574 LECTURE XXIV. 



glands and the spleen. Their function has never been satisfactorily 

 explained. 1 



We must now mention an important kind of cells which are interme- 

 diate between those of the blood and of the tissues. The blood does not 

 communicate directly with the tissue-cells. It neither imparts nutritive 

 material directly to them nor receives the products of metabolism 

 directly from them. In this connection the kidneys occupy an excep- 

 tional position, for in them the blood-vessels of the Glomeruli Malpighi 

 lie directly at the end of a uriniferous tube. The reason for this is clear: 

 in this way the waste-products contained in the blood are given up to 

 the urine as quickly as possible. 



In the other tissues of the body, the exchange of material between the 

 blood and tissue takes place through the lymph. First of all we must 

 consider the formation of lymph. We may say at once that it has two 

 sources of material, the blood on the one hand, and the tissues on the 

 other. Qualitatively it contains the same substances as the blood-plasma. 

 Quantitatively, however, there is a difference between the two liquids. 

 The formation of the lymph, which penetrates into all of the tissues, 

 has been repeatedly referred to a purely physical process, as, for example, 

 that of a filtration. We must admit that unquestionably, purely physical 

 processes do come into play in the giving up of material by the blood 

 and by the tissues to the lymph, and, on the other hand, in the passage 

 of material from the lymph either into the blood or tissues. 2 We have a 

 great many reasons, however, for not being willing to admit that our 

 present knowledge concerning the formation of lymph and all its func- 

 tions is in accordance with the assumption that only physical forces are 

 concerned here. 



In considering processes which are much easier to understand than 

 the formation of lymph, we have constantly encountered phenomena 

 which indicate the existence of a specific activity for every individual 

 cell, whether it be that of secretion or of absorption. We must admit 

 that it is far more difficult to study accurately each individual process 

 when the relations are so complicated as they are in the animal organ- 

 ism. In every case there are a number of different processes which we 

 at present cannot examine very closely. One process crosses another, 



1 See J. Seemann: Die blutbildenden Organe, Ergeb. Physiol. (Asher and Spiro) Jg. 

 3, p. 1 (1904). 



2 See Bayliss and Starling: J. Physiol. 16, 159 (1894). W. Connstein: Virchow's 

 Arch. 135, 514 (1894); Pfliiger's Arch. 59, 350 (1899); 59, 508 (1894); 60, 291 (1894); 

 62, 58, (1895); 63,587 (1896); H. J. Hamburger: Z. Biol. 30, 143 (1893); Arch. 

 Anat. Physiol. 1895, 281; 1897, 132. R. Heidenhain: Pfluger's Arch. 49, 209 (1881); 

 56, 632 (1894); 62, 320 (1895). A. Ellinger: Ergeb. Physiol. (Asher and Spiro) Jg. 

 1 Abt. 1, p. 355 (1902). 



