But a breakdown of these organic masses into tissular surfaces can and must be made if one is to understand the complexity of function and alteration: the hollow organs are provided with mucous membranes, covered with 'a fluid that usually moistens their free surface and that is supplied by small glands inherent in their structure'; the pericardium, the pleura, the peritoneum, and the arachnoid are serous membranes 'characterized by the lymphatic fluid that ceaselessly lubricates them and that is separated by exhalation from the volume of blood'; the periosteum, the dura mater, and the aponeuroses are made up of membranes 'that are moistened by no fluid.'