FOOD OF ANIMALS. 189 



Carbon, ox3'gen, hydrogen and nitrogen. With the exception of 

 the gelatinous tissues, about which chemists are not agreed, they 

 are all compounds of protein. 



During life these tissues arc continually undergoing a change 

 of matter. An act of composition and decomposition goes on 

 within them, so that, while their form remains the same, the mat- 

 ter of which they are composed is continually changing. This 

 composition and decomposition of the tissues constitutes the act of 

 nutrition. 



The body of an animal thus changing its materials, while its 

 Ibrm remains unchanged, has been compared to the waters of a 

 lake into which streams are emptying and from which other streams 

 are flowing, while its smooth surface reveals nothing of the changes 

 going on within its depths. 



Let us see now w'hat is the composition of the streams flowing 

 into the animal (its food,) — what are the changes they undergo in 

 its interior, and what is the composition of the streams as they 

 issue from it — (the excretions.) 



Nutrition takes place by a reaction between the blood and the 

 organized tissues. From the blood are derived the materials out of 

 which the organs are formed, and the product of their decomposi- 

 tion enters into the mass of this fluid before it is thrown from the 

 system by the excretions. The loss thus sustained is made up by 

 the products of digestion absorbed from, the surface of the stomach 

 ! and intestines. 



From the fact that all the organs are formed from the bloody 

 I this fluid has been called liquid Jlesh, and the flesh liquid blood. 

 Composition of the Blood. 



The blood, which to the naked eye looks like a homogeneous 

 ifluid, is seen under the microscope to be composed of an immense 

 number of circular, flattened disks of about 3 ^„ , of an inch in dia-, 

 meter, floating in a transparent colorless liquid. In these disks 

 is contained the coloring matter of the blood, and it is on them that 

 the air acts in the process of respiration. They arrive at the pul- 

 inonary capillaries loaded with carbonic acid which they there give 

 off", and then absorb oxygen, which they carry to the different 

 prgans. 



The transparent fluid in which the disks float is called liquor 

 Sanguinis. It consists of two parts — serum and fibrin. 



