COURSE OF THE CIRCULATION. 



397 



PERICARDIUM. 



The pericardium is a fibro-serous sac, which encloses and is 

 reflected over the heart, and origins of the large blood-vessels. It 

 consists of a parietal and visceral portion, and is composed of two 

 membranous layers ; one external, dense, and fibrous, the other 

 internal and serous. * The outer fibrous layer enters into the 

 formation of the sac, or free portion only of the pericardium, 

 having no direct attachment to the heart itself. It is made up 

 of fibres, which interlace with each other 

 and form a sac, whose apex embraces the 

 origins of the larger blood-vessels, as tubular 

 prolongations. The basal fibres of the 

 membrane sometimes blend with the dia- 

 phragm, and are also attached to the last 

 hones of the sternum and ensiiorm cartilage. 

 The parietal layer is covered by the pleural 

 folds which form the middle mediastinum. 

 The serous layer is much larger than the 

 fibrous, which it lines, and is reflected over 

 the origin of the blood-vessels, and external 

 surface of the heart, thus forming a closed 

 sac. The reflected portion is often called 

 the ppicardium. Between the parietal and 

 visceral layers is secreted a pale, lubrica- 

 ting serous fluid, the liquor pericardii. 

 The pericardium supports the heart, and 

 is-^lates it fi'oiii the other thoracic viscera. 



Fig. 153. 



nUESE OF THE CIRCULATION. 



Diagram of the circulation 

 in Mammalia and Aves. a, 

 Boundary of the heart, con- 

 taining four cavities ; h. Vena 

 cava (represented as single- 

 here) ; c, Eight auricle ; d. 

 Right ventricle ; e. Pulmon- 

 ary artery ; /, Capillaries of 

 the lungs ; g. Left auricle, 

 receiving the purified blood 

 from the pulmonary vein 

 (only one shown here) ; A, 

 Left ventricle; i. Aorta; j. 

 Somatic capillaries convey- 

 ing the blood again to the 

 veins. 



We have seen that the heart is divided 

 into a right or venous, and a left or arterial 

 portion. In tracing the circulation it matters 

 little where we commence, since wherever it be we come back 

 to the .same point. Briefly, the following is the course of the 

 blood : — The venous blood is carried into the right auricle by 

 the anterior and posterior venaB cavae. It then passes through 

 the right auriculo- ventricular opening into the right ventricle ; 

 thence through the pulmonary artery to the lungs. It returns 

 by the pulmonary veins to the left auricle ; thence to the left 

 ventricle, which propels it through the aorta and its branches 



