64 GENERAL ANATOMY OF MUCOUS MEMBRANES. 



successively in thickness as we examine it in the following 

 order; in the gums, the palate, nasal cavities, stomach, small 

 and large intestines, gall-bladder, urinary bladder, sinuses of 

 the head, and in the excretory ducts : in the last its tenuity 

 becomes extreme. 



The squamous or tesselated epithelium or cuticle which as 

 was remarked vol. i. p. 438, is extended upon this membrane 

 from the skin covering the lips, terminates at the cardiac orifice 

 of the stomach ; that which passes in at the anus and vulva, 

 terminates upon the sphincter muscle and the lips of the 

 womb. In the stomach and intestines, the mucous mem- 

 branes are apparently covered with a coating or varnish 

 of mucous matter, which resembles the epithelium in its chemi- 

 cal composition, and seems to be even susceptible of being 

 converted into it, when dried and hardened by exposure to the 

 air, as has been observed in cases of artificial anus. But when 

 examined under a powerful microscope, the membrane is really 

 found covered with a pellicle or epithelium analogous to the 

 cuticle, which though not provided with vessels, is formed by a 

 deposit or secretion of nucleated cells from the chorion below. 

 These cells, lining hollow passages are constantly moistened 

 with fluids, are not steadily subjected to pressure, and develop 

 themselves in the form of oblong cylinders, arranged in a 

 single series like little columns perpendicularly to the surface 

 of the dermis of the membrane, to which their inner end is 

 attached. This pellicle or epithelium, is accordingly called the 

 cylinder epithelium. Each cylinder or nucleated cell contains 

 a central nucleus, and is broadest at the top. They are all 

 arranged as thickly as may be and in juxtaposition. The epi- 

 thelium here retains to the last its original cellular structure, 

 and is not flattened as upon the skin, &-c., so as to be converted 

 into scales, as in the squamous or tesselated epithelium. The 

 epithelial cells or cylinders are thrown off from time to time 

 and are mixed with the mucus, while their place is supplied 

 by new growths from the surface of the chorion. The cylinder 

 epithelia, as before observed, are met with elsewhere than in 

 the stomach and intestines, viz., in the nostrils, trachea, uterus 

 and gall-bladder. 



