748 A MANUAL OF VETERINARY PHYSIOLOGY 



the nucleus still less is known, while as to the causes which lead 

 to its reproduction complete ignorance prevails. All the elements 

 found in the body of the animal or plant are discovered in the 

 protoplasm. Protein, fat, sugar, starch, and salts exist in this 

 minute speck of life, and yet only form one-quarter of its weight, 

 while the remaining three-fourths are water. Simple in struc- 

 ture as the cell is of which we have been speaking, there is great 

 reason for believing that it does not represent the earliest form 

 of living material. What the connection is, if any, between the 

 most elementary form of living or dead material is at present 

 unknown, but the division of chemistry into organic and inor- 

 ganic is rapidly being broken down under the recent extraordinary 

 advances into the structure of the atom made by modern physico- 

 chemistry. There are some who look upon the mysterious fer- 

 ments as the go-between. We shall glance at these remarkable 

 bodies presently, of which so little is known, but which some 

 believe hold the key to the solution of the problem of the origin 

 of life. 



The organic substances of the body are sharply divided into 

 two important groups — those containing nitrogen and those 

 nitrogen free. To the nitrogenous class belongs the all-important 

 and complex group of proteins ; to the non-nitrogenous class 

 belongs the fat and the small amount of carbohydrate, such as 

 sugar and animal starch, found in the body. The inorganic sub- 

 stances are represented by the elements and their salts, and it is 

 convenient to deal with this group first. 



The Inorganic Bodies. 



The inorganic substances found in the body are water, gases, and 

 salts. Water forms 60 p:r cent, of the whole body, the bulk of which 

 is taken in with the food and water, only a small quantity being 

 produced in the organism. The water supplied to the system 

 furnishes no potential energy, and consequently no nutrition. 



The Gases found are oxygen, nitrogen, hydrogen, carbon dioxide, 

 sulphuretted hydrogen, and marsh gas. Oxygen is the most widely 

 distributed of the elements, forming one quarter by weight of the 

 atmosphere, and eight-ninths by weight of water. By means of its 

 compounds it forms one-half by weight of the earth's crust. It is 

 practically the only element which enters the animal or vegetable 

 body in a free state, and in plants it does so only to a limited extent, 

 for these obtain the bulk of their oxygen through the decomposition 

 of carbon dioxide and water. In the animal body it exists free 

 and combined in some of the body fluids, such as blood ; others, 

 such as lymph, contain only traces, and none can be obtained from 

 the most bulky tissue of the body — i.e., muscle. Of the two 

 great cavities of the body — the chest and abdomen — one is remark- 

 able for containing the oxygen absorbing and distributing apparatus, 

 the other contains the digestive canal, which carries out its work in 

 the entire absence of oxygen. Nitrogen exists largely in a free state, 

 since it forms no less than four-fifths of the atmosphere, while it 



