The Origin and Development of the Lymphatic System. 41 



Before the thoracic and renal segments have met, however, it is most 

 difficult to inject them. 



The early jugular trunk I have never injected directly and only 

 once indirectly from the jugular sac. It will be readily seen that ink 

 injected into the jugular sac usually takes the line of least resistance, 

 the physiological path into the veins. Once in an embrvo 23 mm. 



Pig. 12. — Section through the point of origin of the jugular part of the 

 thoracic duct in an embryo pig 23 mm. long. (Specimen 23a.) Magnified 

 40 times. The left jugular lymph sac and the beginning thoracic duct 

 emptying into it were injected in the specimens; in the drawing the veins 

 are arbitrarily shown as injected and the lymphatics as empty. The thick- 

 ness of the endothelial lining of the lymphatics has been exaggerated. A. = 

 aorta; D. t. = ductus thoracicus; E. = esophagus; N. s. = nervous sym- 

 pathetica; N. v. = nervus vagus; J. 1. s. = jugular lymph sac; V. j. i. = 

 vena jugularis interna. 



long I filled the sac, and on pressing the head forward the ink shot 

 into the thoracic duct. The pressure obviously could not be regulated. 

 In 1911 I reported this specimen (23a) at the meeting of the Ameri- 

 can Association of Anatomists in connection with an unfinished study 

 of the thoracic duct. Dr. McClure requested the specimen for Mr. 

 Kampmeier, who was also studying the thoracic duct in the pig at 



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