ORGANIZATION. 43 



are required, as saliva, gastric juice, and bile: these are 

 secreted by special organs, called glands. Moreover, since 

 not all the food eaten is fitted to make blood, and as the 

 blood itself, in going around the body, acts like a scaven- 

 ger, picking up worn-out particles, we have another func- 

 tion, that of excretion, or removal of useless matter from 

 the system. The kidneys and lungs do much of this; but 

 the lungs do something else. They expose the blood to 

 the air, and introduce oxygen, which, we shall find, is 

 essential to the life of every animal. 



These centripetal and centrifugal movements in the 

 body throwing in and throwing out are so related and 

 involved, especially in the lower forms, that they cannot 

 be sharply defined and classified. It has been said that 

 every Dog has two lives a vegetative and an animal. 

 The former includes the processes of digestion, circulation, 

 respiration, secretion, etc., which are common to all life; 

 the functions of the other, as motion, sensation, and will, 

 are peculiar to animals. The heart is the centre of the 

 vegetative life, and the brain is the centre of the animal 

 life. The aim of the vegetative organs is to nourish the 

 individual, and reproduce its kind; the organs of locomo- 

 tion and sense establish relations between the individual 

 and the world without. The former maintain life; the 

 others express it. The former develop, and afterwards 

 sustain, the latter. The vegetative organs, however, are 

 not independent of the animal ; for without muscles and 

 nerves we could not procure, masticate, and digest food. 

 The closer the connection and dependence between these 

 two sets of organs, the higher the rank. 18 



All the apparatus and phenomena of life may be in- 

 cluded under the heads of 



NUTRITION, 



MOTION, 



SENSATION. 



