536 



BLOOD-VASCULAR SYSTEM OF THE HORSE 



aorticus and enters the abdominal cavity, where it lies below the vertebral bodies 

 and the psoas minor, just to the left of the median plane. It divides under the 

 fifth lumbar vertebra into the two internal iliac or hypogastric arteries. 



From the bifurcation a small vessel, the middle sacral artery (A. sacralis media), sometimes 

 passes backward on the pelvic surface of the sacrum. It becomes lost in the periosteum or joins 

 the coccygeal artery, or in exceptional cases is traceal)le to the sphincter ani externus. 



The calil)er of the aorta is greatest at its origin, which is termed the bulbus 

 aortae. Here it forms three pouch-like dilatations, the sinuses of the aorta (or of 

 Valsalva). These correspond to the cusps of the aortic valve, and the coronary 

 arteries arise from the left posterior and anterior sinuses. At the arch the 



Right 

 Pulmonnrii reins pulmonary 

 artery 



Vena azygos 



Great coronary vein 



Small coronary vein 



Bigiit coronary artery 



Fig. 420. — Cardiac Vessels or Horse, Right Side. 

 Veins are Ijlack, arteries white. 



diameter is about two inches (ca. 5 cm.), and beyond this it diminishes gradually 

 in width. 



It is convenient to divide the aorta into thoracic and abdominal parts. The 

 thoracic aorta (Aorta thoracica) lies within the pericardium to the point of attach- 

 ment of the ligamentum arteriosum, and is inclosed with the pulmonary artery 

 in a prolongation of the cpicardium. Beyond tliis it is ])etwecn the two pleural 

 sacs. It is crossed on the right by the oesophagus and trachea, on the left by the 

 left vagus nerve. The left recurrent nerve winds around the concavity of the arch 

 from left to right, and the vena azygos and thoracic duct lie along the dorsal part 

 of its right face. The trachea causes it to deviate to the left; but beyond this it 

 becomes median. The abdominal aorta (Aorta abdominalis) (Fig. 450) is related 

 above to the lumbar vertebrae, the inferior common ligament, and the left psoas 

 minor muscle; in the hiatus aorticus it is related to the cisterna chyli. On its 

 right is the posterior vena cava, and on its left the left kidney and ureter. 



