THE BLOODVESSELS. 



53 



is enabled to move freely in the cavity so formed; an aqueous 

 fluid, the liquor pericardii, lubricates it at all times.* 



The general form of the heart is that of a cone or irregular 

 pyramid reversed ; it is almost wholly flesh, hollow, and in 

 man about the size of the closed hand or fist. In all the 

 mammalia and in birds the heart is composed of four cavi- 

 ties, namely, a right auricle and ventricle, and a left auricle 

 and ventricle ; these are separated from each other by a ver- 

 tical septum (Fig. 40), cutting off all communication between 

 those of the right side and those of the left. 



Each auricle communicates only with its corresponding 

 ventricle (Figs. 40 and 42). 



The two ventricles occupy the lower part of the heart, and 

 do not communicate with each other, but with their cor- 

 responding auricles. The orifices are called the auriculo- 



VenaCavaSup. Art.Pulm. Aorta. Art.Pulm. 



Pulmonary Veins. 

 Eight Auricle. 



Tricuspid Valve. 

 Inferior Vena Cava. 



Eight Ventricle. 



Pulmonary Veins. 



Left Auricle. 

 Mitral Valve. 



Left Ventricle. 



Septum. Aorta. 

 Fig. 40. Theoretical Section of the Heart in Man. 



ventricular orifices, right and left. The left cavities of the 

 heart contain arterial blood ; the right, venous. The auricles, 

 having to propel the blood only into the ventricles, are not so 



* The pericardium is a fibro-serous membrane. By investing the surface 

 of the organs, as well as the walls of the cavities containing them, the serous 

 membranes provide for the friction of such organs as are in constant motion, 

 as the brain, heart, lungs, and abdominal viscera : analogous membranes 

 are found in the articulations. 



