16 TOPOGRAPHICAL ANATOMY OF THE 



mediastinum presents a cribriform or lace-like appearance in the 

 adult horse. The numerous small openings place the right and 

 left pleural cavities in communication with each other; and this 

 is not without clinical significance, for it means that a pleural effu- 

 sion into one pleural cavity will find its way into the other. 



The precardial mediastinum contains the large blood vessels con- 

 necting the heart with the head, neck and thoracic limbs, the 

 terminal part of the thoracic duct, the vagus, recurrent, phrenic, 

 sympathetic and cardiac nerves, the oesophagus and trachea, and 

 the thymus and lymph glands. 



The cardial mediastinum contains the heart and its enveloping 

 pericardium, the commencement of the arterial and the end of the 

 venous systems, the oesophagus, the termination of the trachea, the 

 vagi, left recurrent, phrenic, cardiac and pulmonary nerves, and the 

 thoracic duct. 



The postcardial mediastinum contains the aorta, the azygous 

 vein, the vagi, left phrenic and sympathetic nerves, the thoracic 

 duct, and the oesophagus. 



The thymus. — If the horse is more than two or two and a half 

 years old, there is little chance of seeing more than a trace of the 

 thymus.^ When present and of good size, the organ is a greyish 

 lobulated body lying in the precardial mediastinum ventral to the 

 trachea and the large vessels, and flattened laterally in conformity 

 with its place of location. It is often possible to show that the 

 organ consists of two thin, flattened lobes (right and left) pressed 

 close to each other at the median plane. 



At its maximum development the thymus extends for some 

 distance beyond the first rib into the neck. The opposite end of 

 the organ slightly overlaps the pericardium. 



The lungs (Pulmones).^ — Each lung is a soft, crepitant, spongy 

 organ occupying a considerable part of one side of the thoracic 

 cavity, and, as has been seen, in intimate association with the 

 pleura of that side. It is also very precisely moulded upon all 

 the other thoracic contents, even such a small object as the 

 phrenic nerve being accommodated in its own impression on the 

 surface of the organ. It must be remembered, however, that the 

 fresh lung, or one that has not been absolutely successfully hardened 

 in situ, loses most or many of its surface markings, as well as its 



^ Qii/jLos (thymos) [Gr.], a warty excrescence (from its likeness to a bunch 

 of thyme-ilower), the sweet-bread. 



2 Pulmo [L.], (apparently borrowed from the Gr. irXev/j-wv (pleumon), for 

 Trvev/xuv (pneumon)), the organs of respiration, the lung. 



