THE THORAX. 119 



lations of the contents to the parietes, which have been 

 already discussed, both as far as physical diagnosis and 

 injury are concerned; and considerations with respect to 

 aneurism of the aorta or its great trunks are subjects 

 rather for systematic surgery. Since operative proceed- 

 ings on the oesophagus, trachea, or great vessels are insti- 

 tuted in the neck, the chief anatomical bearings of these 

 structures will be found described in the chapter on the 

 Surgical Anatomy of the Neck. 



The cavity of the thorax is most conveniently described 

 for reference as divisible into two pleural cavities, sepa- 

 rated in the lower four-fifths of the chest, by the three 

 mediastina, and in the upper fifth by the great vessels 

 springing from the arch of the aorta, a region called by 

 Professor Wood, the cervico-thoracic. The posterior 

 w r all of this division, he makes the three upper dorsal 

 vertebrae and their inter vertebral substances, and the in- 

 tervertebral substance between the third and fourth the 

 anterior the manubrium sterni, and upper fourth or fifth 

 of the anterior mediastinum ; on either side the apices of 

 the lungs and pleurae. 



The anterior mediastinum is bounded as follows : An- 

 teriorly, by the sternum and left costal cartilages; pos- 

 teriorly, by the pericardium, for the lower three-fourths 

 or four-fifths, and for the rest, by the cervico-thoracic 

 region ; laterally, by the pleura. The middle mediastinum 

 contains the heart and ascending portion of the aorta 

 and arch, phrenic nerves and vessels, and is limited 

 behind by fibrous pericardium and the obliterated ductus 

 arteriosus. 1 The posterior mediastinum is limited above 



1 Vide Professor Wood, F.R.S , on Kelations of Aorta : " Journal 

 of Anatomy and Physiology," vol. iii. 



