48 



the rest of its extent the heart is free, but is surrounded by a 

 fibre-serous membrane, called the pericardium, which iioi 

 nicivly forms a shut sac for containing the heart, but gives to 

 its surface a smooth covering, by which it is enabled to move 

 freely in the cavity so formed; an aqueous fluid, the liquor 

 pericardii, lubricates it at all times.* 



Artery of 

 the Arm. 



Right Lung. 



L?ft Lung. 



od / a vg 



Fig. 30. Lungs, Heart, and Principal Vessels in Man.t 



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 

 vertical septum (Fig. 31), cutting off all communication be- 

 t \\cen those of the right side and those of the left. 



* The pericardium is one of the serous membranes, of which several exist 

 in the body. By investing the surface of the organs, as well as the walls of 

 the cavities containing them, they provide for the friction of such organs as 

 are in constant motion, as the brain, heart, lungs, and abdominal \i-<vni : 

 analogous membranes are found in the articulations. 



t od, vd, right auricle and ventricle; vg, left ventricle; a, aorta; 

 oo, carotid arteries; cc, veua cava inferior; vj, jugular veins ; t, trachea. 



