540 BLOOD-VASCULAR SYSTEM OF THE HORSE 



2. The superior or deep cervical artery (A. cervicalis profunda) arises in front 

 of the dorsal or by a coinniun trunk with it. It crosses the a'sophgaus (left side), 

 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 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 nuchse, covered by the complexus. Its terminal branches 

 anastomose with branches of the occipital and vertebral arteries in the region of the 

 axis. Numerous collateral ))ranches are detached to the extensor muscles of the 

 head and neck, the ligamentum nuchae, and the skin, and anastomoses occur with 

 the dorsal artery above and the vertebral artery l_ielow. 



The artery sometimes emerges through the second intercostal space. 



3. The vertebral artery (A. vertebralis) arises from the brachial (or brachio- 

 cephalic) ojjposite the first intercostal space and passes upward and forward. 

 On the left side it crosses the a?sophagus, on the right the trachea. Emerging from 

 the thorax it passes between the longus colli internally and the scalenus externally, 

 under the transverse process of the seventh cervical vertebra, and continues along 



Branches of 

 occipital iirterij _^ Vertebral artery 



Fig. 430. — Vertebr.\l Artery op Horse. (After Schmaltz, Atlas d. Anat. d. Pferdes.) 



the neck through the series of foramina transversaria, between which it is covered 

 by the intertransversales colli. Emerging from the foramen of the axis, it crosses 

 the capsule of the atlanto-axial joint, and joins the retrograde branch of the occi- 

 pital artery under cover of the great oblique muscle of the head. At each inter- 

 vertebral 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 supi)ly the deep extensor muscles of the head and neck, and 

 anastomose with the deep cervical and occipital arteries. The ventral liranches 

 supply chiefly the scalenus, longus colli, intertransversales, and rectus capitis 

 anterior major. The artery is accompanied by the vertebral vein and a sym- 

 pathetic nerve trunk. 



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

 which the artery passes. 



4. The internal thoracic artery (A. thoracica s. mammaria interna) is a large 

 vessel which arises from the ventral side of the brachial opposite the first ril). It 

 curves downward and backward, being at first on the inner surface of the rib, and 

 then crosses the lower part of the first intercostal space and passes under the trans- 

 versus thoracis muscle. It runs backward under cover of that muscle over the 

 chondro-sternal joints to the eighth costal cartilage, where it divides into asternal 

 and anterior abdominal branches. At each intercostal space two collateral 



