184 HUMAN PHYSIOLOGY 



intestine is concerned, into an outer layer in which the 

 fibers have a longitudinal direction and an inner and 

 thicker one in which they are transverse or circular. 

 The cells in these layers are of the smooth type (Chapter 

 IV). 



Inside' the muscular coats there is more or less loosely 

 woven tissue rich in blood-vessels and nerves. Still 

 ,within this arid next to the hollow of the canal is the 

 epithelium or mucous membrane which calls for a careful 

 description. It is a single layer of cells which are indi- 

 vidually prismatic in form with their long dimension 

 vertical to the free surface. All that enters the body 

 from the alimentary system must pass through or be- 

 tween these cells. The term mucous is used because this 

 epithelium is overlaid with a slimy film more or less 

 protective in function, the product of certain specialized 

 cells. 



The lining epithelium is not to be thought of as a 

 smooth expanse; it is raised into many prominences and 

 depressed into many pockets. The pockets of the 

 epithelium are the minute glands to which reference has 

 already been made. We must now take pains to make 

 clear the relations of such glands. Many variations in 

 their shape are to be found but we may assume that a 

 fair type is furnished by a slender cylindric pit. Such 

 a pit may be likened to a well, the cells in its walls being 

 quite suggestive of regularly ordered masonry. But 

 it must be pointed out that the cells close in the bottom 

 of the gland where no stones would usually be laid in 

 a well. 



The secretion from a gland is produced by the cells 

 which bound the cavity. These cannot go on discharg- 

 ing water and dissolved substances unless their losses 

 are made good. They must receive supplies from the 

 lymph which lies at their submerged extremities. The 

 lymph itself is a limited source and must be renewed by 

 the blood which is led through a network of fine vessels 

 in close proximity to the secreting cells. A gland ap- 



