192 OUTLINES OF EQUINE ANATOMY. 



a network that the meshes are not so large as the vessels 

 themselves ; and to bring about more complete aeration of 

 the blood, in consequence of the close connection of the 

 air-cells, each layer of capillaries is in apposition on both sur- 

 faces with the thin wall of an air-cell, which has here become 

 but little more than a layer of basement membrane covered 

 by simple tessellated epithelium. Thus the air and the 

 blood do not come into direct contact, but interchange of 

 gases takes place by the process of osmosis through the 

 layers of epithelial cells of the air- sacs and of the capillaries 

 and their respective basement membranes. Two arteries 

 pass to the lungs ; of these the bronchial, derived by a 

 root which it shares with the oesophageal, from the posterior 

 aorta or from the sixth intercostal artery, is the nutrient 

 vessel of the organ ; while the pulmonary artery is the func- 

 tional vessel, which brings that • blood which requires aera- 

 tion (and which will be shortly noticed) . The blood obtained 

 •through these two vessels seems to become mixed in the 

 capillaries, and returns either by the pulmonary veins into 

 the heart, or by the bronchial veins into vena azygos, and 

 so indirectly into the anterior vena cava. The lym])liatics 

 of the lungs are numerous and converge towards certain 

 bronchial lymphatic glands, which may be found as grey 

 nodular bodies at the root of the lungs. The nerves are 

 the pneumogastric and the sympathetic, which intermix 

 their fibres intimately in forming the bronchial plexus at 

 the root of the lung. All these structures are united 

 together by areolar tissue, to which the term jpare7ichyma 

 is applied; but this term is very indefinite, sometimes 

 being used to indicate the whole lung-tissue as opposed to 

 the pleura, its use therefore should be discontinued. If 

 we now remove the exposed lung by cutting through its 

 root we shall see the HEART, in situ. It is enclosed in a 

 dense membrane, the pericardium, and presents a base and 

 an apex ; and to its base, which is superiorly placed, vessels 

 from above, from behind, and from before, are attached, 

 and serve to sling it in position. The pericardium consists 

 -essentially of a membrane composed of white fibrous tissue, 

 fibrous pericardium, which extends from these vessels at 

 the base of the heart, in a downward direction, to become 

 attached to the upper surface of the ensiform cartilage 

 and posterior part of the sternum. It is covered externally 

 by the layers of pleura forming the middle mediastinum, 



