THE PERICARDIUM 925 



The bronchial arteries, one on the right side, and two on the left, are the 

 nutrient vessels of the knigs. The right springs from the first aortic intercostal 

 artery or from the upper left bronchial artery. The two left bronchial arteries are 

 branches of the descending thoracic aorta. Some of tlieir branches ramify in an 

 open jilexus outside tlie hmg beneath tiie i)leura; another set supply the inter- 

 lol)ular areohir tissue; whilst a third set of branches supply the bronchial tubes 

 witli their lining membrane and muscular walls, the walls of the vessels, and the 

 lironchial glands. The ramifications of this set reach and mingle Avith the pul- 

 monary vessels, and thus a small quantity of the blood conveyed by the bronchial 

 arteries is returned as arterial through the pulmonary veins to the heart. 



The bronchial veins consist of a superficial and a deep set, which join, to 

 foiin a single vessel on each side; the right bronchial vein oi)ens into the vena 

 azygos major, and the left into the left superior intercostal vein. 



The lymphatics form a suj^erficial and a deep set. 



The nerves nrc dcrixcd frnni the anterior and ]»(jsterior pulmonary plexuses. 



THE ORGANS OF CIRCrLATlOX 



THE PEEICARDIUJH 



The pericardium is a cone-shaped, fibro-serous sac surrounding the heart, with 

 its apex above, and its base below and adherent to the diaphragm. The fibrous 

 layer is very strong and inelastic, and is composed of interlacing fibres. Below, 

 its connection with the central tendon of the diaphragm is close and intimate. It 

 is very firmly bound to the caval opening, but loosely attached and easily separable 

 elsewhere. Above, it is lost on the sheaths of the great vessels, all of which receive 

 distinct investments, with the single exception of the inferior vena cava, which 

 pierces it from below. 



The aorta. su])erior vena cava, botli divisions of the jjulmonary artery, with the 

 ductus arteriosus, together with the four pulmonary veins, are all ensheathed in 

 tliis manner. The fibrous portion of the pericardium, through the sheaths pro- 

 longed over the great vessels, ultimately becomes continuous above with the deep 

 cervical fascia. Two slight bands of fibrous tissue — the sterno-pericardial bands 

 or ligaments — connect the front of the pericardium, above and below, with the 

 posterior surface of the sternum. 



The serous layer is smooth and glistening. Its parietal portion lines the 

 inncn- surface of tlie fibrous bag. The visceral portion is retiected over the surface 

 of the heart and the roots of the great vessels, the two layers being in close contact, 

 and moistened with a thin secretion to allow the free movement of the heart. A 

 seri(;s of pouches or sinuses are thus formed at the line of reflexion. Between the 

 inferior vena cava and lower left ])ulmonarv vein, the oblique sinus ascends 

 behind the left auricle, partially investing the pulnK)nary arteries and the pul- 

 monary veins of the right and left sides, between which it passes. Small ])ouches 

 from the main sac dip betAveen the veins to meet the inflexions from the sides of the 

 oVjli(jue sinus just noticed, thus completing the serous coverings to these vessels. 



Pouches can be further traced between the inferior vena cava and lower pulmo- 

 nary vein of the right side, between the superior vena cava and iii^i^er ]>ulmonarv 

 vein of the same side, and also between the left ])ulmonary artery and left U]tper 

 pulmonary vein. 



A tubular prolongation, moreover, surrounds both the root of the pulmonary 

 artery and the aorta in common, and these are the only vessels whicli can be said 

 to have a complete and continuous investment. 



There is, therefore, a connnunication behind them between the riglit an<l left 



