MODERN BIOLOGY 597 



manifestations of the body have been studied, inter alia, by J. Meisenheimer, 

 ofLeipzig,whoexperimentedonthelarvie of butterflies, and by Eugen Stein- 

 ACH, of Vienna, who studied vertebrates from the same point of view. Stein- 

 ach has succeeded by means of operations in influencing the sexual character; 

 the genital glands of male and female animals have been interchanged, with 

 the result that both the outward appearance and the sexual behaviour of the 

 animals have been correspondingly altered. Otherwise, Steinach is best known 

 for his having resumed rejuvenation experiments similar to those of Brown- 

 Sequard, but more carefully carried out both in theory and in their details. 

 These experiments have won him a fame that, owing to the nature of the 

 problem, has become associated with much of the glamour that surrounded 

 his predecessor; moreover, they appear already to have exhibited defects that 

 have rendered them impossible of realization in practice. 



Internal secretion and rejuvenation experiments 

 In the mean time our knowledge of internal secretion in other spheres has 

 increased with great rapidity. So-called endocrine glands have been discov- 

 ered in large numbers; among them may be mentioned the suprarenal cap- 

 sules, hypophysis, the thymus and the thyroid gland; and our knowledge 

 of their functions has at the same time been extended. But other organs have 

 also become known as producers of internal secretions, or hormones, as they 

 are also called; the small intestine produces such a secretion, which is termed 

 "secretin" and which, when conveyed through the blood to the pancreas, 

 induces secretion in that gland; another similar substance is produced in con- 

 nexion with the pregnancy of female animals and causes the segregation of 

 milk, and a third is the product of a special cell-category in the pancreas 

 and has a definite influence upon the metabolism of the body, in that its 

 absence induces diabetes. On the whole, many of these internal secretions 

 have become known only by indirect means, through the diseases that arise 

 if the organs which produce them are injured or removed. 



Serology 

 In modern times serology forms a separate field of research, embracing one 

 of the most important chapters in practical medicine. Pasteur should be 

 named as its founder, while later Behring, Ehrlich, and their pupils have 

 particularly distinguished themselves in that subject. It has been established 

 that the danger of the disease-producing bacteria lies in the fact that in the 

 course of their multiplication in the body they produce special isolatable 

 chemical compounds having a specific poisonous effect, which have been 

 given the name of toxins; the body reacts against these by forming similarly 

 specific elements, antitoxins, which counteract them. To isolate these latter 

 and to use them as a counter-poison has in many cases proved the only w'ay 

 for medical science to cure infectious diseases. By injecting the bacterial poi- 

 son into experimental animals, these latter have been allowed to produce 



