144 THE AMERICAN MONTHLY [May, 



gle layer of columnar living cells, which, by their ab- 

 sorptive affinity for the staining fluids, are plainly shown 

 to be active living cells. These cells like epidermis gen- 

 erally, grow most rapidly on the lower layer, where they 

 are nourished from the blood in the capillaries directly 

 beneath them. Their multiplication results in crowding, 

 on the lower level, which pushes up some while it leaves 

 the rest. Some cement substance secreted by the cells 

 causes them to cohere, and since they are less well nour- 

 ished they degenerate in form and composition from the 

 columnar through the rounded to the flattened shape, 

 probably as a result of the pressure which is brought to 

 bear on them from the outer surface. The removal of 

 cells from the surface as a result of friction there, is made 

 good by the fresh arrivals sent up from below, as a result 

 of the multiplication which is constantly going on in 

 the lower living layer. 



The structure of the glands in the middle coat p. 143 is 

 similar to that of the salivary gland, it is a typical race- 

 mose gland, the wedge shaped cells, which line the alve- 

 oli leave a central lumen into which their secretion can 

 be drained into the duct and carried out to the surface 

 where it is to be of service. The duct on the other hand 

 is lined with cells whose shape and position does not 

 adapt them especially for work as secreting agents, there 

 is thus here a case of that division of labor and diversity 

 of form which is so prominent a fact in biology. 



The most of the alimentary canal is lined with a mus- 

 cular coat composed of unstriped muscular tissue, the 

 structure of which will be more particularly pointed out 

 in connection with the stomach and intestine. Some of 

 that tissue enters into the wall of the lower end of the 

 gullet, but the most of the wall is composed of striped 

 muscle. The striped muscle is used most commonly in 

 the body, in the formation of the muscles which are em- 

 ployed to move the bones. The reason for this is that 



