432 The Submaxillary Gland 
sympathetic ganglia that are embedded in the fibrous tissue forming 
the main interspaces. Even at this period there is no connection between 
the circulation in one lobule and that of its neighbor, save occasionally 
at the lobular hilus where lobules of the first order have been incom- 
pletely separated by membranae limitantes into ultimate lobules. Fig. 7 
shows an incomplete silver injection of the circulation at this stage. The 
arteries are seen as definite round tubes while the veins in the photo- 
graph appear collapsed. Owing to the lack of depth, many of the finer 
relationships of the blood-vessels are lost. The alveolar plexuses are 
clearly shown, together with the beginning of the demarcation of the 
ultimate lobules by the branches of the lobular arteries. 
In a pig 22 centimeters long the arrangement of the blood-vessels 
has not been altered in any marked degree. Each of the six divisions 
of the branches of the arteria submaxillaris has been formed. The 
growth subsequent to this stage, therefore, lies more in the differentiation 
and complexity of the lobule rather than in the general plan of the 
organ, a fact which is confirmed by the study of stained specimens. 'The 
plexus about the main submaxillary duct as it enters the gland becomes 
more complex. Branches derived from the main artery anastomose and 
ramify a short distance, and then break up into a dense capillary plexus 
formed by irregular polygonal spaces which entirely embrace the duct, 
lying just external to the basement membrane. Venous radicals formed 
in this plexus unite and flow into larger units which form an irregular 
venous plexus lying on top of the capillary plexus. Emissary veins from 
this plexus empty into the venae comites. The double plexus can now 
be followed as far as the interlobular ducts, where the two layers of ves- 
sels are replaced by one. In the development of the circulation the 
growth of the vessels like the growth of the ducts has been entirely 
centrifugal.” If one takes a thick tangential section of the gland and 
views it with the stereoscopic microscope from above, the outlines of 
the lobules are clearly shown and the branching and radiating vessels 
are always found in the center looking much like the branches of a small 
tree viewed from above. At birth the relations of the last stage remain 
5In another communication, Archiv fiir Anat. u. Phys. Anat. Abth., 1903, the 
writer has called attention to the centrifugal growth of the ducts of the submaxillary 
with reference to the formation of the lobes and lobules of the first and second 
orders. These structures always occupy the center of the lobes and the center of 
the lobules, from which point they radiate out towards the capsule or the limiting 
membranes. The entire arrangement of the frame-work of the submaxillary depends, 
in fact, upon physical factors. involved in successive action of this principle of 
centrifugal growth upon the fibrils that compose the supporting tissue of the organ. 
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— 92 
