108 THE ANATOMY OF THE HOBSE. 



to entirely obliterate the interval between it and the heart, a condition 

 which interferes not only with respiration but also with the movements 

 of the heart. 



The base of the heart has its mid point a little to the right of the 

 mesial plane of the body, and is distant about six inches from the 

 spine, to which it is suspended by the great systemic vessels. 



The apex of the heart lies over the posterior extremity of the sternum, 

 and slightly to the left of the mesial plane. 



THE NERVES AND VESSELS OF THE LEFT SIDE OF THE THORAX (PLATE 22). 



Podtion. — It will be most convenient to lower the trunk from its 

 suspended position, and lay the thorax flat on a table, with the left side 

 upwards. 



Directions. — Sever the insertion of the scalenus into the 1st rib, and 

 then remove that bone by sawing through its lower extremity and 

 disarticulating its costo-vertebral joints. In these operations take care 

 not to cut the vessels or nerves to the inner side of the rib. In order 

 to follow many of the nerves and vessels of the thorax, but little dis- 

 section is necessary, as they show distinctly through the transparent 

 pleura which covers them. The phrenic, pneumogastric, and cardiac 

 nerves should be found in the anterior mediastinum, and traced back- 

 wards ; the sympathetic chain will be seen at the roof of the cavity, 

 extending under the costo-vertebral articulations. 



The Left Phrenic or Diaphragmatic Nerve is formed at the root of the 

 neck by the union of three branches (Plate 3), the smallest of which is not 

 constantly present. The inconstant branch is from the inferior primary 

 branch of the 5th cervical nerve, the others are furnished by the corre- 

 sponding branches of the 6th and 7th cervical nerves. The nerve, as 

 thus formed, enters the chest between the first pair of ribs, passing 

 between the axillary artery and the origin of its inferior cervical branch. 

 Continuing backwards between the layers of the mediastinum, it crosses 

 the common trunk of the dorsal, superior cervical, and vertebral veins, 

 and the pericardium at the level of the common aorta. Behind the 

 heart it passes under the root of the lung, through the posterior 

 mediastinum, and is distributed to the left half of the diaphragm 

 (muscular rim and pillar), of which it is the motor nerve. 



The Left Pneumogastric, Vagus, or 10th Cranial Nerve. At the 

 entrance to the chest this nerve lies on the trachea, at the upper edge 

 of the cephalic trunk, and a little below the sympathetic. It crosses 

 in beneath the arch of the left axillary, in company with a cardiac nerve. 

 It is continued backwards across the angle of separation between the 

 anterior and posterior aortse ; and crossing the root of the latter vessel, 

 it reaches the root of the lung, where it divides. The upper division is 

 continued backwards to fuse above the oesophagus with the corresponding 



