48 TOPOGRAPHICAL ANATOMY OF THE 



nerves pass to each side of the heart. The right nerves, two or three 

 in number, gain the heart by penetrating between the trachea and the 

 cranial vena cava. They are distributed to the termination of the 

 cranial vena cava, both atria, and the right part of both ventricles. 

 From one to three left cardiac nerves pierce the pericardium close to its 

 connection with the aortic arch, and supply the left part of both 

 ventricles, as well as a portion of the right atrium. They are connected 

 with branches of the right nerves. 



N. RECURRENS. — The right recurrent nerve ^ leaves the vagus about 

 the level of the second rib, turns round the costo-cervical arterial trunk, 

 and thus gains the right ventral border of the trachea which it follows 

 into the neck. In the neighbourhood of the thoracic inlet the recurrent 

 nerve is related to the bicarotid arterial trunk. 



The left recurrent nerve does not leave the vagus until this has 

 reached the aorta. It bends round the aortic arch, between this and 

 the left bronchus and beneath the bronchial lymph glands, and then 

 follows the left ventral border of the trachea through the precardial 

 mediastinum into the neck. 



The recurrent nerves furnish filaments to the cardiac (rami cardiaci), 

 tracheal (rami tracheales), and oesopliageal (rami oesophagei) plexuses. 

 At the entrance to the thorax each nerve is connected with the caudal 

 cervical ganglion of the sympathetic, as will be seen later. 



Dissection. — The heart may now be removed from the chest. Cut 

 across the brachio-cephalic trunk, the aortic arch, the vena azygos and 

 the caudal vena cava close to the point at which each pierces the 

 pericardium. 



The aortic semilunar valves should be displayed by slitting open the 

 commencement of the aorta. The cut should be made between two 

 cusps so as to avoid unnecessary injury. 



The semilunar valves (Valvulre semilunares). — At the entrance 

 to the aorta and pulmonary artery there are three semilunar valves, all 

 constructed on the same principle and arranged in such a manner as to 

 prevent the regurgitation of blood from the artery into the ventricle, 

 but offering no obstacle to the flow of blood in the normal direction. 

 Each valve, as its name indicates, is crescentic, with a fixed convex 

 border and a free concave edge. The convex border is attached to one 

 of three crescentic projections of a fibrous ring that surrounds the exit 

 from the ventricle. The free margin projects into the lumen of the 

 artery and, though concave, is not markedly so. In the middle of this 

 border a small nodular thickening (nodulus valvules semilunaris) can be 

 detected in the aortic semilunar valves, but is either very inconspicuous 



^ Eecurrens [L.], running back. Fibres composing the nerve, having passed down 

 the neck in the vagus, run back to the larynx- 



