I . - ■''ill. 



sufficiently that the sac can be injected indirectly throu« 



11. 

 The process by which a lymphatic plexus becomes a sac has been 

 called confluence or cavernization by Ranvier (117-118). He says 

 iliai when two lymph vessels lie in contact the endothelial wall between 



11 disappears (possibh ii i- retracted) and thus large sacs develop. 

 It i- exceedingly interesting that this pi es no! take place when 



lymphatic endothelium rests on venous endothelium, as is the ca 

 "I the jugular lymph sac. 



The si interesting in the formation of the jugular sac is 



.mi 111 fig. ', from an embryo 18 nun. long. To injecl the sac al 

 this stage ii i- again important to note the plan of the blood capillai 



In fie. ii it will I"' noted that there is a tiny 1.1 1 capillary which cuts 



across the lymphatic area from the superficial plexus to the deeper 

 veins and divides the lymphatic area into two parts, a ventral jugular 

 part and a dorsal part. By the time the embryo is from 16-20 nun. 



ig the path of \h\- slender vessel is occupied by a considerable plexus 



.1 1 vessels and i.\ nerves as well. The sprouts from the main 



jugular sac (fig. '. i have grown up into the dorsal lymphatic area and 



there developed into an abundant plexus. It i- this plexus which can 



• injected, as is shown by the glass tube which pierces the arm bud. 



'I'h iginal sa< is shown just lateral to the shadow of the internal 



jugular vein. This is difficult to inject by direct puncture, because 

 the blood capillary plexus over it has become double and is excessively 

 abundant. Moreover the sac is now just mesial to the developing 

 externa] jugular vein and li.- very close to it. 'The blood capillary 

 plexus of the groove shown in fig. 6 is now deeper in. and the entire 

 area of the groove so dense a vascular area that no canula can enter the 



■ 



zone without filling the hlnml capillaries lint juM ventral tn the 

 groove in the dorsal lymphatic area almost every puncture will fill the 

 lymphatics, provided it avoids the superficial blood capillaries. This 

 dorsal lymphatic area is destined to be the posterior triangle of the 

 iie.k: even in this earlj stage it looks translucent and continues to do 

 «... for it is always uncovered by muscle, lying between the trapezius 

 and the sternocleidomastoid muscles (fig. 11 i. 



A description of the form of the sac in fig. 1 will make the basis 



i the des< ription of the peripheral lymphatics of the head, neck and 



thorax worked out by A. II. (lark (24) and given in section V. Phi 



primitive sac is the portion above the arm hud and lateral to the 



