4 PHYSIOLOGY OF FARM ANIMALS [CH, 



to eat up little particles of food, to excrete or get rid of waste 

 products, to grow in size, and lastly, at a certain stage in its 

 life-history, to reproduce bj^ undergoing a process of simple 

 division into two. In order to perform these functions it is 

 essential that it should receive a supply of food, in just the 

 same way as an engine cannot be made to work unless it is 

 provided Avith fuel. In the latter case the fuel undergoes 

 combustion Avhich liberates heat. So also in the case of the 

 animal the energy is derived from the complex food material, 

 which undergoes a process of slow oxidation, thereby breaking' 

 down into simpler substances and setting free the energy necessary 

 for the discharge of the vital functions. The changes which the 

 food or its constituents undergo in the amoeba or any other 

 organism are classed together under the term ' metabolism ' : 

 those of them Avhich relate to the building up of the food into 

 the material of the body are referred to as 'anabolic' or 'as- 

 similative ' ; while the changes which are associated with activity, 

 resulting from a breaking down of complex substances into more 

 simple ones, are known as ' katabolic' or 'dissimilative.' 



An amoeba consists of a single cell, that is to say, a minute 

 mass of protoplasm containing within it a certain specialised 

 portion known as the nucleus which can generally be readily 

 identified under the microscope by its more intense staining 

 capacity. The nucleus is essential to the life of the cell. There 

 are many different kinds of unicellular organisms, varying in 

 form but all resembling the amoeba in their general plan. Higher 

 in the scale we find groups of amoeba-like cells, each with their 

 nucleus, aggregated together Avith little or no division of function. 

 Such an arrangement occurs in the simpler kinds of sponges. 

 But in the majority of multicellular animals whole groups of 

 cells are separated off to subserve particular functions; and 

 these form the various tissues. The body of a higher animal is, 

 however, derived from a single cell essentially similar to that of 

 an amoeba, and this, in the process of mdividual development, 

 undergoes a long series of divisions in the progress of Avhich the 

 nuclei also divide. The products of division, that is to say, the 

 cells with their contained nuclei, become gradually specialised to 

 form the different tissues— bone, cartilage, muscle, nerve, con- 

 nective tissue, etc. Thus the outer layer of cells becomes adapted 

 for protection and for receiving the impressions produced by 



