The Process of Life. 21 



gation into tissues and organs, it remains true that the body 

 of one of the higher animals is composed of cells, together 

 with certain cell-products, horny, calcareous, or other. 

 The simplest animals, called protozoa, are, however, uni- 

 cellular, each organism being constituted by a single cell. 



We must notice that, even during periods of apparent 

 inactivity — for example, during sleep — many life-processes 

 are still in activity, though the vigour of action may be 

 somewhat reduced. When we are fast asleep, respiration, 

 the heart-beat,* and the onward propulsion of food through 

 the alimentary canal, are still going on. Even at rest, the 

 living animal is a going machine. In some cases, however, 

 as during the hibernating sleep of the dormouse or the 

 bear, the vital activities fall to the lowest possible ebb. 

 Moreover, in some cases, the life-processes may be tem- 

 porarily arrested, but again taken up when the special 

 conditions giving rise to the temporary arrest are removed. 

 Frogs, for example, have been frozen, but have resumed 

 their life-activities when subsequently thawed. 



Let us take the function of respiration as a starting- 

 point in further exemplification of the nature of the 

 life-processes of animals. 



The organs of respiration, in ourselves and all the 

 mammalia, are the lungs, which lie in the thoracic cavity 

 of the chest, the walls of which are bounded by the ribs 

 and breast-bone, its floor being formed of a muscular and 

 movable partition, the diaphragm, which separates it from 

 the stomach and other alimentary viscera in the abdominal 

 region. The lungs fit closely, on either side of the heart, 

 in this thoracic cavity ; and when the size of this cavity is 

 altered by movements of the ribs and diaphragm, air is 

 either sucked into or expelled from the lungs through the 

 windpipe, which communicates with the exterior through 

 the mouth or nostrils. It is unnecessary to describe 



* It has before been noticed that the organs themselves have their 

 periods of rest. The rhythm of rest and repose in the heart is not that of 

 the activity and sleep of the organism, but that of the contraction and 

 relaxation of the organ itself. 



