26 
HEMOLYMPH NODES OF THE SHEEP 
the arterial side. It is usually just as easy to distinguish an injected 
hemal from a lymphatic node injected from the vascular system; unless, 
perhaps, some difficulty might be experienced in case of a lymph node in 
which extravasation of the injection mass into the parenchyma or lym- 
phatics had occurred. For, aside from the fact that small lymph nodes 
readily manifest color changes, the injection mass as a rule entirely fills 
the arteries or veins, is not at all or but slightly mixed with blood, is 
confined in vessels of regular calibre with closed walls, and consequently 
has not those irregular outlines characteristic of injections in hemal 
nodes. Moreover, vascular injections of the lymphatic nodes always 
show a very characteristic arborization, (fig. 9), the distribution of the 
injection mass throughout the specimen is quite uniform, and the capsule 
is rarely also surrounded by a network of fine injected capillaries, a ver- 
itable retia mirabilia. It should be mentioned in this connection, how- 
ever, that Weidenreich [37] stated that the vein, and especially the artery, 
frequently branches as it approaches the hilus of hemolymph nodes, send- 
ing branches to the capsule and thus giving the whole a very artistic ap- 
pearance, which may cause confusion in embryonic nodes. 
These differences in the behavior of hemal and lymphatic nodes upon 
injection can easily be accounted for, it seems to me: for in the lymph 
nodes the injection mass merely has to penetrate permanent channels of 
gradually changing calibre, lined throughout by endothelium and sur- 
rounded by tissue, which can easily yield , because the lymph sinuses, 
which form a comparatively large portion of the total volume of the node, 
are easily compressible. In hemal nodes, on the contrary, the injection 
mass, instead of traversing a closed permanent system, must apparently 
traverse very irregular, often collapsed, and no doubt to a certain extent 
discontinuous because evanescent, channels of greatly varying calibre, 
which often have but minute inter-communications, and which also com- 
municate indirectly with the blood spaces of the node. It is obvious, to 
be sure, that improvements in technique may invalidate some of these 
statements , but they are in harmony with results obtained with all solu- 
tions, suspensions and masses used in the present series of experiments. 
Although the artery and vein generally enter the node together, in 
external form the hemal nodes of the sheep are usually oval or spheroid- 
al, and, as previously stated, without a definite hilus. It is true that the 
larger nodes generally have a more definite hilus , but accessory veins 
usually do not have accessory hiluses. After piercing the capsule, the 
artery and vein generally part company at once, and only exceptionally 
traverse the substance of the node as contiguous structures. (Fig. 10.) 
