13 



APPEARANCE OF THE LIBERATED ENERGY IN 



VARIOUS FORMS 



The energy freed in the disintegration of the body-substance 

 appears, as we have seen, in various processes. The most char- 

 acteristic and important of these are contraction, chemical work, 

 excretion, secretion, and the conduction of impulses. Contraction 

 is the process by which mechanical movements are carried out. 

 In it a portion of the living substance changes in shape but 

 not in size, growing shorter in one direction but thicker in others. 

 This may easily be felt in the working of any of the great muscles 

 of the human body, as when the well-known biceps, in short- 

 ening to pull up the forearm, grows at the same time thicker. 

 It should be noted that the opposite of contraction, as used in 

 this sense, is not expansion but relaxation. Instances of chemical 

 activity are seen in the formation of the constituents of the 

 many juices which are used for various purposes in the body. 

 Thus the gastric juice, by which food is digested and dis- 

 infected in the stomach, contains among other substances hydro- 

 chloric acid, whose formation in face of the alkalinity of the 

 blood involves very considerable chemical work. Other examples 

 of liquids formed for special purposes are the spittle or saliva 

 which helps in the swallowing and digestion of food, tears which 

 wash clean the surface of the eyes, and so forth. The regions 

 in which materials are thus formed are known as glands. Again, 

 a part of the energy liberated in the body is used in the discharge 

 of materials from the substance of the body. We have seen that 

 in the process of disintegration there arise waste products of 

 which the body gets rid ; with these it casts out poisonous or 

 excessive materials absorbed from the food. We have just seen 

 also that certain activities of the body consist in the chemical 

 manufacture of materials which are not purely waste but have 

 their uses to the body. The casting out from the substance of 

 the glands of the materials of these two classes, and of the 

 water in which they are dissolved, is a necessary part of the 

 working of the bodily machine. The harmful or excessive products 

 are got rid of because they are injurious, and the products of 

 chemical manufacture are removed in order to be of use else- 

 where. Both kinds of material are accordingly shed, sometimes 

 upon the surface of the body, but usually into tubes known as 

 ducts, in which they flow to the required locality. This shedding 



