LUBRICATING TISSUES 



393 



Hum that lines the whole intestine. Another example, which does not 

 so well illustrate the principle, is seen hi the sac-shaped mucous glands 

 which open out on to, 

 and are derived from, 

 the olfactory epithe- 

 lium of the nasal cav- 

 ity. This epithelium 

 is hardly simple, how- 

 ever, and a still sim- 

 pler and better 

 example is seen in 

 the many tubular 

 mucous glands that 

 open into the lumen 

 of the uterus of many 

 mammals, as the 

 cat. 



Of mucous glands 

 which are derived 

 from a stratified epi- 

 thelium we have many 

 examples, and such 

 glands may be noticed 

 in the mucous glands 

 of the Amphibia. 

 These are each com- 

 posed of a few typical 

 mucous cells that are in communication with the exterior by means of 

 a short duct that is also, of course, constructed of the epithelial cells. 

 In many cases the duct is constructed of a single cell, and the 

 cells immediately connecting it with the body of the gland are specialized 

 to control the flow of the secretion. When the epidermis is shed in the 

 periodical molts of the animal, the cell or cells that form the duct are 

 shed with it. This form of gland is well shown in Figure 357 from the 

 toad. The gland-cells in this and the following case appear to represent 

 the basal layer of the stratified epithelium from which the gland was 

 developed. 



Such a simple form of invaginated gland is also represented in the 

 higher forms (mammals) by the mucous glands found in the posterior 

 portion of the tongue. Here a large number of glands like those found 

 in the salamander are joined together and empty the secretion out through 

 a common duct that has many branches. Still larger collections of the 

 same structures form some of the large, salivary glands that lie far back 



FIG. 357. A many-celled, succular, mucous gland from the 

 skin of a toad, str.ep., stratified epithelium on surface; 

 sec.ep., secreting epithelium in gland; b.m., basement mem- 

 brane; d., duct; Pg.c., pigment cells. X 520. 



