378 Venous System of Didelphys Marsupialis 
follows the course of the ascending cervical artery and opens into the 
dorsal surface of the common jugular near its union with the sub- 
clavian vein. 
The internal jugular vein (V. j. i.) presents no unusual conditions 
except that it is an exceedingly small vessel as compared with the size 
of the external jugular (V. j. e.).’ 
The external jugular vein (V.j.e.) begins near the angle of the 
iower jaw and is formed, on each side, through the union of three 
veins: The V. maxillaris externa (V. m. e.), the V. maxillaris interna 
(V. m. i.) and a vein which may be designated as the V. submaxillaris 
(V. s. max.). The external and internal maxillary veins often unite 
to form a V. maxillaris communis before joining the submaxillary 
vein. 
The external jugular vein (V.j.e.), along most of its course, lies 
quite superficially and instead of passing into the thoracic cavity on 
the dorsal side of the clavicle, as is usually ‘the case in mammals, it 
forms about it, on each side, a complete venous ring so that one part 
of the external jugular les ventral (v.) and another dorsal (do.) to the 
clavicle (cl.).” Hochstetter, 96, has described the presence of a some- 
what similar venous ring in Ornithorhynchus and states that the por- 
tion of the ring which lies dorsal to the clavicle is much larger than 
the ventral and forms the main trunk of the external jugular. In 
Didelphys there is not much difference in size between the dorsal and 
ventral portions of the ring, as, in the majority of cases observed, they 
were found to be subequal in size. 
The formation of such a venous ring about the clavicle, so far as 
the writer knows, has not been hitherto described for Didelphys nor 
for any marsupial. In a specimen of Petrogale recently examined by 
ihe writer no indication of such an annulus was present, so that in all 
probability this feature is not of common occurrence among the mar- 
supials. 
Of the veins opening into each external jugular the following are the 
most important: Beginning craniad, (1) one or two veins which return 
blood from the postauricular region (V. a. p.); (2) the V. transversa 
7 This is also the case in Petrogale (Beddard, g5, and the writer). 
8 As a matter of fact two venous rings are formed about the clavicle in Didelphys ; 
the one mentioned above, which is formed exclusively by the external jugular vein 
and another (X.), much larger, which is formed through a confluence of the cephalic 
vein (V.cep.) with the axillary (V.ax.) and external jugular (V.j.e.) veins. The last 
mentioned or outer ring (X.) isnot uncommonin mammals. Itis a prominent feature 
in the three-toed sloth (Bradypus tridactylus) and is sometimes met with in man (Nuhn). 
