CH. xxii.] FORMATION OF LYMPH. 317 



Gley have obtained evidence of the presence of nerves, causing both dilata- 

 tion and constriction of the duct. These are contained in the sympathetic 

 chain below the first thoracic ganglion. The effect of stimulation is 

 principally dilatation. 



Relation of Lymph and Blood. 



The volume of blood in the body remains remarkably constant. 

 If the amount is increased by injection of fluids, at first its 

 specific gravity is lessened, but in a short time, often in a few 

 minutes, it returns to the normal. The excess of fluid is got rid 

 of in two ways : ( i ) by the kidneys, which secrete profusely ; and 

 (2) by the tissues, which become more watery in consequence. 

 After the renal arteries are ligatured, and the kidney is conse- 

 quently thrown out of action, the excess of water passes only into 

 the tissues. 



On the other hand, a deficiency of blood (for instance, after 

 haemorrhage) is soon remedied by a transfer of water from the 

 tissues to the blood through the intermediation of the lymph. 



In severe haemorrhage life has often been saved by injection of 

 saline solution into the vessels, or by transfusion from another 

 person. The transfer of the blood of another animal to the 

 human vascular system is usually dangerous, especially .if the 

 blood has been defibrinated, for the serum of one animal is usually 

 poisonous to another, producing various changes, of which a 

 breakdown of the corpuscles (hsemolysis) is the most constant 

 Wgn. 



Formation of Lymph. 



Carl Ludwig taught that the lymph flow is conditioned by two 

 factors : first, differences in the pressure of the blood in the 

 capillaries and of the fluid in the tissue spaces, giving rise to a 

 filtration of fluid through the capillary walls; and secondly, 

 chemical differences between these two fluids, setting up osmotic 

 interchanges through the wall of the blood-vessel. 



0*>noi*. The phenomenon of the passage of fluids through animal 

 membrane, which occurs quite independently of vital conditions, was first 

 demonstrated by Dutrochet. The instrument which he employed in his 

 experiments was named an endoxmoineter. One form of this, represented in the 

 figure (fig. 295), consists of a graduated tube expanded into an open-mouthed 

 bell at one enri, over which a portion of membrane is tied. If the bell is tilled 

 with a solution of a salt say sodium chloride and is immersed in water, the 

 water will pass into the solution, and part of the salt will pass out into the 

 water : the water, however, will pass into the solution much more rapidly 

 than the salt will pass out into the water, and the diluted solution will rise 

 in the tube. It is to this passage of fluids through membrane that the terms 

 ilinl i/xi* and itminwix are applied. 



The nature of the membrane used as a septum, and its affinity for the 



