IO MAMMALIAN ANATOMY 
Remove the sternum by severing its attachment to the dia¬ 
phragm and to the most anterior pair of ribs, and thus lay open 
ventrally the whole length and width of the thoracic cavity, the 
extent of which should be carefully noted. Study and identify 
the contained viscera in situ . 
Note the central position of the heart and observe the large 
blood vessels which lead into and from it. Note that the parietal 
pericardial membrane forms a loose sac about the heart. Care¬ 
fully lay this open and observe that the reduplicature which con¬ 
stitutes the visceral pericardial membrane is closely applied to 
the surface of the heart. Note the dark red appearance of the 
thin-walled anterior chambers of the heart, the auricles, due to 
their distension with blood, and the paler appearance of the thick- 
walled, muscular posterior chambers, the ventricles. In life the 
lungs, here seen in a collapsed condition, are distended with air 
and practically fill the pleural cavities. (Cf. specimen in which the 
lungs have been artificially inflated.) 
If the heart and lungs be gently drawn to the right side, the 
tubular oesophagus may be seen extending lengthwise through the 
dorsal mediastinal space. The air passages connecting with each 
lung unite dorsal to the heart to form the trachea. Since this is 
concealed in the thoracic region by the heart and large blood 
vessels as well as by the thymus gland, which in most mammals 
(but not in the guinea pig) is located in the mediastinal space 
anterior to the heart, it is advisable to first locate the trachea in 
the neck region. This may be done by carefully separating the 
glandular masses of the neck and then separating the two delicate 
ribbon-like muscles which extend lengthwise upon either side of 
the median line immediately over the trachea. The trachea will 
be readily recognized by the presence of cartilaginous rings in its 
walls. Dorsal to the trachea and in direct contact with it lies 
the oesophagus. The trachea and oesophagus may now be traced 
posteriorly to the narrow anterior orifice of the thoracic cavity 
through which they pass in company with the large blood vessels 
which supply the anterior region of the body. 
The trachea will be found to extend anteriorly the whole length 
of the neck. Its anterior end is differentiated to form the larynx. 
The oesophagus similarly extends the whole length of the neck. 
