CHARACTERISTICS OP THE ANIMAL BODY 19 



common to the above-mentioned organs and the nervous system 

 lies with the soft organs. 



Vertebrates are divided into the following five classes: (1) 

 Fishes; (2) Amphibians; (3) Reptiles; (4) Birds; (5) Mammals. 



The mammals are of most importance and include the horse 

 and our other large domesticated animals. As a class, mammals 

 are air-breathing, warm-blooded, produce their young alive and 

 nourish them for a variable period on a secretion from the mam- 

 mary glands, and have appendages from the skin in the form of 

 hairs. They are divided into a great many orders. Of these, the 

 horse and ass belong to the ungulates. All the members of this 

 order are characterized by having their back teeth adapted to 

 crushing and grinding their food, in being destitute of a clavicle, 

 in never possessing five digits, and in having the terminal bone of 

 each digit invested by a thick horny covering or hoof. 



THE CHEMISTRY OF THE ANIMAL BODY 



It is practically impossible to determine the chemical composi- 

 tion of living organic matter because changes are constantly going 

 on in it. Analyses of dead tissues show that the compounds are 

 very complex, yet not more than twelve separate elements can be 

 demonstrated. This is a relatively small number when compared 

 to the eighty or more chemical elements known to science. Of 

 these twelve elements only the following five are present in con- 

 siderable quantities — carbon, hydrogen, oxygen, nitrogen, and 

 sulphur — which together constitute 99 per cent, of the body weight. 

 These are linked together in different ways to form molecules of 

 enormous size. Inorganic salts of sodium, potassium, calcium, 

 magnesium, and iron, in the form of chlorids, sulphates, phos- 

 phates, and carbonates, are found in the ash and make up the 

 remaining 1 per cent. 



Carbon enters the animal's system as a constituent of organic 

 compounds, and leaves it either as carbonic acid gas or in simple 

 organic compounds, such as urea. 



Hydrogen occurs in nature in combinations like water and 

 ammonia. It is in these forms that it is taken up by plants and 

 converted into a constituent of organic compounds. Hydrogen 

 is given off by the body in the same forms as acquired, that is, as 

 water and in combination with nitrogen to form ammonia, and as 

 a constituent of organic compounds. 



