190 OUTLINES OF EQUINE ANATOMY. 



tlie external serous layer of tlie pericardium. This is tlie 

 'middle onediastinum, while those portions anteriorly and 

 posteriorly placed to the heart are the anterior and pos- 

 terior mediastina. The anterior mediastinum, therefore, 

 contains the following structures ; anterior aorta and two 

 arterise innominatse with the commencement portion of the 

 branches given off by them ; the anterior vena cava with 

 the terminating portions of some of its component veins ; 

 part of the trachea and oesophagus, pneumogastric, phrenic, 

 recurrent, and sympathetic nerves and thoracic duct, and 

 superiorly the terminating portion of longi colli (right and 

 left) ; interiorly, the thymus gland or its remains. The 

 middle mediaslinum is mainly occupied by the heart en- 

 closed in the pericardium, and superiorly may be dis- 

 tinguished the large vessels which it receives or gives off 

 (including vena azygos, which it receives indirectly), and 

 which serve to attach it to the under surface of the spine. 

 Also we here see the oesophagus situated on the right side 

 of the base of the heart ; the trachea terminating in 

 bifurcating to form the right and left bronchi, and the 

 thoracic duct winding from the right to the left side towards 

 its termination. From the supero-lateral parts of this 

 mediastinum also we find the visceral pleura projecting, to 

 surround the right and left lungs. Through this medias- 

 tinum also the two pneumogastrics, the left recurrent and 

 the two phrenic nerves, run. The posterior mediastinu^l 

 is much longer suj)eriorly than inferiorly, and is j^erforated 

 inferiorly. At its superior part are three vessels which, 

 after passing through or while going to hiatus aorticus, 

 course along the under surface of the spine. On the 

 right side vena azygos, on the left the posterior aorta ; 

 centrally the thoracic duct which seems to be simply a 

 space between the two, so thin and uncoloured are its 

 walls. Lower down the oesophagus accompanied by the 

 two pneumogastric nerves courses through this medias- 

 tinum towards the foramen sinistrum, with it also runs the 

 left lohrenic nerve. The right 2?lirenic nerve with posterior 

 vena cava passes through a sj^ecial fold of pleura situated 

 between two lobes of the right lung. 



We have now examined the pleura and have seen that 

 it is divided into a parietal and a visceral portion ; the 

 former consists of the costal and phrenic, the latter of the 

 pulmonary and raediastinal portions. In health these parts 



