132 PRACTICAL ZOOLOGY. 



3. Note that the heart is, normally, in the plane of the 

 mediastinum, and that this membrane entirely sepa- 

 rates the two halves of the chest. 



4. Take the heart between the thumb and finger to feel 

 how easily it slips about in its sac, the pericardium. 



5. Cut away the mediastinum and the pericardium, and 

 note the appendages, auricles, at the large end, or 

 base, of the heart. 



6. Cut away the breastbone entirely, press the ribs out 

 to the sides, and dissect away a thin layer of muscles 

 covering the windpipe, so that the blood-vessels which 

 run forward from the heart into the neck may be 

 traced. Carefully pick away a fatty-looking body, 

 the thymus gland, in front of the heart and trace the 

 following arteries. 



7. The main artery, the aorta, is a whitish, thick-walled 

 tube. Springing forward from the centre of the base 

 of the heart, it soon arches over to the left (of the 

 animal) and runs along the middle of the dorsal wall 

 of the chest-cavity, piercing the diaphragm, as noted 

 in studying the abdominal cavity. 



8. At the arch the aorta gives off two branches; the 

 first of these soon subdivides, giving off a branch to 

 the right fore limb, the right subclavian artery, a 

 branch running along each side of the windpipe, the 

 right and left common carotid arteries (called com- 

 mon because each as it nears the head divides into 

 the internal carotid and the external carotid). The 

 second branch from the arch is the left subclavian 

 artery. 



9. Just outside of the common carotids on each side are 

 the white, thread-like pneumogastric nerves. 



