132 THE BLOOD 



question which has been surrounded with considerable difficulty. It is 

 thought by Ludwig and many of his followers that the process involved 

 is merely one of nitration. Certainly the blood pressure in the capillaries 

 is in the main greater than that of the pressure of the lymph in the surround- 

 ing tissues, and this positive pressure will contribute so much to the direct 

 ingredients of the blood-plasma through the capillary walls. It is true, as a 

 matter of experiment, that anything which contributes to an increase in the 

 capillary pressure is very apt to produce an edema of the corresponding 

 tissues. Since the colloidal materials represented by the proteid are non- 

 diffusible, one would by this theory expect to find a diminished percentage 

 in the lymph, which is true, though not to the extent which the theory demands. 



Heidenhain was the first to question the adequacy of the blood 

 pressure and filtration hypothesis. He showed that many of the conditions 

 under which lymph formation takes place are not sufficient to produce filtra- 

 tions of the material found. He advanced the hypothesis that the living 

 endothelial lining of the blood-vessels exerted a secretory activity in lymph 

 production. He discovered that various substances known as lymphagogues 

 when introduced into the circulatory system produce a remarkable increase 

 in the flow of lymph from the thoracic duct. Further, he noticed that the 

 concentration of the lymph was changed, i.e., increased. It has been sug- 

 gested that these substances act to change the normal resistance of the endo- 

 thelial cells, and this has been offered as a criticism. Nevertheless many 

 drugs act to increase the flow of lymph in a way which cannot be presumed 

 to be other than normal, i.e., they stimulate the physiological processes going 

 on in the endothelial cells. Such observations contribute strongly to the 

 view advanced by Heidenhain. Many investigations have been brought to 

 the support of the hypothesis that lymph formation is largely a process of 

 secretion, yet it seems at the present time that we cannot wholly deny that 

 filtration and osmosis play a part in the processes. Certainly the permea- 

 bility or activity of the endothelial lining of the blood-vessels varies greatly 

 at different times in the life of an individual, and this variation in function is 

 associated with the marked change in the character and quantity of lymph 

 produced. 



The second factor in lymph formation, the activity of the tissue in taking 

 up or discharging materials into the lymph-mass, must not be ignored alto- 

 gether. 



LABORATORY EXPERIMENTS FOR THE EXAMINATION OF 



THE BLOOD. 



i. Microscopical Examination of the Blood. Mount a drop of frog's 

 blood in 0.7 per cent sodium chloride and examine with the low power of a 

 compound microscope. The red corpuscles will appear as oval nucleated 



