4 MANUAL OF ELEMENTARY ZOOLOGY 



will be found to have undergone the same changes as the 

 air in a vessel in which an animal has been stifled. Fishes 

 and other aquatic animals use the oxygen which is held in 

 solution in the water in which they live. (2) The nitrogenous 

 waste matters may be identified in the urine by chemical 

 analysis. (3) The formation of water is less easily demon- 

 strated, because the bulk of the water lost to the body 

 has been taken in as such through the mouth to perform 

 certain indispensable functions, one of which is the washing 

 out of the nitrogenous waste, but a careful comparison of 

 the quantities of water which enter and leave the body 

 shows that more goes out than can be accounted for by 

 what has entered. 



The energy freed in the disintegration of the body- 

 substance appears, as we have seen, in various 

 ano* of'the wavs - The most characteristic and important 

 liberated of these are contraction, chemical work, ex- 

 varicnis 'forms, cretion, secretion, and possibly 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 wider 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 shortening to pull up the forearm, grows at the 

 same time wider. 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 disinfected 

 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 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. Lastly, a part of the energy liberated in the 

 body is used in the removal from the substance of the body 

 of the chemical products of its activity. We have seen that 

 in the process of disintegration there arise waste products 

 of which the body gets rid. We have just seen also that 



