THORAX AND ABDOMEN OF THE HORSE 41 



When well developed, the common brachio-cephalic trunk may be 

 as much as 6-7 cm. in length. Leaving the aortic arch within the 

 pericardium, it runs in an oblique cranio-dorsal direction, slightly to the 

 left of the median plane of the body, ventral to the trachea and dorsal 

 to the cranial vena cava. Opposite the second intercostal space or the 

 third rib the trunk ends by dividing into two vessels of unequal size, 

 the left subclavian and brachio-cephalic arteries. 



A. SUBCLAVIA SINISTRA. — The left subclavian ^ artery is the smaller 

 of the two terminal branches of the common brachio-cephalic trunk. It 

 passes to the thoracic inlet in a curved course the convexity of which is 

 dorsal, and, at the border of the first rib, becomes the axillary artery. 

 To the right of the subclavian artery lie the trachea, the oesophagus 

 and the thoracic duct ; while its left face is crossed by the common 

 venous trunk formed by the union of the vertebral, deep cervical and 

 costo-cervical veins. The concavity of the arch formed by the sub- 

 clavian artery is related to the left phrenic, cardiac and vagus nerves. 



The branches of the left subclavian are: (1) The costo-cervical 

 artery ; (2) the deep cervical artery ; (3) the vertebral artery ; (4) the 

 internal thoracic artery ; (5) the omo-cervical trunk ; and (6) the 

 external thoracic artery. 



(1) A. costocervicalis. — The costo-cervical- artery is a short vessel 

 that crosses the left face of the trachea, oesophagus and longus colli 

 muscle, with a slight degree of obliquity, to reach the vertebral end 

 of the second intercostal space. Here it divides into the transverse 

 artery of the neck and the supreme intercostal artery. 



(a) The transverse artery of the neck {A. transversa, colli) at once 

 leaves the chest by passing between the second and third ribs, and 

 traverses the surface of the ilio-costalis and longissimus dorsi 

 muscles. Its branches supply the ventral serratus, longissimus cervicis, 

 spinalis dorsi et cervicis, multifidus, rhomboid and trapezius muscles. 

 One branch runs in a dorsal and cranial direction underneath the 

 splenius muscle and anastomoses with branches of the deep cervical 

 artery. 



(b) The supreme intercostal artery (A. intercostalis supremo) 

 travels in a caudal direction in the groove between the bodies of the 

 thoracic vertebrae and the longus colli muscle in company with the 

 sympathetic trunk. It furnishes the second, third and fourth (some- 

 times also the fifth) intercostal arteries. 



(2) A. cervicalis profunda. — The deep cervical artery leaves the 

 convex side of the curve of the left subclavian close to the origin of the 



' Sub [L.], under, near. Clavicula [L.], dim. of cJavus, a nail, or elavis, a key. 

 ^ Costa [L.], a rib. Cervicalis [L.], pertaining to the neck (cervix). 



