6 Florence B. Sabin. 



3. Discovery of Endothelium. 

 In contrast to these hypothetical relationships of lymphatics, the 

 work of von Kecklinghausen (119) gave the first definite theory which 

 was actually hased on the interpretation of direct observations of the 

 lymphatic capillary. Through the use of silver nitrate solutions re- 

 duced in direct sunlight, he discovered that the lymphatics are lined by 

 a definite cell layer of endothelium. This is clearly one of the most 

 important discoveries in histology. Von Kecklinghausen interpreted 

 the silver pictures to mean that the finest lymphatic capillaries were 

 lined by an epithelium, but that these capillaries communicated by 

 wide openings with Saftcanale or lymph radicles, which formed 

 the real rootlets of the lymphatics. Subsequently he described more 

 definite openings or stomata in the serosa of the diaphragm (120). 



Fig. 3. — Diagram to show von Recklinghausen's theory of the relation of 

 lymphatics to tissue spaces. All of the tissue spaces are cross-hatched to 

 indicate hypothetical lymphatics. 



The von Eecklinghausen figures seemed to offer a perfectly satisfac- 

 tory demonstration of the relations of open lymphatics, especially in 

 view of the fact that the great majority of anatomists already believed 

 in open lymphatics. But it is of great interest to follow the writings 

 of His at this period. His (44), who was just publishing in favor of 

 open lymphatics, and saw von Eecklinghausen's paper as his own was 

 going to press, at first recognized in von Eecklinghausen's descriptions 

 a confirmation of his own views. On careful studying of the silver 

 pictures, however, His (45) saw that the discovery of endothelium was 

 the very step which, instead of proving that lymphatics are open and 



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