86 ALLEN 
17 and 18; O.N.V.).—Each of these veins has its origin 
directly behind the maxilla, and, following caudad along the 
ventral side of the corresponding artery, passes behind the 
nasal sac, where it receives 2 veins coming from the nasal sac. 
The smallest and most cephalic one is designated as the 
anterior nasal sac vein (P|. III, figs. 17 and 18; N.S.V.,,). In 
the specimen from which figs. 17 and 18 were drawn this 
vessel arose from 6 anterior radial veins (see fig. 17). Each of 
these radial veins runs along the outer or distal edges of the 
secondary filaments of one of the primary filaments, and from 
each of these secondary filaments there comes a branch, which 
receives the capillaries from the inner connective tissue layer 
of that secondary filament and from that portion of the primary 
or radial filament. These radial filament veins unite with one 
another at their bases and finally terminate in the anterior nasal 
sac vein, which empties into the main orbito-nasal trunk. In 
like manner the larger posterior nasal sac vein (P1. ILI, figs. 
17 and 18; N.S.V.,,) arises from 8 posterior radial veins, 
which take their origin from the secondary filament veins from 
their respective radial or primary filament. The 2 nasal sac 
veins are usually distinctly separated as shown in fig. 17, but 
in a few cases I have noticed that they were connected by a 
longitudinal vein, thus forming a continuous lateral vein into 
which all the radial veins were emptied, and from which the 2 
nasal sac veins had their source. Between these 2 nasal sac 
veins, the orbito-nasal vein anastomoses with a branch of the 
facialis-maxillaris vein (see fig. 18). After leaving the nasal 
sac, the orbito-nasal vein pursues a general caudal course, 
parallel with, but dorsad of, the orbito-nasal artery and mesad 
of the olfactory nerve, and enters the orbit through the olfactory 
foramen in the prefrontal. Once inside the orbit it leaves the 
olfactory nerve and the orbito-nasal artery to pursue, with the 
truncus supra-orbitalis or ramus ophthalmicus superficialis and 
profundus, a sort of dorso-caudal course through the orbit. 
Passing behind the superior oblique muscle it receives the 
inferior oblique vein (Pl. Il, fig. 13; Inf.O.V.), coming up 
from the outside of that muscle, and the swperzor oblique vein 
(fig. 13: Sup. O.V.), coming down from the inside of that 
