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 nuchae, 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 nuchse, and the skin, and anastomoses occur 

 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 oesophagus, 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. — Vertebral 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 



' The artery sometimes emerges through the second intercostal space. 



- In some cases the last cervical transverse process has a foramen transversarium, through 

 which the artery passes. 



