I STRUCTURE AND PHYSIOLOGY OF ANIMALS 35 



glands open always, not into the digestive, but into some part of 

 the ingestive region of the alimentary system. 



The most important function of the liver properly so called 

 is one distinct from the process of digestion ; its secretion the 

 bile has, however, at least a mechanical effect on this process, 

 and assists the secretion of the pancreas in its effects upon fat. 

 In lower forms the organ to which the term liver is commonly 

 applied appears in many cases to combine the functions of a true 

 liver with that of a pancreas, and is thus more appropriately 

 termed hepato-pancreas or liver-pancreas. 



The pancreas secretes a fluid the pancreatic juice which has 

 a very important effect in digestion. It renders substances of the 

 nature of albumins soluble by converting them into modifications 

 termed peptones ; it converts starch into the soluble substance 

 sugar ; it acts on fatty matters in such a way as to convert them 

 into emulsions which are capable of being taken up and absorbed, 

 and it effects the splitting up of part of the fat into fatty acids 

 and glycerine. 



When the food has been acted on by the various digestive 

 secretions, the soluble part of it is fitted to be taken up and 

 absorbed through the wall of the alimentary canal into the blood 

 (in animals in which a blood-system exists), or into the fluid 

 which takes its place. In the higher animals a part of the 

 soluble matter of the food passes directly into the blood contained 

 in the blood-vessels ; while another part is taken up by a set of 

 special vessels, the lacteals, which are a part of the lymphatic 

 system, and reaches the blood indirectly. 



In some of the lower groups of animals there is no system of 

 blood-vessels, and the nutrient matter of the food, absorbed 

 through the alimentary canal, merely passes from cell to cell 

 throughout the body, or is received into a space or series of spaces 

 containing fluid intervening between the alimentary canal and the 

 wall of the body. But in the majority of animals there is a system 

 of branching tubes containing a special fluid the blood, and it is 

 into this that the nutrient matter absorbed from the food sooner 

 or later finds its way. The blood has for one of its principal 

 functions the conveyance of the nutrient matters from the 

 alimentary canal throughout the body, so that the various organs 

 may select from it the material which they require for the carrying 

 on of their functions. To carry out this office the blood is con- 

 tained in a complicated system of branching tubes or blood-vessels. 



The essence of the process of respiration, as we have already 

 seen, is an interchange of oxygen and carbonic acid which takes place 

 between the tissues of an organism and the surrounding medium, 

 whether air or water. During the vital changes which go on in 

 the bodies of all animals, as in Amoeba, oxygen is constantly 

 being used up and carbonic acid being formed. The necessary 



D 2 



