RESPIRATION. Ill 



CHAPTER VII. 



RESPIRATION. 

 STRUCTURE OF THE LUNGS THE THORAX AND ITS CONTENTS. 



THE thorax is Bounded anteriorly by the sternum and cartilages 

 of the ribs, posteriorly by the vertebrae and lesser circle of the ribs, 

 and on either side by the shafts of the ribs and the intercostal mus- 

 cles. Its upper orifice is transversely oval, and allows the exit and 

 entrance of vessels, nerves, and muscles to and from its cavity ; 

 its inferior orifice, or circumference, being much larger, and closed 

 by the diaphragm. 



The thorax contains the heart and lungs, and also several vessels, 

 nerves, glands, &c., to be noticed as we proceed. 



THE PLEURJE 



Are two serous membranes, one on either side, which cover the 

 inner surface of the thorax, and are reflected upon the outer sur- 

 faces of the parts contained in its cavity. That portion of the 

 pleura which lines the thorax, is called the parietal layer, and that 

 which lines the contained parts the visceral layer. Each pleura 

 can be traced in the following manner : From the posterior sur- 

 face of the sternum it passes backwards until it meets with the 

 anterior surface of the pericardium, along the side of which it passes 

 to the anterior surface of the root of the lung ; from this it passes 

 upon the lung, and is reflected over the entire surface of the organ, 

 until it arrives at the posterior surface of its root and of the peri- 

 cardium, from whence it passes upon the sides of the bodies of the 

 vertebrae, reaching as high as the transverse process of the sixth 

 cervical vertebra on the right side, the seventh on the left, and de- 

 scending to the diaphragm, the thoracic aspect of which it covers ; 

 it finally lines the ribs and intercostal muscles, until it arrives at 

 the portion which was opened, and which corresponds to the pos- 

 terior aspect of the sternum. 



Ligamentum latum pulmonis (one on either side) is merely a tri- 

 angular fold of pleura, formed by the reflection of the membrane 

 from the lower edge of the root of the lung upon the diaphragm. 



