12 TOPOGRAPHICAL ANATOMY OF THE 



rib to that of the sixth, and because of the cranial and ventral slope of 



the diaphragm, the greatest dorso-ventral (vertical) diameter of the 



cavity occurs on a level with the diaphragmatic attachment to the 



sternum. The slope of the diaphragm is also responsible for the fact 



that the dorsal wall is almost twice as long as the ventral wall (sternum). 



The decree of curvature of the ribs causes the transverse diameter to 



increase from the first rib to the last ; though, in some animals, this 



diameter remains fairly constant from the eleventh rib to the last. 



The chief organs contained within the thorax are the heart and the 



two lungs. The heart lies between the lungs and is enclosed in a fibro- 



serous sac, the pericardium. The lungs fill the greater part of the 



thoracic cavity, and, except where it is connected with the heart by 



large vessels, and where it is joined to the trachea by a bronchus, each 



lung lies free in its own side of the thorax. A serous membrane, the 



pleura, covers each lung and lines the corresponding part of the 



thoracic wall. Where the two pleurse come into contact with or 



approach each other in or near the median plane, they form the 



mediastinal septum, a partition in which all the thoracic contents, with 



the exception of the lungs, the caudal vena cava and the right phrenic 



nerve, are contained. Part of the septum and some of the features 



of the pleural cavity must now be displayed. That part of the pleura 



that covers the lung is known as visceral ; the rest of the membrane is 



the parietal pleura. 



Dissection. — On both sides of the body the ribs from the second to 

 the fourteenth inclusive must be cut through with bone forceps about 

 their angles. The same ribs are, in like manner, to be separated from 

 their cartilages. Then, with a knife, remove en masse the portion of 

 the chest wall thus indicated. The greatest care is necessary in con- 

 ducting this dissection, otherwise the lung, which, in a properly 

 hardened subject, is closely applied to the inner surface of the chest 

 wall, will be injured. 



The pleura. — Before any further dissection is undertaken, the 

 disposition of the pleura^ must be ascertained. That part of the 

 membrane which is applied to the ribs, and in which a large window 

 has just been made, is known as the costal 'pleura (pleura costalis). If 

 this be traced in a ventral direction it will be found to approach or 

 reach the middle line of the sternum, where it meets its fellow 

 membrane of the opposite side of the thorax. From the sternum each 

 membrane is reflected dorsalwards as the mediastinal fleitra (pleura 

 mediastinalis). In some regions the mediastinal pleurae are intimately 

 related to each other, and a thin double membrane, the mediastinal 

 septum (septum mediastinale), is produced. In other regions, however, 



1 irXfvpd (pleura) [Gr.], the side, rib, the membrane that lines the chest. 



