70 AN AMERICAN TEXT- HOOK OF PHYSIOLOGY. 



to the tissues of various glands in amounts greater than could be explained if we sup- 

 posed that the lymph of these tissues was derived solely by filtration from the blood- 

 plasma. (See p. ~'l for an illustration.) Another important conception in this con- 

 nection is the possibility that the capillary walls may be permeable in different degrees 

 to the various soluble constituents of the blood, and furthermore the possibility that 

 the permeability of the capillary walls may vary in different organs. With regard to 

 the first possibility it has been shown by Roth ' that the blood-capillaries are more per- 

 meable to the urea molecules than to sugar or NaCl. With the aid of these facts it is 

 possible to explain in Large measure the transportation of material from the blood to 

 the tissues, and vice versa. For example, to follow a line of reasoning used by Roth, we 

 may suppose that the functional activity of the tissue-elements is attended by a con- 

 sumption of material which in turn is made good by the dissolved molecules in the 

 tissue-lymph. The concentration of the latter is thereby lowered, and in consequence a 

 diffusion stream of these substance- is set up with the more concentrated blood. In this 

 way, by diffusion, a constant supply of dissolved material is kept in motion from the 

 blood to the tissue-elements. On the other hand, the functional activity of the tissue- 

 elements is accompanied by a breaking down of the complex proteid molecule with the 

 formation of simpler, more stable molecules of crystalloid character, such as the sul- 

 phates, phosphates, and urea or some precursor of urea. As these bodies pass into the 

 tissue-lymph they tend to increase its molecular concentration, and thus by the greater 

 osmotic pressure which they exert serve to attract water from the blood to the lymph, 

 forming one efficient factor in the production of lymph. On the other hand, as these 

 substances accumulate in the lymph to a concentration greater than that possessed by 

 the same substances in the blood, they will diffuse toward the blood. By this means the 

 waste-products of activity are drawn off to the blood, from which in turn they are 

 removed by the action of the excretory organs. 



Diffusion of Proteids. — This simple explanation on purely physical grounds of the 

 flow of material between the blood and the tissues can only be applied, however, at 

 present to the diffusible crystalloids, such as the salts, urea, and sugar. The proteids of 

 the blood, which are supposed to be so important for the nutrition of the tissues, are prac- 

 tically indiffusible, so far as we know. It is difficult to explain their passage from the 

 blood through the capillary walls into the lymph. Provisionally it may be assumed 

 that this passage is due to filtration. The blood-plasma in the capillaries is under 

 a slightly higher pressure than the lymph of the tissues, and this higher pressure tends 

 t" Mpieeze the blood-constituents, including the proteid, through the capillary walls. 

 Tin- explanation, however, cannot be said to be satisfactory, and in this respect the 

 purely physical theory of lymph-formation waits upon a clearer knowledge of the nature 

 of the nutritive proteids and their relations to the capillary walls. 



LYMPH. 



LYMPH is a colorless liquid found in the lymph-vessels as well as in the 

 extravascular spaces of the body. All the tissue-elements, in fact, may be 

 regarded as being bathed in lymph. To understand its occurrence in the body 

 one has only to hear in mind its method of origin from the blood. Throughout 

 the entire body there is a rich supply of blood-vessels penetrating every tissue 

 with the exception of the epidermis and some epidermal structures, as the nails 

 and the hair. The plasma of the blood, by the action of physical or chemical 

 processes, the details <d* which are not vet entirely understood, makes its way 

 through the thin walls id' the capillaries, and is thus brought into immediate 



1 Archiv fur Physiohgie, 1899, 8. 416. 



