The Blood and its Circulation 



141 



layers, one over the other. The inner (or visceral) layer 

 covers the external surface of the heart and is reflected 

 back upon itself in order to form, like all membranes of 

 this kind, a sac without an opening. The space between 

 the two membranes is filled with 

 fluid which permits the heart and 

 the pericardium to glide one upon 

 the other with the least friction. 1 



FIG. 70. Diagram illus- 

 trating the Structure of* 

 a Serous Membrane. 



A, the viscus, or organ, envel- 

 oped by serous membrane ; 

 B, layer of membrane lin- 

 ing cavity; C, membrane 

 reflected to envelop viscus ; 

 D, outer layer of viscus, 

 with blood vessels at R 

 communicating with the 

 general circulation. 



213. Serous Membranes. The serous 

 membranes form shut sacs, one portion 

 of each of which is applied to the walls 

 of the cavity which it lines ; the other is 

 reflected over the surface of the organ 

 or organs contained in the cavity. The 

 sac is completely closed, so that no com- 

 munication exists between the serous 

 cavity and the parts in its neighbor- 

 hood. These membranes secrete a 

 fluid, commonly called serum, but which 

 is really a form of lymph. The various 

 serous membranes are \hzpleura, which 

 envelops the lungs ; the pericardium, 

 which surrounds the heart ; and the 

 peritoneum, which invests the viscera of the abdomen. In health 

 the serous membranes secrete only sufficient fluid to lubricate 

 and keep soft and smooth the opposing surfaces. 



/ 214, The Auricles and Ventricles. The heart is a hollow 

 organ, divided into two great cavities or "chambers" by a 

 fixed partition forming a left and a right side. These two 

 cavities are each divided transversely by a movable par- 

 tition into an upper and a lower chamber. These partitions, 



1 A correct idea may be formed of the arrangement of the pericardium 

 around the heart by recalling how a boy puts on and wears his toboggan 

 cap. The pericardium encloses the heart exactly as this cap covers the 

 boy's head. 



