SECRETING GLANDS. 87 



the other. 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 interchange of fluids ever takes place between the two cavities in 

 dropsy of the peritoneum, or in accumulation of fluid in the Fallopian tubes. 

 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." In the parietal portion of the arachnoid there is, according to many 

 anatomists, no serous membrane in the proper sense of the term ; but the dura 

 mater is merely lined with a layer of epithelium, the basement-membrane being 

 here indistinguishable. In other situations, the following parts may be recog- 

 nized as constituting a serous membrane. 1. The epithelium, a single layer of 

 polygonal or pavement-epithelial cells. 2. A structureless basement-membrane. 

 3. The connective tissue and vessels which support the latter, connect it with 

 the parts below, and supply blood to its deep surface. Some of the serous 

 portion of the blood is secreted, or transudes, through the basement-membrane 

 to furnish the special secretion. This latter is, in most cases, only in sufficient 

 quantity to moisten the membrane, but not to furnish any appreciable quantity 

 of fluid*. When a small quantity can be collected, it -appears to resemble in 

 many respects the lymph, and like that fluid coagulates spontaneously, but 

 when secreted in large quantities, as in dropsy, it is a watery fluid containing 

 usually sufficient albumen to gelatinize with heat. 1 



The Mucous Membranes are more complex in their structure than the serous. 

 Their epithelium is of various forms, including the spheroidal, columnar, and 

 ciliated, and is often arranged in several layers (see Fig. 46). This epithelial 

 layer is supported by the corium, which is analogous to the derma of the skin; 

 and is in fact continuous with it at the orifices of the body. The corium con- 

 sists, as it is usually described, of a transparent structureless basement-mem- 

 brane next to the epithelium, supported by a fibro- vascular layer of variable 

 thickness below it, and this merging into the submucous areolar tissue. It is 

 only in some situations that the basement-membrane can be demonstrated. 



The fibro- vascular layer of the corium contains, beside the white and yellow 

 fibrous tissue and the vessels, muscular fibre-cells, nerves, and lymphatics, in 

 various proportions. Imbedded in it are found numerous glands, and project- 

 ing out of it are processes (villi and papillae) analogous to the papillae of the 

 skin. These glands and processes, however, exist only at certain parts, and it 

 will be more convenient to refer for their description to the sequel, where the 

 parts are described in which they occur. Thus the mucous glands are described 

 in the account of the mouth, the stomach, the intestines, &c., the papillae and 

 villi with that of the tongue and the small intestine. 



The Synovial Membranes are analogous in structure to the serous, but differ 

 from them in the nature of their secretion, which rather resembles mucus. 

 They are described in connection with the Articulations. 



SECRETING GLANDS. 



The Secreting Glands are organs in which the blood circulating in capillary 

 vessels is brought into contact with the epithelial cells of a mucous membrane, 

 whereby certain elements are separated ("secreted") out of the blood, and are 

 poured into the mucous cavity. This cavity is arranged in the form of a rami- 

 fying duct, the secreting cells lying in, or touching, the terminal ramifications 

 (or more correctly the commencing radicles) of the duct. 



In size these glands vary extremely: thus the liver weighs nearly four 

 pounds, while many of the mucous glands are only visible to the naked eye 

 when distended with secretion ; and they vary not less in structure. Thus the 



1 The resemblance between lymph and serum led Hewson to the belief that the serous cavities 

 are sacs into which the lymphatics open. 



