260 VERTEBRATES. 



THE LIFE PROCESS IN VERTEBRATES. 



The life process is essentially the same in all animals, 

 differing in different animals only in the means by which 

 it is carried out. The perfection of the means the 

 adaptation of the means to new and higher conditions 

 of life has made the vertebrates the dominant group 

 among animals. 



I. Nutrition. The organs subservient to nutrition have 

 been grouped as organs (1) of digestion, (2) of circulation, 

 (3) of respiration, and (4) of excretion; and to these 

 might be added (5) organs of prehension, for grasping food, 

 and (6) organs of mastication, for chewing it, although the 

 organs of the last two groups are not exclusively sub- 

 servient to the nutritive function, and are not present in 

 all vertebrates. 



1. Digestion takes place in the alimentary canal, parts 

 of which are specialized for the reduction of the food, and 

 other parts for the absorption of it when digested. Four 

 digestive secretions are poured upon the food normally 

 in its course, saliva, from the salivary glands ; gastric 

 juice, from glands in the walls of the stomach ; bile, from 

 the gall bladder of the liver ; and pancreatic juice, from 

 the pancreas. The greater part of the digested food is 

 absorbed through the walls of the intestine, to be passed 

 into the circulation. In the highest vertebrates it is col- 

 lected by the lacteals of the mesentery into the thoracic 

 duct, and poured thence directly into the venous system. 



2. Circulation is effected through a closed system of 

 blood vessels, consisting of the heart, arteries, veins, and 

 capillaries. Through this system the food dissolved in the 

 blood is carried to every vascular part of all the tissues. 

 The three types of circulation found in the vertebrates 

 studied are roughly outlined in the accompanying dia- 



