360 HISTORY OF THE HUMAN BODY 



ing development, but are here transitory structures and do 

 not survive embryonic life. The division of the subvertebral 

 space into two lateral thoracic ducts is inaugurated in croco- 

 diles and turtles, and becomes definite in birds ; in these the 

 chyle from the intestines is collected and emptied into the 

 veins at the juncture of jugular and subclavian. There are 

 also in all Sauropsida posterior connections with the venous 

 system, but only in reptiles do pulsating hearts persist at these 

 points. 



The above phylogenetic history of the lymphatic system is 

 well recapitulated during the embryonic development of mam- 

 mals, and the adult condition is best understood by tracing 

 the steps in this development (Fig. 104). As in the case of 

 the circulatory system, the preservation of so many phyloge- 

 netic steps in this developmental history is doubtless due to 

 the continual functional activity of this system from an early 

 embryonic period, its usefulness at all stages preventing the 

 customary degeneration of transitory structures.* The lym- 

 phatic system first appears in the form of a pair of tiny diver- 

 ticula which bud out from the venous system at the angle 

 formed by the meeting of the jugular and subclavian veins. 

 From these develops an anterior pair of lymph-hearts, in lo- 

 cation similar to those of lower forms, but without muscular 

 walls; and from these chambers as centers, definite lymphatic 

 vessels begin to develop, growing from their free ends and 

 gradually invading the surrounding tissues. At a slightly 

 later period a pair of posterior hearts appears, and from these 

 in the same way there grow out branching lymphatics. 



It is at this period that the thoracic ducts make their ap- 

 pearance, starting from the vessels connecting the lymph- 

 hearts with the veins and growing posteriorly, following the 

 aorta. There are two of these, which at first are about equal 

 in size, but soon the left one gains the superiority, and branches 

 to supply each side, while the right one remains small. The 



* An embryo in which the lymphatic system is not in full functional 

 activity becomes oedematous, and in extreme cases the result is a spheri- 

 cal ball, without indication of the normal shape. 



