31- Florence E. Sabin. 



sufficiently large so that the sac can be injected indirectly through 

 them. 



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

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

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

 them disappears (possibly it is retracted) and thus large sacs develop. 

 It is exceedingly interesting that this process does not take place when 

 lymphatic endothelium rests on venous endothelium, as is the case 

 of the jugular lymph sac. 



The most interesting stage in the formation of the jugular sac is 

 shown in fig. 7 from an embryo 18 mm. long. To inject the sac at 

 this stage it is again important to note the plan of the blood capillaries 

 In fig. 6 it will be noted that there is a tiny blood 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 mm. 

 long the path of this slender vessel is occupied by a considerable plexus 

 of blood vessels and by nerves as well. The sprouts from the main 

 jugular sac (fig. 7) have grown up into the dorsal lymphatic area and 

 there developed into an abundant plexus. It is this plexus which can 

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

 The original sac 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 

 external jugular vein and lies 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 blood capillaries. But just ventral to 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 

 neck; even in this early stage it looks translucent and continues to do 

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

 and the sternocleidomastoid muscles (fig. 17). 



A description of the form of the sac in fig. 7 will make the basis 

 for the description of the peripheral lymphatics of the head, neck and 

 thorax worked out by A. H. Clark (24) and given in section V. The 

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



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