456 



THE ANATOMY OF THE HORSE 



CHAPTER XXI 



THE THORACIC ORGANS AND THEIR APPENDAGES 



CONTENTS OF THE THORAX — THE BLOOD — GENERAL PLAN OF THE CIRCULATION— THE HEART 

 AND ARTERIES— THE VEINS — PHYSIOLOGY OF RESPIRATION — MECHANISM OF THE 

 PULMONARY APPARATUS — THE NASAL ORIFICES AND CAVITIES — THE LARYNX — THE 

 TRACHEA AND BRONCHI — THE LUNGS— PULMONARY GLANDS, ETC. 



CONTENTS OF THE THORAX 



The THORAX, or chest, is that cavity formed by the bodies of the dorsal 

 vertebrae superiorly ; by the ribs and their cartilages with the connecting 

 muscles laterally ; by the sternum inferiorly ; by the diaphragm poste- 

 riorly ; and by the inner margins of the first ribs and body of the first 



dorsal vertebra anteriorly. It contains 

 the central parts of the important organs 

 of circulation and respiration, and gives 

 passage to the oesophagus, as it connects 

 the pharynx with the stomach. As 

 these lie within it, they are allowed to 

 play freely in performing their func- 

 tions, by being enveloped by smooth 

 serous membranes, called the ^:)^eM?"a 

 and the pericardium, the latter being 

 also protected by a fibrous layer. A 

 section of the thorax, as shown in the 

 plan. Fig. 68, will give some idea of the 

 relative situation of these organs and 

 their investments, as well as of the 

 shape of the cavity itself in this direc- 

 tion. The heart is shown at A, lying 

 between the two bags of the pleura, in 

 the space called the mediastinum. The 

 lungs are shown at B B, covered by a 

 fine serous membrane (H Ji), pleura ptd- 

 monalis, except at their roots, where the air-tubes and blood-vessels pass into 

 their substance. This portion of the pleura is continuous with the serous 

 membrane lining the ribs (G G, pleura costalis), which thus allows them to 

 expand and contract freely, by allowing one surface to glide against the 

 other. Thus, the pleura on each side covering the lungs, and reflected 

 thence to the inside of the ribs, and the thoracic side of the diaphragm, 

 forms a shut sac or bag, which in the natural state contains only sufficient 

 serum to lubricate its walls ; but in disease this is often increased to an 

 enormous extent, ending in dropsy of the chest, or in a collection of pus 

 when the membrane is greatly inflamed. The shape of the thorax in a 

 longitudinal direction is shown at Fig. 69, in which its posterior wall, the 



Fio. 68.— Sectional Plak of Thorax and its 

 Contents (XHRonGH the Girth-place). 



A. Heart. 



B. B. Lungs. 



C. E. D. F. Walls of the thorax. 

 G. G. Pleura costalis. 



H. H Pleura pulmonalis. 



