228 VERTEBRATE ZOOLOGY 



almost immediately makes a sharp turn to the animal's left and 

 passes to the mid-dorsal line of the thoracic cavity, where it runs 

 posteriorly just beneath the vertebral column to the hinder end 

 of the body cavity; it is thus composed of two divisions, the 

 thoracic aorta and the abdominal aorta. 



Study the thoracic aorta and its branches, but do not follow 

 them out of the thoracic cavity. 



The two small coronary arteries leave the aorta at its base and 

 supply the walls of the heart with blood. If the left auricle is 

 turned forward, the base of these arteries will be seen ; later, when 

 the heart is dissected, they may be traced to the aorta. 



The innominate artery is the first large vessel that passes for- 

 ward from the arch of the aorta, and lies parallel with the precaval 

 vein ; note the nerves which lie along its left side. It gives off the 

 small mediastinal artery and then divides into three branches: 

 the right and the left carotid artery, which carry blood to the 

 head, and the right subclavian artery, which goes to the right 

 foreleg. 



Immediately to the left of the innominate artery is the left 

 subclavian artery. It passes forward parallel with the innominate 

 and goes to the left foreleg. 



Joining the aortic arch with the pulmonary artery is the short 

 ligamentum Botalli. It is the rudiment of an artery called the 

 ductus Botalli, which in the embryo joins these vessels and turns 

 the blood of the right ventricle into the aorta instead of permitting 

 it to go to the lungs. In young cats this ligament is large ; in old 

 cats it is small or wanting. 



Turn the left lung over to the right and observe the aorta as 

 it passes posteriorly in the mid-dorsal line of the thoracic cavity. 

 Ten pairs of small intercostal arteries will be seen arising from 

 its dorsal side and passing to the muscles of the back. Two small 

 bronchial arteries also branch off, either from the aorta, near the 

 fourth intercostals, or from these intercostals themselves, and 

 extend along the bronchi to the lungs. A number of small arteries 

 also go to the oesophagus. 



Exercise 17. Draw the ventral aspect of the heart and the thoracic 

 blood vessels just mentioned. 



