118 DIGESTION 



2. By these four ste~ps the sustaining power of food is gradu- 

 ally brought into exercise and the vital machinery kept in 

 working order, somewhat after the manner of the steam-engine. 

 To operate the latter, the force imprisoned within the coal and 

 water is set free and converted into motion by the burning 

 of the fuel and the vaporization of the water. It will be seen, 

 however, when we come to study these operations in the human 

 body, that they are conducted silently and harmoniously, with 

 marvellous delicacy and completeness, and without that friction, 

 and consequent loss of power, which attend the working of 

 the most perfect machinery of man's invention. 



3. Glands. Situated in all parts of the body are certain 

 organs, called glands, which aid the work of nutrition in its 

 various stages. Each gland consists of many active cells, all 

 alike in general structure and function. These cells may be 

 compared to the skilled laborers in a large factory. Like 

 these, the cells take the " raw material," as it comes to them 

 through the blood and lymph, separate it, and excrete on their 

 free surface the finished product. This general process is 

 called secretion. 



4. The product of the salivary glands is called saliva, that 

 of the liver is called bile, that of the lachrymal glands is 

 called tears, and so on. It is evident that if much material 

 is to be secreted by a gland, many working cells are neces- 

 sary. In order that many cells may be brought into as small 

 a space as is possible, they are arranged on the surface of a 

 tube or sack. Glands are of two types, tubular and racemose. 

 If the general plan of arrangement is that of a tube, either 

 simple or compound, the gland is called a tubular gland. 

 The perspiratory glands are good examples of this type. If 

 the cells are arranged along a surface which swells out at 

 its upper end, like a sack, the gland is called sacular or race- 

 mose. The sebaceous glands are racemose. Either type may 

 be simple or compound. 



2. Sustaining power of food ? Simile of the engine ? Operation in the human- body ? 



3. Use of glands? Constitution? What is the process of secretion ? 



4. What is the product of the salivary glands called ? Of the liver 2 Of the lachrymal 

 glauds ? Name the two types of glands. 



