4 Florence B. Sabin. 



2. Morphology of the. Lymphatic System. 



It will be proved in the following pages that the lymphatic problem 

 is closely connected with, or rather is a fundamental part of, the 

 vascular problem, so that the study of the one throws light on the 

 other, or. to put it more strongly, that the same kind of evidence is 

 needed to solve both problems. The fundamental morphology of the 

 lymphatic system has, however, been put on a more satisfactory basis 

 than that of the vascular system, for it has been seen in the living 

 embryo that the first lymphatics bud off from the veins. Moreover, 

 lymphatics begin at a comparatively late stage, long after the forma- 

 tion of the blood islands has ceased. 



II. HISTORICAL. 



1. Vasa Serosa. 



Until the primary origin of any system is known our cenceptions of 

 it are necessarily hazy and vague and this is nowhere better illus- 

 trated than in connection with the lymphatic system. The views of 

 the eighteenth century may well be summed up in the hypothetical 

 "vasa serosa" of Boerhave (13), Haller (41) and others, which were 

 tiny channels too small to allow corpuscles to pass, supposed to connect 

 arteries, veins and lymphatics at their tips. The conception which 

 underlies vasa serosa may be traced back to the experiments of Nuck 

 (100), who injected air into the arteries and found it returning in 

 the lymphatics, as may be represented in the diagram of fig. 1. Vasa 

 serosa meant the idea that the arteries finally branched into vessels too 

 tiny to carry the corpuscles, but the term likewise represents a whole 

 series of vague conceptions, such as Bichat's (12) absorbents and ex- 

 halants, which sought to make definite some idea of the nature of 

 lymphatics. 



2. Lymphatics with Open Mouths. 



In the latter half of the eighteenth century the conception of definite 

 "vasa serosa" was modified through the work of William Hunter (50) 

 and Munro, who believed that lymphatics began with open mouths. 

 The views of these English observers, as shown by Cruikshank (23), 

 involved the idea that the mouths of the lymphatics opened directly 

 onto the surface of the body, into the cavity of the intestine and the 

 air sacs of the lungs, as well as into the connective tissues. The theory 

 was also involved with discussions of the lymphatics as the exclusive 



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