34 THORAX 



the anterior surface of the root of the left lung. When the incisions have 

 been made, the anterior flaps can be turned anteriorly and the posterior flaps 

 downwards. None of the flaps must be removed, for it will be necessary 

 to replace them in position at a later stage of the dissection. 



When the flaps marked out by these incisions are turned aside the 

 dissectors will find that, on the right side, they have exposed the greater 

 part of the right atrium (see Fig. 12). They should note that the area of 

 the atrium which is exposed is separated into two parts by a vertical sulcus, 

 the sulcus terminates, which runs from the anterior face of the cardiac end 

 of the superior vena cava to the anterior aspect of the terminal part of the 

 inferior vena cava. This sulcus divides the atrium into a posterior part, 

 the sinus venosus, and an anterior part, the atrium proper, whose upper 

 and anterior part is prolonged medially to the anterior surface of the heart. 

 On the left side the greater part of the heart exposed by the reflection of 

 the pericardial flaps is the left ventricle, but in the upper part of the 

 area the auricle (O.T. auricular appendage) of the left atrium is seen. 

 Anterior to it lie the stem of the pulmonary artery and the upper part 

 of the anterior portion of the right ventricle. A line of fat, in which lie 

 the inter ventricular branch of the left coronary artery and the accompany- 

 ing vein, indicates the position of the septum between the left and right 

 ventricles (Fig. 13). 



After the dissection is completed and the dissectors have carefully noted 

 the relative positions of the various structures which have been exposed, 

 they should proceed to study the phrenic nerves, which have been retained in 

 position by the strips of pleura on their lateral surfaces (see Figs. 12 and 13). 



Nervi Phrenici. Each phrenic nerve arises in the neck 

 from the cervical plexus, receiving fibres from the third, 

 fourth, and fifth cervical nerves. It descends on the scalenus 

 anterior muscle and, at the root of the neck, passes anterior 

 to the subclavian artery and posterior to the corresponding 

 vein, but on the left side, as it leaves the scalenus anterior, 

 it lies anterior to the subclavian artery and posterior to the 

 commencement of the innominate vein. As it enters the 

 upper aperture of the thorax it crosses the internal mammary 

 artery, passing from its lateral to its medial side, then 

 it descends along the lateral border of the mediastinum, 

 anterior to the root of the lung, to the diaphragm where 

 it breaks up into branches. The majority of the branches 

 pass between the muscular fibres of the diaphragm and, 

 after communicating with the abdominal sympathetic nerve 

 fibres which form the diaphragmatic plexus, they are 

 distributed to the muscle from its lower surface. The re- 

 lations of the phrenic nerves in the thorax are different 

 on the two sides, and the left phrenic nerve, as a whole, is 

 on a plane somewhat anterior to the right. 



The right phrenic nerve descends along the lateral borders 

 of the right innominate vein and the superior vena cava to 

 the point where the latter enters the pericardium, then along 



