it obviously more liquid in com- 

 position than normally. In valvular heart-disease, owing to defects in 

 the pump which cripple a prompt circulation and so cause venous stasis, 

 edema is a frequent symptom; thus people formerly were said to die of 

 "the dropsy." Bright 's disease of the kidneys shows a similar effect, 

 but here, owing to the incomplete excretion of waste from the blood, the 

 capillary walls probably become diseased, thus allowing of the too great 

 escape of more or less abnormal plasma into the tissues. In the ankles 

 and under the eyes the mechanical conditions are such that here the 

 distention makes its first appearance. 



Other matters concerning the lymph are discussed in the previous 

 chapter where the composition and functions of the circulating liquids 

 are described. The most important part of the truth about the lymph, 

 namely, the exact chemical reactions which take place between it and 

 the tissue-cells, still remains a blind secret. From this view-point the 

 physiology of the lymph is almost the whole science of organic metabolism. 



The Functions of the Veins, compared with those of the arteries and the 

 capillaries, are simple and easily described. One might almost say that 

 the veins have only one function, namely, to return the blood to the heart 

 that it may be sent out again to perform its duties in the capillaries under 

 the control of the arteries. The veins outside of this requirement have 

 nearly negative qualities. The walls are tough, that they may stand 

 when' necessary much pressure. They are lined with endothelium to 

 reduce the friction and to perform functions doubtless of a chemical 

 nature on the blood. They are thin-walled partly because the low 

 pressure within them does not require them to have the strength coming 

 from greater thickness, and partly that they may collapse promptly 

 when severed and thus prevent death from air sucked into the heart 

 from wounds. They are capacious (two or three or even more times as 

 spacious as the arteries), in order that the friction of the blood within 



