330 CLINICAL APPLIED ANATOMY. 



with those of the mediastina. The connective tissue of the 

 retropharyngeal space, which lies between condensed layers 

 known to anatomists as the prevertebral and retropharyngeal 

 fasciae respectively, is directly continuous with the connective 

 tissue of the posterior mediastinum surrounding the oesophagus. 

 Perforation of the latter may give rise to emphysema in these 

 layers. The connective tissue which surrounds the trachea is 

 encased by a condensed lamina known as the pretracheal fascia, 

 and this fascia blends with the fibrous pericardium after encasing 

 the innominate vein. As the result of difficult tracheotomies or 

 wounds of the cervical fascia, air may be sucked or driven into 

 the layer of connective tissue which surrounds the trachea 

 under the pretracheal fascia. It then passes into the superior 

 mediastinum, inflating the connective tissue spaces in this 

 region, travels onwards in the connective tissue on the front of 

 the pericardium, and laterally invades the connective tissue 

 sheaths of the pulmonary roots, ultimately reaching the inter- 

 stitial connective tissue of the lungs. Doubtless a valvular 

 arrangement of the wound leads to this inflation of the connective 

 tissue by violent respiratory efforts. Similar results may follow 

 the passage of a tracheotomy tube into the connective tissue 

 between the pretracheal layer and the trachea. 



The extension of the emphysema may occur in the reverse 

 direction. When an air cell ruptures into the connective tissue 

 of the lung the air first gains access to the connective tissue of the 

 lung and its root, and then spreads to the mediastinum, pericardial 

 connective tissue, and ultimately to the neck, whence it may 

 overflow into the subcutaneous connective tissue also. 



Invasion of the mediastinum by gas-producing bacteria 

 occasionally produces a spreading emphysema. 



The presence of air in the extra-pericardial tissue may closely 

 simulate pericardial friction ; the air in the adjacent connective 

 tissue of the lung gives rise to crackling sounds similar to the 

 consonating crepitations of pneumonia ; air in the connective 

 tissue on the front of the thorax gives rise to a crackling which 

 can be heard with the stethoscope and appreciated by the hand. 



