LYMPHATIC SYSTEM OF THE HORSE—THE THORACIC DUCT 697 
large vein in the proximal part of the leg, two lower down. They arise from a 
number of anastomosing radicles on the front of the capsule of the hock joint, 
chiefly as the continuation of the perforating tarsal vein. A large branch from the 
saphenous vein concurs in the formation of the somewhat plexiform arrangement 
on the flexion surface of the hock. 
The posterior tibial vein (V. tibialis posterior) is commonly double (Fig. 583). 
It arises at the level of the tuber calcis, where it has a communication with the 
recurrent tibial vein. It is a satellite of the artery. 
The medial or great metatarsal vein (VY. metatarsea dorsalis medialis (Figs. 
583, 585) arises from the venous arch above the sesamoids at the fetlock, but is 
practically the upward continuation of the medial digital vein. It ascends along 
the medial border of the suspensory ligament, then in the groove on the proximal 
part of the large metatarsal bone to the capsule of the hock joint, and is continued 
by the saphenous vein. The small middle dorsal metatarsal vein (V. met. dorsalis 
media) joins this vein or the saphenous at the hock. 
The lateral metatarsal vein (V. metatarsea dorsalis lateralis) is the very small 
satellite of the great metatarsal artery. It is nconstant, but may be double in the 
proximal part of the metatarsus. 
The medial plantar vein (V. metatarsea plantaris superficialis medialis) is very 
small, and is not always present. It ascends along the medial border of the deep 
flexor tendon in front of the corresponding nerve. It is connected below with the 
medial digital vein, and above with the deep metatarsal or the lateral plantar vein. 
The lateral plantar vein (V. metatarsea plantaris superficialis lateralis) arises 
from the venous arch above the fetlock, but appears to be the upward continuation 
of the lateral digital vein. It ascends along the lateral border of the deep flexor 
tendon in front of the plantar nerve, and is connected with the deep metatarsal 
vein at the proximal part of the metatarsus by a transverse branch. It then passes 
upward along the deep flexor tendon in relation to the plantar nerves and the 
medial tarsal artery, and is continued by the recurrent tibial vein. 
The deep metatarsal vein (V. metatarsea plantaris profunda medialis) arises 
from the plantar venous arch, above the fetlock, passes forward between the branches 
of the suspensory ligament, and ascends on the posterior face of the large meta- 
tarsal bone. At the proximal end of the metatarsus it is connected with the lateral 
plantar vein by a transverse branch, thus forming the deep plantar arch (Arcus 
plantaris profundus). It then passes, as the perforating tarsal vein (V. tarsea 
perforans), through the vascular canal of the tarsus and forms the chief radicle of 
the anterior tibial vein. 
The distal plantar venous arch (Arcus venosus plantaris distalis) and the digital 
veins are arranged like those of the thoracic limb. 
LYMPHATIC SYSTEM OF THE HORSE 
THE THORACIC DUCT (Figs. 553, 554) 
The thoracic duct (Ductus thoracicus) is the chief collecting trunk of the lym- 
phatic system. It begins as an elongated irregular dilatation, the cisterna chyli, 
which is situated between the right side of the aorta and the right crus of the 
diaphragm at the first and second lumbar vertebre. The duct enters the 
thorax through the hiatus aorticus and runs forward on the right of the median 
plane, between the vena azygos and the aorta, covered by the pleura. At the sixth 
or seventh thoracic vertebra it inclines somewhat ventrally, crosses obliquely over 
the left face of the cesophagus, and passes forward on the left side of the trachea 
to the inlet of the thorax. The extrathoracie terminal part passes downward and 
