=The Peritoneum.=—During life and in the uncut corpse the peritoneal cavity is air-tight. It is not a real cavity, as muscular tension and atmospheric pressure permit no vacant space to form. When the surgeon or embalmer opens the abdomen, the peritoneal cavity is at that moment produced. The peritoneum is the largest serous membrane in the body. In the male it is a closed sac, a part of which is applied against the abdominal sides, while the remainder is reflected over the contained viscera. In the female it is not a closed sac, since the free extremities of the fallopian tubes open directly into the peritoneal cavity. _The parietal peritoneum_ is that portion applied against the abdominal sides. _The visceral peritoneum_ is that portion reflected over the viscera. The peritoneum consists of two sacs. _The greater sac_ lines the greater part of the abdominal cavity as almost all of the viscera are covered by it. _The lesser sac_ is placed behind the stomach. These two sacs communicate with each other by a narrow orifice called the _Foramen of Winslow_. The peritoneum, as it covers different organs or sets of organs, receives special names. _The lesser omentum_ consists of two layers, these split to envelope the stomach. _The greater omentum_ consists of four layers. Two of these layers extend from the stomach and together with two other layers of the same structure which envelope the transverse colon, form an apron for the intestines. _The mesentery_ consists of two layers which invests the small intestines. Between the two layers of the mesentery we find the blood vessels, nerves, lacteals, and glands, leading to and from the intestines. The mesentery is fan shaped, and is attached to the second lumbar vertebra. The length of the mesentery fan is about eight inches from commencement to termination at intestine. It extends the whole length of the intestines, which is about twenty feet. CHAPTER IX. THE VASCULAR SYSTEM. =The Vascular System.=—The vascular system is composed of the organs immediately concerned in the circulation throughout the body of the fluids which convey to the tissues the nutritive substances and oxygen necessary for their metabolism and carry from them to the excretory organs the waste products formed during metabolism. The system is usually regarded as being composed of two portions, the one consists of organs in which circulate the red fluid which we term blood, and called the _blood vascular system_, while the organs of the other contain a colorless or white fluid known as lymph or chyle, and is known as the lymphatic circulation. { arteries { Blood vessels { capillaries The vascular { The blood vascular system { { veins system { { Heart { The lymphatic circulation