Some general observations concerning the abdominal cavity are necessary before the description of the digestive system continues.
The abdomen is the portion of the trunk that lies caudal to the diaphragm. It contains the largest of the body cavities, which is continuous at a plane passing through the sacral promontory and the pubic brim with the more caudal and very much smaller pelvic cavity (see Fig.
2.2). The more cranial (intrathoracic) part of the abdominal cavity is protected by the hindmost ribs and costal cartilages and is rather restricted in the variations in its size; the more caudal part is supported by the skeleton only on its dorsal aspect and is therefore more variable. The pelvic cavity has the most extensive bony support and the most constant size, although even here a certain latitude is allowed by changes in the soft tissue components of its walls (see Fig. 29.25A and B).The structure of the abdominal and pelvic walls has been described with the locomotor apparatus. Comparative features, including conformation and the factors that influence it in different species, are considered in later chapters. The abdominal and pelvic cavities contain the peritoneal sac; the stomach, small and large intestines, and associated liver and pancreas; the spleen; the kidneys, ureters, bladder, and urethra (in part); the ovaries and most of the reproductive system in the female and a smaller part of the reproductive tract in the male; the adrenal glands; and many nerves, blood vessels, and lymph nodes and vessels.