1066 



PHYSIOLOGY 



they can be shown also to have some effect in the same direction on the 

 intestinal and skin capillaries. In fact the injection or ingestion of these 

 bodies often gives rise to a copious eruption of nettle-rash, i. e. swellings of 

 the skin due to an increased exudation of lymph into the meshes of the 

 cutis. 



An increased lymph flow from the thoracic duct may be produced also 

 by the injection of large amounts (10 to 40 grm.) of innocuous crystalloids, 

 such as dextrose, urea, or sodium chloride, into the circulation. In this 

 case the lymph becomes much more dilute. The explanation of the action 

 of these bodies is very simple. We have already seen that injection of 

 large amounts of dextrose into the circulating blood raises the osmotic pres- 

 sure of this fluid. The blood therefore imbibes water from the tissues and 





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JOmiiiutes 



InJ. of mussel extract 



FIG. 488. Changes in lymph flow in portal, inferior cava, and arterial pressures, 

 resulting from injection of a member of the first class of lymphagogues (extract 

 of mussels). (STARLING.) 



swells up, i. e. a condition of hydraemic plethora is brought about as surely as 

 if several hundred cubic centimetres of normal salt solution were injected 

 into the circulation. This increase in the total volume of the blood causes 

 a rise of pressure throughout the vascular system arteries, capillaries, 

 and veins and the increased capillary pressure, combined with the watery 

 condition of the blood, induces a great transudation of lymph, especially 

 in the abdominal organs (Fig. 489). The lymph is more watery because the 

 blood also is diluted. That the action of these bodies is purely mechanical 

 is shown by the fact that, if the rise of capillary pressure be prevented by 

 bleeding the animal immediately before the injection, the increase in the 

 lymph flow is also prevented (Fig. 489, B), although the concentration of the 

 sugar or salt in the blood is still greater than in the experiments in which 

 bleeding was not performed. 



MOVEMENT OF LYMPH 



In the frog the circulation of lymph is maintained by rhythmically con- 

 tracting muscular sacs, which are placed in the course of the main lymph 



