The =vena hemiazygos= usually takes the place of the vena azygos. It
lies along the left side of the aorta and the bodies of the thoracic
vertebræ, turns down across the left face of the aorta and left
pulmonary artery, runs back over the left auricle and opens into the
great coronary vein or the right atrium. It receives the intercostal
veins.
Two =jugular veins= occur on either side. The =internal jugular vein=
(V. jugularis interna) is a relatively small vessel which accompanies
the carotid artery. It arises by occipital, laryngeal, and thyroid
radicles, receives tracheal, œsophageal, and muscular branches, and
joins the external jugular near its termination. It is sometimes absent,
but in some cases it appears, on the other hand, to be large enough to
interfere with venesection practised on the external jugular. The
=external jugular vein= (V. jugularis externa) is very large and
corresponds to the single jugular of the horse. It is separated from the
carotid artery in the greater part of its course by the
sterno-cephalicus and omo-hyoideus muscles.
The =inferior cerebral vein= usually does not unite with the occipital;
the latter is continued by the internal jugular vein.
The =orbital veins= form a network between the periorbita and the
muscles of the eyeball. This plexus communicates with the cavernous
sinus and with the superior cerebral vein. It is also drained by the
=frontal vein= which runs in the supraorbital canal and groove and joins
the angular vein of the eye.
The =dorsal nasal vein= is usually double.
The =superior labial vein= usually joins the infraorbital.
The =vena reflexa= is absent and the radicles which are received by it
in the horse go to the internal maxillary vein.
The =sublingual vein= is very large.
The =veins of the thoracic limb= differ chiefly in the distal part; the
special features are as follows:
The =dorsal digital veins= ascend on the front of the digits and are
connected with the other digital veins by transverse branches. They
unite near the fetlock to form the =dorsal metacarpal vein=. This runs
upward on the anterior face of the metacarpus and carpus, inclines to
the inner surface of the radius, and joins the accessory cephalic or the
cephalic vein.
The =volar digital veins= are larger than the dorsal. They lie on the
interdigital surfaces of the digits and unite in the interdigital space
to form a trunk which is a satellite of the volar common digital artery.
The =internal= and =external digital veins= lie in front of the
corresponding arteries. They are connected with the volar digital vein
by a large branch which passes between the flexor tendons and the first
phalanx. At the distal end of the metacarpus each inclines forward and
anastomoses with the volar common digital vein to form the volar venous
arch. The inner vein is continued as the internal volar metacarpal vein
along the inner border of the suspensory ligament, and becomes a
satellite of the radial artery in the forearm, while the outer one is
continued on the posterior face of the metacarpal bone by two irregular
veins, the external and middle volar metacarpals. The latter anastomose
freely with each other and with the inner vein. They unite below the
carpus or join the volar common digital vein.
The =accessory cephalic vein= is the upward continuation of the dorsal
metacarpal vein and is much larger than in the horse.
The =posterior vena cava= is partially embedded in the medial border of
the liver. Its abdominal part has a thicker wall than in the horse. Its
affluents correspond to the arteries of which they are satellites. The
=renal veins= are large and thick-walled; they run obliquely forward and
join the vena cava at an acute angle. The left one is much the longer.
Two =middle sacral= veins usually accompany the artery.
The =veins of the mammary glands= deserve special notice. They converge
to a venous circle at the base of the udder, which is drained chiefly by
two pairs of veins. The =subcutaneous abdominal vein= (anterior mammary
or “milk” vein) is very large in animals of the dairy breeds and its
course along the ventral wall of the abdomen is easily followed. It is
usually flexuous. It emerges at the anterior border of the udder about
two or three inches (ca. 5 to 8 cm.) from the linea alba, runs forward
(deviating a little outward), dips under the panniculus, passes through
a foramen in the abdominal wall about a handbreadth from the median
plane, and joins the internal thoracic vein. The =external pudic vein=
(middle mammary vein) is also of considerable size. It ascends in the
inguinal canal as a satellite of the artery and joins the external iliac
vein. The right and left veins are connected at the posterior border of
the base of the udder by a large transverse branch. From the latter
arises the =perineal vein= (posterior mammary vein), which runs medially
upward and backward to the perineum, turns around the ischial arch, and
joins the internal pudic vein. In the male these veins are relatively
small.
The deep veins of the thigh and leg resemble those of the horse, but
there is no recurrent tibial vein.
The =saphenous vein= is much smaller than in the horse.
The =recurrent tarsal vein= (external saphenous) is large. It is the
upward continuation of the external plantar metatarsal vein and
anastomoses with the anterior tibial and saphenous veins. It arises on
the outer face of the hock, ascends at first in front of the tendo
Achillis, then crosses the latter externally, passes up between the
biceps femoris and semitendinosus, and joins the posterior femoral vein.
There are three chief metatarsal veins. The great =dorsal metatarsal
vein= arises at the distal part of the metatarsus by the union of the
dorsal digital vein and a large branch from the venous arch above the
sesamoids. It ascends superficially between the long and lateral
extensor tendons and becomes the chief radicle of the anterior tibial
vein. The =internal plantar metatarsal vein= arises from the venous arch
above the fetlock, ascends along the inner border of the suspensory
ligament, passes through the vascular canal of the metatarsus and
centro-tarsal (as the perforating tarsal) and joins the anterior tibial
vein. The =external plantar metatarsal vein= is larger than the
preceding. It passes superficially over the outer face of the hock and
is continued by the recurrent tarsal vein.
The =digital veins= differ from those of the fore limb chiefly in that
the dorsal vein is large and the plantar absent or small.
The =portal vein= is formed usually by the confluence of two radicles,
gastric and mesenteric. It receives the right gastro-epiploic vein and
veins of the pancreas. The gastric vein is the largest affluent. It is
formed by the junction of three veins, the left of which receives the
splenic vein. The anterior mesenteric vein is also formed by the
confluence of three radicles; it usually receives the small posterior
mesenteric vein. The portal tributaries are in general satellites of the
corresponding arteries.