THE RABBIT. 13 



compounds, containing nitrogen and sulphur. The albumin 

 of white-of-egg is a typical proteid, and illustrates one of 

 the peculiar properties of proteids, that of coagulating or 

 becoming solid and insoluble when heated. Similar proteids 

 are the myosin of muscle (meat), gluten of wheat, and 

 legumin of peas, etc. It is the last two and similar vege- 

 table proteids that concern the rabbit. Allied to the 

 proteids are such compounds as gelatin and others, 

 characteristic of certain animal tissues. 



(2) Carbohydrates are compounds of carbon, hydrogen, 

 and oxygen, the two latter being present in exactly the 

 proportions in which they occur in water. Examples are 

 starch, sugar, and cellulose (the substance of cotton, and one 

 of the main constituents of most plants). 



(3) Fats are organic compounds, containing (like the 

 carbohydrates) no nitrogen, but with less oxygen than the 

 carbohydrates. They are compounds of glycerine with 

 one or other of the " fatty acids." 



(4) Mineral salts, such as sodium chloride, the carbonates 

 and phosphates of calcium, potassium and sodium, etc. 



(5) Water. 



3. The Problem of Digestion. The great bulk of the 

 compounds in the rabbit's food, as well as of those forming 

 its body, come under these five headings. But most of 

 these, as they occur in the food, are either solid and in- 

 soluble in water, or if liquid do not mix with water and 

 cannot diffuse through organic membranes. Hence the 

 problem of digestion is to change these compounds into such 

 forms that they can readily pass into the blood of the rabbit 

 through the bounding membranes of the alimentary canal. 

 ,| This is effected by certain fluids, formed either by the walls 

 (I of the alimentary canal or by certain organs called glands, 

 which open by ducts into it ; all these fluids contain small 

 quantities of organic compounds of the class called ferments, 

 and these are the active agents in the change. We shall 

 discuss the structure and action of glands a little more 

 fully in a subsequent chapter. Here we will simply say 

 that they are organs forming each its characteristic' fluid or 

 secretion, and sending it by a conduit, the duct, to the 



