110 THE ANATOMY OF THE HORSE. 



Position and relations. — In order to studyt hese, the heart should now 

 be restored to its natural position within the pericardium. 



The position of the heart may be expressed with regard to the skeleton 

 as follows : — It lies beneath the bodies of the dorsal vertebrse from the 

 4th to the 10th inclusive ; it responds to the four ribs behind the 2nd ; 

 and it is placed above the sternum from about the 3rd ohondro-sternal 

 joint to within an inch of its posterior e.xtremity. 



In a medium-sized animal the most anterior part of the heart (right 

 auricula) is distant about four or five inches from the entrance to the 

 chest ; the posterior border at its upper part is separated by about the 

 same interval from the tendinous centre of the diaphragm, but at its 

 lower part it is only about an inch in front of the insertion of the rim of 

 the diaphragm across the ensiform cartilage ; and, during great distension 

 of the abdominal viscera, the diaphragm may be driven forwards so as 

 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 OP THE LEFT SIDE OF THE THORAX (PLATB 22). 



Position. — 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 \inder 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 corresponding branches of the 6th and 7th cervical nerves. The 

 nerve, as thus formed, enters the chest between the first pair of ribs, 



