THE PRODUCTION OF LYMPH 589 



fluid for examination. Consequently we have no direct means 

 of knowing the exact composition of the materials interchanged 

 between a tissue cell and tissue fluid on the one hand, and 

 the tissue fluid and blood capillary on the other. 



The nearest approach to tissue fluid which is available for 

 examination is the lymph which can be collected from one of 

 the main lymphatic trunks of the body. The flow along the 

 thoracic duct and the main trunk of the neck is continuous, but 

 in the case of a limb it is necessary to perform passive move- 

 ments of the limb in order to obtain a continuous flow. 



The lymph in a large lymphatic vessel represents the overflow 

 from any or all the interstitial spaces which the vessel drains. 

 Since, as we shall see, material passes back from the tissue fluid 

 direct into the blood capillary, it follows that lymph in a 

 lymphatic is not the same as tissue fluid. In composition it 

 must be different from that of any tissue fluid, and it will 

 vary according to 'the different combinations of tissues from 

 which it is being derived. In quantity, while it is conceiv- 

 able that an active interchange might go on between the 

 tissue fluid, the tissues and the blood without any overflow from 

 the tissue spaces, an increased overflow into . the lymphatic 

 vessels must have been preceded by an increased formation of 

 tissue fluid. 



In spite of these necessary differences between the fluid in 

 the tissue spaces and that in the lymphatic vessels, both may 

 be referred to generally as lymph. 



The subject of lymph formation and absorption is one not 

 merely of physiological interest, but also of great pathological 

 and medical importance. In many conditions of circulatory 

 disturbance and of inflammation fluid accumulates in the 

 subcutaneous tissues and the serous cavities. Dropsy is also 

 liable to occur in certain forms of kidney disease, and a localised 

 oadema of the skin may take place in certain individuals after 

 eating shell-fish, strawberries, &c., and in other less well-defined 

 conditions. We cannot hope to understand these pathological 

 oedemas until we have arrived at an accurate knowledge of the 

 process of normal lymph production and the conditions which 

 regulate it. 



