The vertebral canal has been opened by sawing off the arches. The
nerve-roots are cut on one side and the spinal cord turned over to
right. _1_, Ventral or middle spinal artery; _2_, reinforcing
branches from vertebral, intercostal, or lumbar arteries (according
to region); _3_, longitudinal vertebral sinuses; _a_, ventral
surface of spinal cord; _b_, dura mater (cut); _c_, nerve-roots;
_d_, superior common ligament. (After Ellenberger, in Leisering’s
Atlas.)
]
The =cephalic vein= (V. cephalica) arises at the inner side of the
carpus as the continuation of the internal superficial metacarpal vein.
It runs upward on the deep fascia of the forearm at first in the furrow
between the flexor carpi internus and the radius. Toward the middle of
the forearm it inclines gradually forward on the inner surface of the
radius, accompanied by a cutaneous branch of the median nerve, and
arrives at the insertion of the biceps. Here it detaches a large branch
(Ramus communicans) which passes upward and backward over the inner
insertion of the biceps, the posterior radial artery, and the median
nerve, and joins the brachial vein. The vein to this point is often
termed the internal subcutaneous vein of the forearm (V. cephalica
antebrachii). It is continued (as the V. cephalica humeri) in the furrow
between the mastoido-humeralis and the anterior superficial pectoral
with a branch of the inferior cervical artery, crosses the deep face of
the cervical panniculus, and opens into the terminal part of the jugular
or the brachial vein. It receives an =accessory cephalic vein= (V.
cephalica accessoria), which arises from the carpal network, runs upward
along the inner border of the extensor carpi, turns along the lower
border of the brachialis, passes under the superficial tendon of the
biceps and joins the cephalic.
[Illustration:
FIG. 466.—DISSECTION OF PECTORAL REGION AND ANTERIOR PART OF ABDOMINAL
WALL OF HORSE.
_1_, Jugular vein; _2_, loose connective tissue of axillary space;
_3_, ascending branch of inferior cervical artery; _4_, descending
branch of same; _5_, cephalic vein; _6_, median nerve; _7_, ulnar
nerve; _8_, brachial artery; _9_, brachial vein; _10_, external
thoracic or “spur” vein; _11_, anterior abdominal artery and vein;
_12_, branches of sixth cervical nerve; _13_, cutaneous branch of
axillary nerve; _14_, cutaneous branch of musculo-cutaneous nerve;
_15_, cervical panniculus; _16_, sterno-cephalicus; _17_, scalenus;
_18_, mastoido-humeralis; _19_, anterior superficial pectoral; _20_,
posterior superficial pectoral; _21_, anterior deep pectoral; _22_,
posterior deep pectoral; _23_, abdominal panniculus; _24_, obliquus
abdominis externus; _25_, rectus abdominis; _26_, coraco-brachialis;
_27_, tensor fasciæ antibrachii; _a_, prescapular lymph glands; _b_,
prepectoral lymph glands; _c_, axillary lymph glands; _d_, cubital
lymph glands; _c.x._, xiphoid cartilage. (After Schmaltz, Atlas d.
Anat. d. Pferdes.)
]
The deep veins of the forearm are variable. Commonly two =posterior
radial veins= (Venæ medianoradiales) accompany the artery of like name,
one in front and one behind. A third vein arises by radicles emerging
from the proximal part of the deep flexor; it joins the posterior
satellite of the posterior radial artery or forms one of the roots of
the brachial vein. The =common interosseous vein= joins the posterior
satellite. The =anterior radial vein= (V. collateralis radialis) is a
satellite of the artery. The =ulnar vein= is usually double at its
proximal part, and communicates with the deep brachial vein.
There are three chief =metacarpal veins=. The =internal metacarpal vein=
(V. metacarpea volaris superficialis medialis) arises from the volar
venous arch above the fetlock. It is the largest vein of the region and
lies in front of the large metacarpal artery. It separates from the
artery at the proximal end of the metacarpus, passes upward on the inner
part of the posterior surface of the carpus, under cover of the
superficial layer of the posterior annular ligament, and is continued as
the cephalic vein. It communicates at its proximal part with the origin
of the posterior radial veins by a short but relatively large branch.
The =external metacarpal vein= (V. metacarpea volaris superficialis
lateralis) arises from the venous arch above the fetlock and passes
upward behind the external border of the suspensory ligament in front of
the external branch of the median nerve and accompanied by a small
artery. At the proximal end of the metacarpus it is connected with the
deep metacarpal vein by two transverse anastomoses which pass across the
suspensory ligament. It then passes upward as a satellite of the
external volar metacarpal artery and concurs in the origin of the ulnar
and posterior radial veins. The =deep metacarpal vein= (V. metacarpea
volaris profunda medialis) arises from the venous arch, passes forward
between the two branches of the suspensory ligament, and ascends on the
posterior surface of the large metacarpal bone. At the proximal end of
the latter it communicates with the other metacarpal veins, ascends with
the small metacarpal artery, and concurs in forming the radial veins.
The =venous arch= (Arcus venosus volaris), from which the metacarpal
veins arise, is situated above the sesamoids of the fetlock between the
suspensory ligament and the deep flexor tendon. It is formed by the
junction of the two digital veins.
The =digital veins=, internal and external (V. digitalis medialis,
lateralis), drain the venous plexuses of the foot. They arise at the
upper edge of the lateral cartilages and ascend in front of the
corresponding arteries.
It is convenient to recognize the following venous plexuses of the foot,
which, however, communicate very freely:
1. The =coronary plexus= encircles the upper part of the foot. It is
attached to the terminal part of the extensor tendon, the lateral
cartilages, and the bulbs of the plantar cushion.
2. The =dorsal= (or laminal) =plexus= covers the dorsal or wall surface
of the third phalanx in the deep layer of the matrix of the wall of the
hoof. It forms the circumflex vein of the third phalanx or vein of the
distal border of the third phalanx, which corresponds to the artery of
like name.
3. The =volar plexus= is in the deep layer of the matrix of the sole of
the hoof and on the deep surface of the lateral cartilages. It
communicates around the inferior border of the third phalanx with the
dorsal plexus and through the lateral cartilages with the coronary
plexus.
The deep vein of the third phalanx accompanies the terminal part of the
digital artery. It drains the =intraosseous plexus=.