Fig. 10. 



PAVEMENT EPITHE 



54 ARRANGEMENT OF SEROUS MEMBRANES. 



it contains very few nerves. The smooth surface of the mem- 

 brane forms one unbroken plane, being neither raised into 

 villi, nor depressed into follicles ; and its basement-membrane 

 is covered with a single layer of flat epithelium-cells, which 

 are closely applied to it and to each other, 

 like the pieces of a pavement (fig. 10). It 

 is with such a membrane that every one of 

 those great cavities is lined, which contains 

 important viscera ; and it is also continued 

 on to the outer surface of these viscera, 

 so as to afford them an external coating 

 over every part save that by which they 

 are attached. Thus the heart is suspended 

 freely, by the large vessels proceeding from 

 CELLS OF^SEROUS its summit, within a bag or sac of fibrous 

 MEMBRANE. membrane peculiar to itself, which is termed 



the pericardium. The cavity of this bag is completely lined 

 by the serous membrane (fig. 11, p'\ which closely embraces 



the vessels, and which then 

 bends down over the surface 

 of the heart, so as to enclose 

 it in the envelope p. Hence 

 it will be seen that this 

 membrane, whilst including 

 the heart, and allowing it to 

 communicate with its vessels, 

 forms a completely shut sac; 

 and it may be likened to a 

 common double cotton or 

 woollen night-cap, which has 

 a similar cavity between its 

 two layers, the head being 



j-'Ag. ** J-'i A. VJJVAUO. vx i nr* JL X.AJ.VAAI.'IIJ J -- _ ... ^^ 



a a, auricles ; v v, ventricles ; b, pulmonary really On the GUtSlue 01 

 artery ; c, aorta; pp>, pericardium. ^ whi]jgt geeming to be 



within the envelope. The two layers of the pericardium, 

 though separated in the diagram for the sake of distinctness, 

 are really in mutual contact, save when separated by the in- 

 terposition of fluid poured out in disease. Each of the lungs, 

 in like manner, is suspended in a closed sac of its own, termed 

 the pleura; and the surface of the lung is covered by a serous 

 membrane, which is reflected over the wall of the pleural cavity. 



Fig. 11. DIAGRAM OF THE PERICARDIUM. 



