426 The Submaxillary Gland 
that terminate in the venae principales. In ducts of the interlobular 
order there is the same arrangement of the blood-vessels around the 
ducts, except that the elements of the venous plexus are not so large 
nor is the connection between them so frequent. As one progresses up- 
ward from the main ductus submaxillaris to the lobular ducts, there is 
a gradual simplifying of the duct circulation, until, in the ducts of the 
sublobular order, we have afferent arteries derived from the Aa. sub- 
lobulares running part way around the duct, breaking up into capillaries 
which reunite in small anastomosing veins, that pass around the walls 
of the ducts to terminate in the venae comites. Alternations of artery 
and veins in these instances are not uncommon and arterioles and veins 
are in all cases above the capillary plexus. Fig. 3, which shows the 
circulation in a duct of this order, gives the relations beautifully. It 
should be noted, however, that one of the venae comites has been re- 
DARLIN Ue nee ERE 
A RON tang mance 
Fia. 3. Blood-vessels about a sublobular duct. The artery is the upper of the 
two vessels. One of the venae comites has been left out for the sake of clearness. 
moved for the sake of clearness. The lobular and intralobular ducts 
show a still simpler form of duct circulation. Small arterioles almost 
immediately break up into capillaries when they are derived from 
the lobular or intralobular arteries. These quickly reunite again into 
veins that terminate into venules which empty directly into intra- 
lobular veins. This gradual simplification of the circulation about 
the ducts gives a characteristic demonstration of their age, and, in the 
angiogenesis of the submaxillary circulation, parallels can be drawn 
between the circulation about the ducts at different periods of embryonic 
life and the conditions found in the vascular supply of ducts of the differ- 
ent orders. In the very earliest stages the circulation of the main ductus 
submaxillaris is much like that about the adult lobular duct. At a some- 
what later stage is approximates that of the duct of the sublobular order, 
while still later there is a similarity to that of the interlobular system, 
and finally the highest development is attained with the complete forma- 
tion of the arterial and venous plexuses which embrace the main ductus 
