THE THEORY OF OSMOTIC PRESSURES. 2S9 



square centimetre. The absolute force of human muscle is 

 8000 grams, so we see that the mammalian kidney can 

 exert a force six times greater than the maximum per- 

 formance ot voluntary muscle. 



Of course these results absolutely disprove Ludwig's 

 old mechanical theory of urinary secretion. According to 

 this theory the urine is formed by a process of filtration 

 from the glomerular blood-vessels. The fluid thus furnished 

 is free from proteid and less concentrated than the blood- 

 plasma. Since a blood pressure of 200 mm. Hg = 272 

 metres water, and 1 A= 1227 metres water, the highest 

 possible difference between dilute urine and blood could 

 only be A = 0*022 C. Dreser finds, however, that after 

 beer drinking and in diabetes insipidus the urine secreted 

 may have A = 0*16 C, i.e., a difference between A of blood 

 and A of urine of '4. Hence the production of urine by the 

 glomeruli is also attended with the doing of work, and must 

 therefore be looked upon as a process of secretion. Ludwig 

 further considered that the dilute urine was concentrated 

 on its passage through the tubules by the osmotic passage 

 of water into the concentrated blood-plasma. This, how- 

 ever, is also impossible (as Hoppe Seyler showed long 

 ago), since the urine as normally secreted is much more 

 concentrated than the blood -plasma, and the difference 

 may, as mentioned above, attain to enormous amounts 

 (A of blood = - o"56°C. and A of urine = - 47°C). 



In a paper published a few months ago, 1 Heidenhain 

 applies the same method to deciding the question whether 

 the absorption of salts and water from the intestines de- 

 pends merely on the osmotic difference between intestinal 

 contents and blood, or whether this act is carried out by 

 the walls of the mucous membrane in opposition to the 

 osmotic differences. In his experiments about 100 ccm. of 

 salt solutions of varying concentration were introduced into 

 a loop of the ileum a little above the iliacaccal valve, and 

 the loop then returned to the abdomen. Half an hour 



1 " Neue Versuche iiber die Aufsaugung im Dunndarm." Pfliiger's 

 Archiv, lvi., p. 579. 



