THE PERITONEUM. 



175 



infolding of the mucosa, submucosa and greater part of the muscular tunic, 

 a portion of the longitudinal muscle passing over the creases externally. 

 The shortness of the muscular bands, into which the longitudinal muscle is 

 condensed in front and behind, serves to maintain these transverse folds. 

 Where the peritoneum is wanting, as it is except over part of the anterior 

 aspect of the rectum, the serous coat is replaced by a fibrous one composed 

 of fibro-elastic tissue. 



THE PERITONEUM. 



The peritoneum, the serous membrane lining the abdominal cavity and 

 covering more or less completely the therein contained organs, consists of a 

 connective tissue stroma and the surface layer of mesothelium. The latter 

 is a single layer of plate-like cells (Fig. 25) irregularly polygonal in form 

 and of varying size, whose contours are mapped 

 out, after staining with silver nitrate, by delicate 

 sinuous dark lines that correspond with the particles 

 of reduced silver in the intercellular cement-sub- 

 stance. Each cell encloses an oval flattened nucleus, 

 usually somewhat eccentrically placed, that is almost 

 invisible until tinged with some appropriate dye. 

 The size and form of the mesothelial plates vary 

 much with the tension to which they are subjected; 

 when unduly stretched, they are often imperfect or, 

 indeed, displaced. 



The stroma consists of a feltwork of con- 

 nective tissue bundles of variable fineness, those 

 of the parietal being commonly more robust than 

 those of the visceral peritoneum. The deeper part 

 of the stroma contains numerous elastic fibres, 

 which are most abundant and developed in the 

 parietal sheet, where they form a distinct network. 

 Seldom in the mesenteries but constantly in the 

 omenta, the stroma undergoes partial absorption, 

 whereby larger or smaller openings, fenestra, result 

 (Fig. 38). In this manner what originally was a 

 continuous sheet becomes a fenestrated membrane, 

 over which the mesothelium stretches as an un- 

 broken covering, investing the trabeculae as well 

 as the parts still retaining the character of membranes. The nuclei of the 

 connective tissue cells and those of the mesothelial plates are seen inter- 

 mingled. Although all the important peritoneal folds, as the mesenteries, 

 omenta and many of the so-called ligaments of the viscera, theoretically 

 include two layers of serous membrane and an intervening layer of connective 

 tissue prolonged from the body-wall, in which course the vessels and nerves 

 supplying the organs, such duplicatures consist essentially of a general 

 connective tissue stroma-layer covered on each side by a stratum of meso- 

 thelium. Wherever two peritoneal surfaces are brought into permanent 

 contact, the mesothelium disappears and the serous character of the attach- 

 ment is lost, the union henceforth being one of fibrous tissue. Where readily 

 movable, as over most parts of the abdominal and pelvic walls and many folds, 

 the attachment of the peritoneum to the subjacent parts is effected by a layer 

 of fat-laden subserous tissue. This fibro-elastic layer varies in thickness, 

 but in many places, as over the liver, stomach or intestine, where the perito- 



Peritoneal 

 coat 



:i-~.J Adipose 

 J tissue 



5 Vein 



Artery 



FIG. 218. Longitudinal sec- 

 tion of an epiploic appendage. 



X22. 



