10.14.96 dr.dodson peritoneal cavity handout: the peritoneal cavity refs: sisson and grossman pp 87-95; 100-103 dyce pp 123-126; 145-150 peritoneal cavities are lined w/peritoneum. nature of these cavities is they are lines w/serous cavities and the abdominal peritoneal cavity is EMPTY. there is nothing in there except a bit of fluid. the organs are not IN the abdominal peritoneal cavity. fist in ballon analogy. organs may be SUSPENDED within the peritoneal cavity, but topographically they are outside of it. organs suspended in the cavity are called INTRAPERITONEAL, but they are suspended by peritoneal membranes. Other organs (kidney) attached to body wall - RETROPERITONEAL.- covered on one surface only by peritoneum. note that cats/horses/cows/goats kidneys are more intraperitoneal than retroperitoneal. bladder- when full, is intraperitoneal, when empty is generally retroperitoneal. so the line is kind of blurred. development of peritoneum....back to the embryo. this is the first time in a while that embryology is being taught this semester.....three layers of developing embryo....ectoderm, mesoderm, endoderm. early in development, there is an invagination of the early ectoderm which closes off as the neural tube; and the endoderm grows down from both sides and closes off a tube as well, which is the gut. so a tubular animal is forming. in between ectoderm and endoderm is the mesoderm,which is incredibly important. and part of the mesoderm grows down and adheres to the ectodermal body wall, and part of it grows down and adheres to the endodermal gut. the outer sheet is the parietal peritoneum, and the internal part is the visceral peritoneum, and the space in between is the peritoneal cavity. mostly a 'potential' space, in that there isn't really empty space unless there is free gas in there. there are no organs in the peritoneal cavity, except sometimes an ovum escapes and gets lost and wanders free in there instead of being caught by the fallopian tube. early gut= archenteron enteron=gut dysentery mesentery at this stage we have two mesenteries - ventral and dorsal. heart and liver are present as well. note that there is one peritoneal cavity running the length of the body, at firsst - no abdomen v thorax. but now we get a separation developing ventrally between the heart and the liver. this separation is called the transverse septum. it starts as a membrane. and tissue begins then to fold in from the sides, forming the pleuropericardial membrane. so, soon, it's two walled off cavities, the abdomen and the thorax. the lungs develop into the thoracic cavity where the heart is already present. eventually you see development of the diaphragm, which provides the definitive separation between the thorax and abdomen. the development of the diaphragm happens late in the development, though. in birds, separation of cavities is purely membranous. diaphragm found only in mammals. a major portion of the diaphragm is tendinous - called "central tendon" or central tendinous region in the adult. this is remnant of embryologic transverse septum. otherwise, diaphragm is largely muscular. the central muscle of the diaphragm is a neck muscle, not a thoracic or abdominal muscle. it is derived from ventral neck musculature. the phrenic nerve, which innervates the diaphragm, arises from cervical nerves 5,6 and 7. so the diaphragm must have migrated down from the neck embryologically. the neck, too, is an invention. fish don't have necks - their hearts are between the gills, just caudal to the head. differentiation of neck only occured after animals came out of the water into land. the neck developed phylogenetically in that neck musculature developed as the heart migrated backwards. so, diaphragm is made of central tendon, a neck muscle, and some minor representation of the intercostal muscles. going back to our abdomens....you have the gut tube and it begins to swell cranially in the foregut, becoming a primitive stomach. ventral to the stomach, certain organs start developing - liver, bile duct, pancreas(pancreas actually grows part dorsal and part ventral to stomach). we already know gut is suspended by mesentery. so, liver, gall bladder, and R lobe of pancreas develop in the ventral mesentery, and the L pancreas develops in the dorsal mesentery. grossly, you don't really see the stomach hanging from a dorsal mesentery. the stomach undergoes considerable modification and rotation - in the horizontal plane, developing a curvature - the lesser curvature being the "inside" of the curve, facing the liver, and the greater curvature, the "outside" of the curve. the stomach also rotates in a transverse plane, such that the "dorsal mesentery" which is now the mesogastrium is stretched out. there is a pocket like structure next to the stomach. the dorsal mesogastrium continues to stretch and grow until eventually you have a very long - absurdly long - structure, which is no longer a "supporting" structure, really, because it is too long and stretchy to support anything, but it is now called the greater omentum, which is a hypertrophy, if you will, of the mesogastrium. a lacey structure with an opening in the middle, the omental bursa. omentum= apron so, the greater omentum goes from dorsal body wall to greater curvature of the stomach, but it's very large and stretchy and doesn't do much to support the stomach. the lesser omentum goes from the lesser curvature of the stomach to the liver to the ventral body wall. ventral omentum bet. liver and stomach = hepatogastric ligament. the remnant of ventral mesentery bet liver and body wall is falciform ligament. caudally, the ventral mesentery is the median ligament of the bladder. three expressions of ventral mesentery in adult dog are hepatogastric ligament, falciform ligament, and median ligament of bladder. note that when you have a reflection of the parietal peritoneum toward an organ, it is referred to as a LIGAMENT, a reflection of peritoneum off the body wall toward an organ. the attachement bet diaphragm and liver = coronary ligament. moving on to the pelvic cavity. two ways to view this. cross section and longitudinally. still have the same tube (gut) running through body - rectum; also reproductuive organs (uterus) and urinary bladder. these organs exit to the outside world through their respective openings caudally, and they also protrude into the peritoneal cavity. note that the peritoneal lining does NOT go all the way caudally through the bony pelvis. it forms a series of pouches (see diagram). there is peritoneum suspending the rectum, uterus and bladder pubovesicular pouch- between pubis and bladder vesicogenital pouch- between bladder and uterus rectogenital pouch - between uterus and rectum bladder suspended by median ligament to pubis, and lateral ligaments to pelvis uterus supported by broad ligaments rectum supported by mesorectum non-peritoneal space in which rectum etc pass through is called the ischeorectal fossa. this is an important space. don't forget it. dodson likes it. this is the space ADJACENT to the rectum, caudal to the pouches. the internal obturator and the levator ani and coccygeal and the anal glands are in this fossa.