devoted most of his activity to the far more difficult task of experimenting 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 Ludwig 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 importance 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 peculiarities 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 mechanism 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 determined quantitatively its amount and its variations, and by histological 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 movements, but also the internal or tissue respirations. He was very successful 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
