294 SPECIAL ANATOMY. 



brane firmly attached to it. The heart lies also on the surface, not in the ca- 

 vity of the sac, and is loosely surrounded by the non-inverted portion of the 

 pericardium, so that it can move freely therein. The inverted, internal layer 

 of the pericardium passes over the base of the heart, since it leaves uncovered 

 a narrow stripe only of the left auricle, on the great vessels (ven. cava super., 

 aorta and Art. pulmonal.') into the free, external layer, and surrounds the ves- 

 sels like a sheath. To the external surface of the free layer we find a thin 

 fibrous lamina firmly attached, which at the anterior inferior part is united 

 with the tendon of the diaphragm (three and a half inches transversely, one 

 to two inches from before to behind, close to the Sternum) and also passes 

 over upon the vascular trunks. The cavity of the pericardium secretes a 

 humid vapour, which in disease, or after death, becomes fluid, as lig. Peri- 

 cardii; its walls are smooth. 



The shape of the pericardium is a cone, the base of which lies upon the 

 speculum Helmontii, of the diaphragm, the apex looking upwards (to the 

 great vessels). Its anterior surface is covered by pleura, and touches the 

 sternum with only a small rhomboidal piece covered by areolar tissue (cor- 

 responding to the right half of the heart). The posterior surface is separated 

 from the vertebral column by the mediastin. post, with oesophagus, Aorta, 

 duct, thoracic., &c. At the sides the Nn. phrenici and Artt. phrenic, sup. pass 

 between pericardium and pleura. 



Vessels: Artt. pericardiacce , or branches of aorta descend., mammar. in- 

 tern., phreniccB super., bronchiales, thymicce and wsophagece. 



Veins : pass with the arteries, and open partly into Ven. azygos. 

 Lymphatics : go to the glands on the V. cava superior, and into the due- 



tus thoracicus. 

 Nerves : have not hitherto been found. 



531. Circulation, course of the blood, Circulatio sanguinis, 



differs according as to whether the lungs (after birth) effect the change of 

 the dark red blood into the bright red, or the placenta (before birth) is the 

 medium for the exchange of the consumed, for a blood containing nutritious 

 substances. 



532. I. Circulation after birth. 



It is divided into two sections : 



1. Smaller circulation, course of the pulmonary blood. The venous blood 

 collects from the vv. caves and coronaricB cordis in the right auricle of the 

 heart, passes into the right ventricle, thence into the pulmonary artery and 

 lungs. From the capillary rete of the lungs it comes forth as arterial blood, 

 and returns through the pulmonary veins to the heart, but to the left 

 auricle. 



2. Greater circulation, course of the blood through the body. Pressed by 

 the left auricle into the left ventricle, thence into the aorta, the bright red 

 blood streams through all arteries (with the exception of the pulmonary), 



