282 HISTOLOGY 



of the food eaten. The calcium carbonate glands of the earthworm are 

 cesophageal glands which have arisen, as is held by some investigators, 

 to supply an alkali to neutralize the great amount of acid taken in with 

 the food. Many of the so-called salivary glands of vertebrates have been 

 differentiated as structures to supply fluids that mechanically aid diges- 

 tion by furnishing a fluid for softening and lubricating the food. These 

 glands are known by the character of the secretions given off as mucous 

 glands. The mucous glands are represented by the palatine, the dou- 

 denal (Brunner's) glands, and the glands of the large intestine. These 

 glands vary in complexity, but all elaborate a thick mucus, which serves 

 primarily to lubricate the food on its passage into and out of the ali- 

 mentary canal. 



The regions of the alimentary tube, which are little more than 

 passageways for food materials, form another class of accessory tissues. 

 These regions are always tubular, with lumina that are narrow when not 

 functioning and with highly developed walls. The muscular coats are 

 thickened and supported by an intensified connective tissue. The 

 blood supply in these regions is less than in the more active digestive 

 regions of the alimentary tube. The epithelium of a conducting region 

 is always heavy and suited to withstand abrasion. In some cases a 

 heavy cuticle of chitin or other dense material is formed. When this 

 protecting cuticle is developed, the epithelium is simple and columnar. 

 Resistance to abrasion in the absence of a cuticle is met by stratification 

 of the lining cells. In most cases there are frequent strata of the rest- 

 ing cells. In many cases certain digestive and accessory glands pour 

 digestive and lubricating fluids into these conducting tubules. The 

 pharynx and oesophagus are examples of such structures. 



Each cell of an absorbing tissue must come into actual contact with 

 the food, and consequently we find these epithelia in the central cavity 

 of the digestive tract to which the food is confined; on the other hand, 

 as long as the digestive tissues have a canal leading from them to the 

 lumen of the alimentary tube, they may retreat as glands to remote and 

 various regions of the body. Digested foods vary much less than foods 

 not digested. Because of these two facts we find absorbing cells and 

 tissues much less differentiated than digestive cells and tissues. 



Absorbing cells are not well defined. Indeed, it may be said that 

 all cells coming in contact with digested food may to a certain degree 

 be absorptive, despite any peculiar structure they may have. The most 

 efficient absorbing tissue is one that presents the greatest number of 

 living cells in contact with digested food. In regions where we have 

 stratified epithelium, therefore, we find the least efficient absorptive 

 tissues. A characteristic absorbing epithelium is always a simple co- 

 lumnar one ; such a tissue stands exposed to the lumen of the enteron ; 



