MUCOUS MEMBRANE. 97 



bone, where the former glides over the latter : and between the skin and various 

 subcutaneous bony prominences. 



The svnovial membranes are composed essentially of connective tissue, con- 

 taining numerous vessels and nerves. It was formerly supposed that these mem- 

 branes Avere analogous in structure to the serous membranes, and consisted of 

 a layer of flattened cells on a basement-membrane. No such cells, however, 

 exist, and the only ones found on the surface are irregularly branched 

 connective-tissue corpuscles, similar to those found throughout the tissue. Here 

 and there these cells are collected in patches and present the appearance of epi- 

 thelium, but do not possess the true characters of an endothelial layer. They 

 are surrounded and held together by an albuminous ground-substance. A 

 further description of the svnovial membranes will be found in the descriptive 

 anatomy of the joints. 



MUCOUS MEMBRANE. 



Mucous membranes line all those passages by which the internal parts com- 

 municate with the exterior, and are continuous with the skin at the various orifices 

 of the surface of the body. They are soft and velvety, and very vascular, and 

 their surface is coated over by their secretion, mucus, which is of a tenacious con- 

 sistence, and serves to protect them from the foreign substances introduced into 

 the body with which they are brought in contact. 



They are described as lining the two tracts the gastro-pulmonary and the 

 genito-urinary : and all, or almost all, mucous membranes may be classed as 

 belonging to and continuous with the one or the other of these tracts. 



The external surfaces of these membranes are attached to the parts which they 

 line by means of connective tissue, which is sometimes very abundant, forming a 

 and lax bed. so as to allow considerable movement of the opposed surfaces 

 on each other. It is then termed the submucous tissue. At other times it is 

 exceedingly scanty, and the membrane is closely connected to the tissue beneath ; 

 sometimes, for example, to muscle, as in the tongue ; sometimes to cartilage, as 

 in the larynx : and sometimes to bone, as in the nasal fossae and sinuses of the 

 skull. 



In structure a mucous membrane is composed of corium and epithelium. The 

 epithelium is of various forms, including the squamous, columnar, and ciliated, 

 and is often arranged in several layers (see Fig. 11). This epithelial layer is 

 supported by the corium, which is analogous to the derma of the skin, and con- 

 sists of connective tissue, either simply areolar or containing a greater or less 

 quantity of lymphoid tissue. This tissue is usually covered on its external surface 

 by a transparent structureless basement-membrane, and internally merges into the 

 submucous areolar tissue. It is only in some situations that the basement-mem- 

 brane can be demonstrated. The corium is an exceedingly vascular membrane, 

 containing a dense network of capillaries, which lie immediately beneath the 

 epithelium, and are derived from small arteries in the submucous tissue. 



The fibro-vascnlsr layer of the corium contains, besides the areolar tissue and 

 is unstriped muscle-cells, which form in many situations a definite layer, 

 called the mus>-uJ<iri* mu<:-o(e. These are situated in the deepest part of the mem- 

 brane, and are plentifully supplied with nerves. Other nerves pass to the epi- 

 thelium and terminate between the cells. Lymphatic vessels are found in great 

 abundance, commencing either by caecal extremities or in networks, and com- 

 municating with plexuses in the submucous tissue. 



Imbedded in the mucous membrane are found numerous glands, and project- 

 ing from it are processes (villi and papilla) analogous to the papillae of the skin. 

 These glands and processes, however, exist only at certain parts, and it will be 

 more convenient to defer their description to the sequel, where the parts are 

 described as thev occur. 



