CARL LUDWIG. 163 



tribution, after studying the minute structure of the kidney, 

 more especially the distribution of its blood-vessels, and 

 after numerous experiments, he attempted to explain the 

 process of urinary secretion on a physical basis. With 

 his pupil Goll, still later, he showed the dependence of 

 urinary secretion on blood pressure, and hence arose the 

 so-called "filtration hypothesis". In 1844 in Wagners 

 Handwdrterduck, vol. ii., p. 628, his researches, extending" 

 to twelve pages, are given under the title " Nieren und 

 Harnbreitung ". Just as it was his earliest work, so a 

 study of urinary secretion cropped up every now and 

 again in his laboratory. Ustimowitsch in 1870 showed 

 how curare diminished urinary secretion, how the secretion 

 was influenced by section of the renal nerves, and above 

 all, from the more modern standpoint regarding the secretion 

 of urine, this other important fact that urea when injected 

 into the blood-vessels — the blood pressure being very 

 low, so low, indeed, that no secretion of urine took place 

 — again caused a secretion of urine. This fact, recorded 

 by Ludwig's pupil, is to-day utilised to explain secretion 

 by the presence of " harnfahigen " substances in the blood. 

 In 1893 Grijus showed that "the kidneys produce heat 

 in proportion to the water they excrete " ; and again we 

 have the important fact brought out in this paper from 

 a comparative study of muscle, salivary gland, and kidney, 

 that while muscle and salivary glands derive their impulse 

 to the production of heat from nerves, in the case of the 

 kidney this is not so ; the kidneys are excited to excrete 

 only when the blood is charged with " harnfahigen " sub- 

 stances. But while this is so, one of the contributions 

 which made him famous was his investigation on the 

 influence of the nervous system on glandular activity. 

 By his classical researches on the submaxillary gland, 

 " Neuen Versuche Liber d. Beihilfe d. Nerven z. Speichel- 

 absonderung," Ludwig completely retransformed the theory 

 of salivary secretion, and of secretion in general. He 

 showed that secretion could no longer be regarded merely 

 as a process of filtration ; on the contrary, it was associated 

 with chemical and thermal changes in the gland and in the 



