634 BLOOD-VASCULAR SYSTEM OF THE HORSE 
the trachea (right side), and the longus colli, and emerges from the thoracic cavity 
by passing through the space behind the first costo-transverse articulation.! In 
the thorax it gives off a small mediastinal branch (A. mediastini cranialis) to the 
mediastinum and the pericardium; also the first intercostal artery (A. intercostalis 
prima), a very small vessel which passes down in the first intercostal space. After 
leaving the thorax the artery passes upward and forward on the spinalis muscle 
and the lamellar part of the ligamentum nuchwe, covered by the complexus. Its 
terminal branches anastomose with branches of the occipital and vertebral arteries 
in the region of the axis. Numerous collateral branches are detached to the lateral 
muscles of the neck, the ligamentum nuche, and the skin, and anastomoses oecur 
with the dorsal artery also. 
3. The vertebral artery (A. vertebralis) arises from the brachial on the left 
side, the brachiocephalic on the right; it begins opposite the first intercostal space 
and passes upward and forward. On the left side it crosses the cesophagus, on the 
right, the trachea. Emerging from the thorax it passes between the longus colli 
medially and the scalenus laterally, under the transverse process of the seventh 
cervical vertebra, and continues along the neck through the series of foramina trans- 
versaria, between which it is covered by the intertransversales colli.” Emerging 
Branches of 
occipital artery 
Vertebral artery 
Fig. 555.—VertTesraL Artery oF Horse. (After Schmaltz, Atlas d. Anat. d. Pferdes.) 
from the foramen of the axis, it crosses the capsule of the atlanto-axial joint, and 
joins the recurrent branch of the occipital artery under cover of the obliquus 
capitis posterior. At each intervertebral foramen a spinal branch (Ramus spinalis) 
is given off which enters the vertebral canal and reinforces the ventral spinal artery. 
It also gives off series of dorsal and ventral muscular branches (Rami musculares). 
The dorsal branches are the larger; they supply the deep extensor muscles of the 
head and neck, and anastomose with the deep cervical and occipital arteries. The 
ventral branches supply chiefly the scalenus, longus colli, intertransversales, and 
rectus capitis ventralis major. The artery is accompanied by the vertebral vein 
and a sympathetic nerve trunk (Nervus transversarius). 
4. The internal thoracic artery (A. thoracica interna) is a large vessel which 
arises from the ventral side of the brachial opposite the first rib. It curves down- 
ward and backward, being at first on the medial surface of the rib, and then crosses 
the ventral part of the first intercostal space and passes under the transversus 
thoracis muscle. It runs backward under cover of that muscle over the chondro- 
sternal joints to the eighth costal cartilage, where it divides into musculo-phrenic¢ 
and anterior abdominal branches. At each intercostal space two collateral branches 
are detached. The intercostal branches (Rami intercostales) ascend in the inter- 
costal spaces and anastomose with homonymous descending arteries. The ventral 
1' The artery sometimes emerges through the second intercostal space. 
2 In some cases the last cervical transverse process has a foramen transversarium, through 
which the artery passes. 
