CHAP, VL] THE CAT'S ALIMENTARY SYSTEM. 189 



19. The PERITONEUM is the large [closed sac, formed of very 

 delicate membrane, which both lines the abdominal cavity and coats 

 its contents. It is one of a class of membranes called "serous," 

 from the nature of the colourless fluid with which their surfaces 

 are moistened, and which is more or less like the " serum" of the 

 blood. Part of the membrane is applied to the inner surface of 

 the walls of the abdominal cavity, and this is called its parietal 

 portion, but it is every here and there reflected from the walls 

 over the viscera contained within them, and such reflected parts 

 are called its visceral or reflected portion. Thus the viscera nowhere 

 enter into the real cavity of the peritoneal sac, while their move- 

 ments can take place without friction because the moist inner 

 surfaces of the peritoneum are everywhere juxtaposed, however 

 complex and complete may be wrappings round which the viscera 

 receive from the inflexions of this highly complex sac. 



The membrane is formed of connective tissue, which is lined by a 

 layer of squamous epithelial cells. 



The layer of epithelium thus lining a serous cavity is called 

 endothelium. The peritoneal serous membrane is attached to the 

 parts to which it is applied by fibres of more or less loose areolar 

 tissue. 



The FORM OR ARRANGEMENT of the peritoneum is exceedingly 

 complex, owing to the contorted and unsymmetrical arrangement of 

 the viscera which it invests, and which fill the abdominal cavity. 

 In development, as will be hereafter seen, the alimentary canal is 

 primitively an exceedingly simple tube traversing the abdominal 

 cavity from before backwards. The peritoneum lining the ventral 

 surface of the abdomen is continued upwards along its sides, nearly 

 to the middle line, whence each lateral layer is reflected ventrally, to 

 embrace closely each side of the alimentary tube, and to meet the 

 reflection of the lateral layer of the other side, upon the ventral 

 surface of the alimentary tube. Thus this tube is enclosed and 

 slung in a fold of membrane, and really lies (as before said) external 

 to the peritoneal cavity ; though the two reflected folds (between the 

 alimentary tube and the back of the abdominal cavity) become so 

 closely applied together as to seem to form but one membrane. 



As development proceeds, the alimentary tube becomes differen- 

 tiated into regions of very different capacities ; while, at the same 

 time, it becomes enormously elongated, contorted, and unsymmetri- 

 cally disposed, and so the membrane which holds it enclosed and 

 attaches it to the dorsal wall of the abdominal cavity, becomes 

 necessarily drawn out and folded in a very complex manner ; and 

 this complexity is increased by the fact that layers of the membrane 

 which are primitively distinct grow together with contact, till they 

 appear to be but one membrane. 



Folds of the peritoneum which retain the primitive condition 

 and still suspend portions of the alimentary canal from the mid- 

 dorsal region of the abdominal cavity, are termed mesenteries ; folds 

 of peritoneum which pass from one viscus to another, are called 



