CARL LUDWIG AND CARL THLERSCH. 343 



devoted most of his activity to the far more difficult task of experi- 

 menting on the living body, a branch of work which in his hands 

 and by his own inventions, the " Kymographion," the " Stromuhr," 

 the mercury pump, etc., gained a precision that up to that time had 

 never been imagined. 



His inaugural thesis, that appeared in 1842, at first opposed by 

 the Marburg faculty, treated of the secretion of the kidneys, a subject 

 to the study of which he returned again and again in later years. 

 Another very systematically arranged line of experiments led Lud- 

 wig in 1851 to the important discovery of the dependence of the 

 secretion of the saliva upon the irritation of the glandular nerves, and 

 to the recognition of the independence of the pressure of the secretion 

 from that of the blood. This discovery was of fundamental impor- 

 tance for the physiology of secretion no less than for that of the 

 nervous system, and it was at that time all the more surprising, since 

 Ludwig's own suppositions had caused precisely the opposite result to 

 be expected. 



Constant objects of intense interest to Ludwig were the pecul- 

 iarities of the circulating blood, its lateral pressure, and its rate of 

 flow, as well as the dependence of these functions on the activity of 

 the heart, on that of the muscles of the body, on the condition of the 

 vascular muscles, and of numberless other factors. It is in this field 

 that Ludwig's delicate graphic apparatus and his precise methods of 

 measurement won their greatest triumphs. No less are our thanks 

 due to him for a great part of the present knowledge of the mechan- 

 ism of the heart's activity. It was he who first gave us an idea of 

 the action of the lymph current in the living organism. He deter- 

 mined quantitatively its amount and its variations, and by histo- 

 logical investigations on the origin of the lymphatic system he threw 

 light on the nature of this extraordinary apparatus. Ludwig labored 

 incessantly to obtain a true understanding of respiration, including 

 not only the gas exchange in the lungs, and the respiratory move- 

 ments, but also the internal or tissue respirations. He was very suc- 

 cessful in studying the activity of organs in the so-called state of 

 survival which is produced by conducting a stream of blood through 

 parts taken from a freshly killed animal, and thus the qualities of the 

 blood, the lymph, as well as of various secretions, are determined and 

 compared, both before and after the passage of the blood. When 

 it seemed to him necessary to extend the anatomical foundations of 

 physiological study, he always undertook this himself, or required 

 students to make anatomical investigations. Among many other 

 histological researches, we are indebted to him, above all, for his 

 classic treatise on the structure of the kidneys. The most careful 

 investigations of the blood-vessels of the eye and those of the inner 



