1144 GENERAL ANATOMY OR HISTOLOGY. 



But this communication is closed during life, except at the moment of the passage 

 of the ovum out of the ovary into the tube, as is proved by the fact that no inter- 

 change of fluids ever takes place between the two cavities in dropsy of the perito- 

 neum or in accumulation of fluid in the Fallopian tubes. 1 The serous membrane 

 is often supported by a firm, fibrous layer, as is the case with the pericardium, and 

 such membranes are sometimes spoken of as "fibro-serous." 



The various serous membranes are the peritoneum, lining the cavity of the 

 abdomen ; the two pleurae and the pericardium, covering the lungs and heart respec- 

 tively ; and the tunicae vaginales, surrounding each testicle in the scrotum. 2 

 Serous membranes are thin, transparent, glistening structures, lined on their inner 

 surface by a single layer of polygonal or pavement endothelial cells, supported on 

 a matrix of fibrous connective tissue, with networks of fine elastic fibres, in which 

 are contained numerous capillaries and lymphatics. On the surface of the endo- 

 thelium between the cells numerous apertures or interruptions are to be seen. Some 

 of these are stomata, surrounded by a ring of cubical endothelium (see Fig. 684), 



FIG. 684. Part of peritoneal surface of the central tendon of diaphragm of rabbit, r prepared with nitrate 

 of silver, s. Stomata. I. Lymph-channels, t. Tendon-bundles. The stomata are surrounded by germinating 

 epithelial cells. (From Handbook for the Physiological Laboratory, Klein.) 



and communicating with a lymphatic capillary ; others ( pseudostomata) are mere 

 interruptions in the endothelial layer, and are occupied by processes of the branched 

 connective-tissue corpuscle of the subjacent tissue or by accumulations of the inter- 

 cellular cement-substance. 



The amount of fluid contained in these closed sacs is, in most cases, only suffi- 

 cient to moisten the surface, but not to furnish any appreciable quantity of fluid. 

 When a small quantity can be collected, it is found to resemble lymph, and like 

 that fluid coagulates spontaneously ; but when secreted in large quantities, as in 

 dropsy, it is a more watery fluid, but still contains a considerable amount of proteid 

 which is coagulated on boiling. 



1 The communication between the uterine cavity and the peritoneal sac is not only apparent in 

 the dead subject, but is an anatomical fact, which is established by the continuity of its epithelium 

 with that covering the uterus, Fallopian tubes, and fimbrise. 



2 The arachnoid membrane, lining the brain and spinal cord, was formerly regarded as a serous 

 membrane, but is now no longer classed with them, as it differs from them in structure, and does not 

 form a shut sac as do the other serous membranes. 



