86 ALLEX 



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 (PL 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 primar}^ 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 (PL III, figs. 

 17 and 18; N.S.V.(2)) arises from 8 posterior radial veins, 

 which take their origin from the secondary filament veins from 

 their respective radial or primar}^ 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 arter}' 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 II, fig. 13 ; Inf.O.V.), coming up 

 from the outside of that muscle, and the superior oblique vein 

 (fig. 13 : Sup. O.V.), coming down from the inside of that 



