THE INTAKE OF MATERIALS AND ENERGY 65 



and most of the vitamins require no special process to prepare them to 

 pass through the walls of the intestine. For them the digestive system 

 merely provides a sufficient area of permeable surface through which 

 they may pass into the blood and lymph of the circulatory system. The 

 carbohydrates, proteins, and fats, on the other hand, are taken into the 

 digestive tract in a form in which they cannot pass through the intestinal 

 walls and must be first digested (broken down) into simpler substances — 

 monosaccharides, amino acids, and fatty acids and glycerol. 



Digestive enzymes. Digestion involves both mechanical and chemical 

 processes, the latter chiefly of a type known as enzyme action. Enzymes, 

 sometimes termed organic catalysts, have the power to hasten enormously 

 certain chemical reactions that otherwise would take place very slowly. 

 The digestive enzymes act upon the carbohydrates, proteins, fats, and 

 certain of their derivatives and break them down into simpler chemical 

 compounds until the final products are able to permeate the walls of the 

 intestine. This process is greatly facilitated and quickened by the me- 

 chanical maceration that is also a part of the digestive process. The 

 complete process and the structures that accomplish it can be best 

 described by following the sequence of events when a meal is taken into 

 the digestive system. 



The mouth and esophagus. Digestion and the digestive system 

 begin with the mouth. As the food is chewed, it is mixed with saliva, 

 the watery product of three pairs of mouth glands. Saliva is nearly 

 neutral in reaction and contains two enzymes, ptyalin and maltase, which 

 act upon certain of the carbohydrates. Chewing not only permits carbo- 

 hydrate digestion to begin but prepares the food for swallowing by break- 

 ing it into smaller pieces and forming it into an easily swallowed paste. 

 The voluntary muscles of the tongue and pharynx perform the action of 

 swallowing, i.e., passing the food through the pharynx into the esophagus. 

 (Since the pharynx is a common part of both the digestive and pulmonary 

 systems and a passageway for both food and air, the air passages that 

 connect with the pharynx are closed in the act of swallowing. The posterior 

 ends of the nasal passages are closed by the soft palate, and the opening 

 of the upper end of the windpipe by a lid known as the epiglottis.) 



The wall of the esophagus, like those of the stomach and intestine that 

 follow it, is composed of five layers of tissue. There is first an inner epi- 

 thelial layer, called the mucosa; this is surrounded by a layer of connective 

 tissue, the submucosa; then come two layers of smooth (involuntary) 

 muscle tissue, 1 and finally, on the outside, there is a smooth, thin, moist 



1 The muscles of the walls of the digestive tract are typical of visceral muscle in 

 general. The cells making up this type of contractile tissue are flattened and spindle- 

 shaped. Unlike skeletal muscle cells, they possess only a single nucleus and are not 

 striated — i.e., they do not possess the cross-bandings so typical of skeletal or voluntary 



