CHAPTER XXIV. 



COMPOSITION AND FORMATION OF LYMPH. 



To understand the occurrence of lymph in the body one has 

 only to bear in mind its method of origin from the blood. Through- 

 out the entire body there is a rich supply of blood-vessels pene- 

 trating every tissue with the exception of the epidermis and some 

 epidermal structures, as the nails and the hair. The plasma of the 

 blood makes its way through the thin walls of the capillaries, and 

 is thus brought into immediate contact with the tissues, to which 

 it brings the nourishment and oxygen of the blood and from which 

 it removes the waste products of metabolism. This tissue fluid 

 in which all the tissue elements are bathed is thus derived directly 

 from the plasma of the blood. We may assume that it is being 

 constantly formed from the blood and accumulates in the tissues 

 under some pressure. But it is also constantly being removed from 

 the tissues by means of the lymphatic vessels. Lymphatic capil- 

 laries ramify through the tissues after the manner of the blood 

 capillaries and collect this tissue fluid and eventually carry it 

 back to the blood-stream. Some authors state that the lymph 

 capillaries are in open communication with the tissue spaces, but 

 the best evidence indicates that they are a closed system of tubes 

 like the blood-vessels.* The lymph capillaries are, however, in 

 contact with the tissue spaces and absorb from them the tissue 

 fluid as it accumulates in these spaces. The term "lymph" is some- 

 times restricted to the liquid after it gets into the lymphatic vessels, 

 but from a physiological point of view it would seem more desir- 

 able to apply this term to the tissue fluid in general, to say that 

 the tissue elements are bathed in lymph and that the excess, so 

 to speak, of this lymph is drained off in the lymph capillaries. The 

 lymph capillaries unite to form larger and larger vessels, and the 

 system after traversing many lymph-nodes and glands is brought 

 together into a large trunk, the thoracic duct, which empties into 

 the veins by a valved opening at the base of the neck where the 

 left internal jugular and left subclavian veins come together. 



*See Sabin, "American Journal of Anatomy," 1, 367, 1902, and 3, 183, 

 1904, and "Science," Aug. 4, 1916; also "General and Special Anatomy of the 

 Lymphatics," from Poirier and Charpy, translated by Leaf, 1904. 



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