It was formerly common to compare the peritoneal cavity with a lymph space. This comparison was unfortunate, as it led to a misconception of the nature of the peritoneum. Thanks to the researches of Dr. Sabin and others, our understanding of the lymphatic system is now much more complete. The comparison of the peritoneum to the lymphatic system was due to the fact that there is a close resemblance in structure between the endothelium of the lymphatics and that of the peritoneum. Functionally the comparison is not apt. The peritoneum is the protective covering of the abdominal viscera—nothing more. In order that this function may be fulfilled, the peritoneum must not only serve as an efficient covering, but provision must be made for the movements of the abdominal viscera. This is accomplished by the viscid fluid with which the peritoneum is constantly covered. The peritoneum, therefore, if it must be