THE LYMPHATIC SYSTEM 599 



The popliteal vein (\'. poplitea) lies along the inner side of. the artery (Fig. 

 459). It is formed by the confluence of anterior and posterior tibial veins. 



Two anterior tibial veins {Yv. til^iales ant(n-iores) usually accompany the artery 

 of like name; the outer vein is much the larger. In other cases there is a single 

 large vein in the proximal part of the leg, two hjw(n- 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. The origin of the chief 

 vein is conn(>ct(Ml with the sajihenous l)y a large anastomotic branch. 



The posterior tibial vein {\. til)ialis posterior) is commonly douljle (Fig. 458). 

 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 internal or great metatarsal vein {V. metatarsea dorsalis medialis) (Figs. 

 458, 460) arises from the venous arch above the sesamoids at the fetlock, but is 

 practically the upward continuation of the internal digital vein. It ascends along 

 the inner border of the deep flexor tendon, then in the groove on the inner aspect 

 of the proximal part of the large metatarsal bone to the capsule of the hock joint, 

 and is continued by the saphenous vein. 



The external metatarsal vein (^^ metatarsea plantaris lateralis) arises from 

 the venous arch above the fetlock, but appears to be the upward continuation of 

 the external digital vein. It ascends along the outer 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 b.y a transverse branch. It then passes 

 upward along the deep flexor tendon in relation to the plantar nerves and the inner 

 tarsal artery and is continued by the recurrent tibial vein. 



The deep metatarsal vein (V. metatarsea plantaris medialis) arises from the 

 plantar venous arch, passes forward between the branches of the suspensory liga- 

 ment, and ascends on the posterior face of the large metatarsal lione. At the proxi- 

 mal end of the metatarsus it is connected with the external metatarsal vein by a 

 transverse branch. It then passes (as the perforating tarsal vein) through the 

 vascular canal of the tarsus and forms the chief radicle of the anterior tibial vein. 



The plantar venous arch (Arcus venosus plantaris) and the digital veins are 

 arranged like those of the thoracic limb. 



The Lymphatic System 



The lymphatic system (Systema lymphaticum) is subsidiary to the venous 

 part of the circulatory system, from which it arises in the embryo. It consists of 

 the lymph vessels and glands. 



The lymph vessels (\'asa lymphatica) contain a colorless fluid, the lymph, 

 which contains numerous Ij'mphocytes.' They resemble the veins in structure 

 but have thinner walls and are provided with more numerous valves. The vessels 

 are sacculated opposite the segments of the valves and have a characteristic beaded 

 appearance when distended. The collecting lymph vessels do not usually form rich 

 plexuses, as veins often do, their branching is more limited and less tree-like than 

 that of the blood-vessels, and their caliber therefore increases less from the periphery 

 toward their termination. All of the lymph is ultimately carried into the venous 

 system by two trunks, the thoracic duct and the right lymphatic duct. Almost all 

 of the lymph passes through at least one group of lym]>h glands before entering the 

 blood-vascular system. 



The lymph glands or nodes (L>'mphoglandula) are intercalated in the course 

 of the lymph' vessels. They vary widely in size, some being microscopic, others 



' The term chyle is often applied to the lymph carried I)y the efferent vessels of the intestine 

 when it contains products of digestion, and these vessels may be designated as lacteals or chyle 

 vessels. 



