54 HENRY MCELDERRY KNOWER 



account of lack of a full series of stages and because the con- 

 nections of the primary sac with the main duct are generally 

 partly hidden by the distention of the central sinus. But the 

 main duct (Cn.D.) persists in the deeper layer, though hidden 

 by the overlying sinus and the slender tubular outlet of the 

 triangular mandibular sac may be traced to it when the speci- 

 men is turned to a favorable position or partly dissected 

 (fig. 21a). Examined in this way, definite views are obtained 

 of the mandibular lymph sac and its outlet through the main 

 collecting duct of the system, with its relations to the deeper 

 submaxillary veins. The smaller tube is shown not to con- 

 nect with the central sinus as appears in figures 21 and 22, its 

 connection with the main collecting duct lying deep, immedi- 

 ately over (superficial to) the submaxillary veins, in the 

 original position of the distal projection of the early sub- 

 maxillary duct from which it appears to arise. 



These deep relations are well shown in figure 21a, from 

 which the central sinus has been dissected off on the left side, 

 L (to right of observer), to expose the connections of the 

 ducts and the underlying pericardial sinus; and except for 

 injury to the left circum-oral tube, the deep relations of the 

 lymphatics are intact. 



The definiteness of these observations seems to offset the 

 lack of a more complete series of stages and justify the con- 

 clusion that the simple branched duct first encountered (fig. 

 20) represents the fundamental (primary) form of the sub- 

 maxillary lymph system ; and that sinuses are later secondary 

 modifications of its features forming a distended system re- 

 tained through later stages. 



The remaining units of the submaxillary sinuses, the peri- 

 cardial sacs, are conspicuous in late stages but data as to their 

 early history are few in these injections. In some specimens 

 the injection flows back through the central sinus to fill a sac 

 (the pericardial sinus) extending back from the posterior 

 wall. This result is shown in figure 19, and on the right, R, 

 of figure 21. Such injections are, however, probably artifacts, 

 not normal, but due to over-pressure from the front, causing 

 breaks in the dividing walls between the sinuses. 



