I 3 2 PHYSIOLOGY FOR BEGINNKKS , HAP. 



and palatability have to be taken into account. Thus peas 

 contain a larger amount of proteids than meat, and are, more- 

 over, very rich in carbohydrates, yet peas are not so valuable a 

 diet as meat, because they are far less digestible. 



The Object of Digestion. From the cavity of the 

 alimentary canal the useful parts of the food the proteids, 

 carbohydrates, fats, salts, and water are absorbed, that is, 

 they pass into the tissue forming the wall of the alimentary 

 canal. The wall of the alimentary canal is richly supplied 

 with blood-vessels and lymphatic vessels, and into these vessels 

 the food materials pass ; the greater part of them, passing into 

 the blood capillaries, reaches the blood stream at once, while 

 the fats, as we shall see, passing into the lymphatic capillaries, 

 reach the blood in an indirect way, by the thoracic duct, 

 but do eventually reach it. The blood then carries the food 

 materials to the tissues of the body. Before the food materials 

 can thus reach the blood they must be rendered capable of 

 passing into the capillaries or lymphatics by being brought into 

 solution or into a state of division into very fine particles. More- 

 over, most of the food materials, if they existed in the blood in 

 exactly the same chemical form as in the food, could not be 

 taken up and used by the tissues. The proteids, carbohydrates, 

 and fats must be brought to the tissues as certain particular 

 proteids, carboh yd rates, and fats, otherwise the tissues cannot 

 take them up assimilate them, as it is called. Therefore the 

 proteids, carbohydrates, and fats of the food have to be changed 

 to the particular proteids, carbohydrates, and fats respectively 

 which are appropriate to the tissues of the body. By the pro- 

 cesses of digestion the food materials are rendered capable of 

 absorption and of assimilation. These changes are chiefly 

 brought about by the action on the food of certain juices which 

 are prepared secreted, as it is called -by certain tissues, and 

 are poured into the cavity of the alimentary canal. There are 

 four chief digestive juices : the saliva, the gastric juice, the 

 pancreatic juice, and the bile. 



The changes which the food undergoes commence in the 

 mouth. It is here masticated by the teeth, and mixed with and 

 acted on by the saliva. 



The Teeth. In the adult there are 32 teeth, 16 in the 

 upper and 1 6 in the lower jaw, the 8 on one side correspond- 



