Peritoneum, in Latin peritonoeum , in Greek περιτóναιον, from περιτείνω, stretched around , a very large membranous envelope in immediate contact with the internal surface of the transverse muscles and the rest of the cavity of the lower abdomen, which it covers, and which envelops the viscera like a sort of bag.

This membrane is usually a fairly tight, yet very supple tissue that can be extended a good deal, and then regain its ordinary extension or that which it had before. This can be clearly seen in pregnancy, in dropsy, and in people with large stomachs due to stoutness or repletion.

The peritoneum seems to be composed, according to its extension, of two large parts, one internal and one external. Several anatomists have considered it as a duplication of two distinct membranous layers, but strictly speaking, only one should be called the membranous layer, namely, the inner portion, which is, as it were, the body of the peritoneum ; the outer part is only a kind of follicular apophysis of the inner one. It is appropriately called the cellular tissue of the peritoneum .

The real membranous layer, generally called the inner layer , is very smooth on the side facing the cavity and the viscera of the lower abdomen, and its inner surface is always moistened by a serosity that seems to ooze through almost imperceptible pores. The pores can be seen by turning over a part of the peritoneum on the tip of one’s fingers, and by tugging at it from one side and another; we can then distinctly see the dilated pores and little drops of liquid coming out of it, even without a microscope.

The sources of these drops and this serosity of the internal sac of the peritoneum are not yet well understood. They may be the result of transudation or sweating, such as is observed when recently killed animals are dissected. The whitish grains that are found in the peritoneum of certain patients who have died of a disease have no effect on the glands, which are said to remain in their natural state.

The cellular tissue or the outer part of the peritoneum is highly adherent to the parts that make up the internal walls of the cavity of the lower abdomen. This cellular tissue does not have the same thickness in all places; what is more, the tissue looks in places like an adipose membrane, since, like the kidneys, it is filled with fat along the fleshy parts of the transverse muscles to which it is attached.

The large blood vessels, namely the aorta and the vena cava, are also contained in the thickness of the cellular part of the peritoneum . In short, this tissue envelops tightly and individually all the parts and organs that are said commonly to be situated in the duplication of the peritoneum .

The main uses of the peritoneum seem to be to cover the cavity of the lower abdomen; to envelop, like a common sac, the viscera contained in this part; to give them their own tunics or envelopes; to form elongations, ligaments, attachments, coils, sheaths, etc.

The fine dew that oozes out all over the inner surface of the peritoneum prevents any disadvantages that might be due to the continual rubbing and the various degrees of jolting to which the viscera in the lower abdomen are exposed in part naturally and in part as a result of different external movements.

Such is the structure of the peritoneum , according to the description of Messrs Douglas and Winslow, which, although very precise, is insufficient to form an idea of it, which is impossible without demonstrating it; speaking generally, all we can say is that it is a pear-shaped sac compressed at the top, wider in the middle, and tapering away obtusely towards the lower parts. From the lower part of the diaphragm, it descends in front of the iliacus and psoas major muscles, continues down in front of the rectum, and turns back upwards above the bladder, in front of the pubic bone and behind the abdominal muscles; the sac is pierced to let through the esophagus and the rectum; it contains within it the liver, the spleen, the pancreas, and the entire volume of the intestines, along with the stomach. The aorta, the vena cava, the thoracic duct, the kidneys, the neighboring vessels, and most of the rectum lie outside the cavity of the peritoneum in the cellular membrane surrounding it, connecting it to the diaphragm, the transverse muscles, the bladder, the muscles connected to the anus, the psoas major, the iliacus, and the tendinous envelopes of the vertebra of the loin. Its outer surface is supported by solid fibers at the front part of the lower abdomen, and the inside is moistened by a vapor that continually transpires.

The peritoneum is so tightly packed with the viscera it contains that it bears the impression of the intestines; it pushes back the ventricle that the diaphragm lowers when we bend down and opposes a certain resistance to the compression of the abdominal muscles on the stomach, which thus find themselves between two different kinds of pressure, since the lower abdomen is completely full. That is why, when the peritoneum is ruptured, especially during life, the viscera come out only with difficulty from the opening produced in the envelope that keeps them in place. Lastly, this membrane contains the smaller vessels, the epigastria, the spermatic plexus, and other neighboring arteries.