Fig. 112. Frontal Section through the Trunk. 



Frozen Section. The anterior surface of the second section of a series of Ver- 

 tical Frontal Sections seen from in front. The Thoracic and Abdotninal 

 Viscera are in their position at extreme expiration which is never reached 

 during life. Atmospheric pressure has driven the intercostal spaces inwards so 

 that the outlines of Pleura and Lung are undulating. 



A frontal section shews much better than a transverse section the different 

 cavities derived from the general (Coelomic) Cavity of the Embryo. 



The formation of the Diaphragm results in a division into Thoracic and 

 Abdominal Cavities. There is really no Thoracic Cavity but a Thoracic Bony 

 Cage. As the pericardium is pushed downwards from the neck into the Coelom, 

 3 completely independent cavities are estabhshed: 



i) Pericardial Cavity, containing the Heart and a part of the large vessels. 



2) Left Pleural Cavity with the Lung. 



3) Right Pleural Cavity with the Lung. 



It is the practice of many anatomists to call the space between the Left 

 and Right Pleural Cavities the Mediastinum, — according to this the Heart would 

 be in the Mediastinum (Middle). But if three independent cavities are recognized 

 there remain two spaces communicating above, one in front of and one behind 

 the heart — the Anterior Mediastinum and the Posterior Mediastinum — . 



The Anterior Mediastinum is further divided by the overlapping right and 

 left pleurae (cf. Fig. 116) between the 2nd and 4th ribs into an upper and lower 

 compartment with free communication. Above the Mediastina communicate freely 

 with the spaces between the structures of the neck. 



The complementary spaces shewn in the figure between the Thoracic 

 Wall and the Diaphragm are only occupied by the Lung during inspiration. 



The Pericardium is a closed sac consisting of a visceral layer closely 

 enveloping the heart and the roots of the large vessels and a parietal layer 

 directly continuous with the former but blending with the Mediastinal Pleura, the 

 Diaphragm and the Anterior Thoracic Wall. 



The Pleural Sacs are two closed cavities, consisting of a visceral layer 

 which at the root of the lung blends with the parietal layer. This parietal layer 

 lines the thoracic wall — (Costal pleura) — the pericardium — (Pericardial pleura) 

 — the diaphragm — (Diaphragmatic pleura) — and the mediastinum — (Media- 

 stinal pleura) — . 



The uppermost part of the pleura which reaches from V2 to 4/5ths inch 

 above the first rib is the Apical Pleura (this is not shewn in the figure because 

 it lies behind the plane of section). (Vide Figs. 70, 121, 126.) 



