VAS DEFERENS—VIREO 1013 
(p. 139, Fig. 1, Z.). Lymphatics and lacteals unite, and generally 
follow the course of the bigger arteries and veins, often surrounding 
them with anastomosing network. The lymph vessels of the tail 
and hinder parts of the body enter the hypogastric veins, and at 
the point of junction a small contractile “lymph-heart” is regularly 
developed in the embryo and persists in many forms until maturity. 
The lymphatic vessels of the trunk and intestines chiefly accompany 
the aorta descendens and the mesenteric and cceliac arteries, finally 
opening into the two superior ven cave, as also do the branches of 
the lymphatic stems that come down from the head and neck, accom- 
panying the jugular veins into which they partly enter. 
VAS DEFERENS, see REPRODUCTIVE ORGANS (p. 784 and fig. 
p. 782). 
VEERY, a name in North America (Nuttall, Man. Orn. 1. 
p. 349) for what is otherwise known as Wilson’s THRusH, Z'urdus 
Suscescens. 
VEIN (adj. venous). Veins are the vessels through which 
the blood flows into the HEART, no matter if this blood be venous, 
or arterial like that which returns purified from the LUNGS through 
the pulmonary veins. The walls of the veins are thinner than 
those of the ARTERIES, and, especially toward the extremities, 
contain numerous valves to hinder the reflux of the blood. 
Similar veins guard the entrance to the heart, but there are none in 
the jugular, trunk and cutaneous veins (see p. 1008). 
VERTEBRAL, see SKELETON (p. 848). 
VIREO, the name of a genus proposed by Vieillot in 1807, and 
long since used as English, for some North-American birds, sometimes 
called GREENLETS. With some allied genera they seem to form 
a small but recognizable Family, very character- 
istic of the “Columbian” Fauna. They are 
mostly inconspicuous in their olive-green plumage, 
but lke the Alaudidx (LARK) are instructive to 
the taxonomer, teaching him not to depend on ESOS 
the number of primary quills, and also shewing Sere ga 
a considerable amount of differentiation of form within certain 
limits, though some of the species are not easily distinguished. 
By most systematists they are supposed to be allied to the Laniidx 
(SHRIKE), and by some are even included in that Family, but 
on grounds that are at least debatable. Baird in 1866 (Rev. Am. 
B. pp. 321-400) insisted forcibly on the distinctness of the Vireo- 
nidx, to which he assigned 7 genera and some subgenera, being 
therein followed in 1873 by Messrs. Sclater and Salvin (Nomencl. 
Av. pp. 11-13); but Sundevall in 1872 (Tentamen, p. 13) had 
