OF VITAL ACTIONS, AND THEIR MUTUAL DEPENDENCE. 69 



CHAPTER II. 



GENERAL VIEW OF THE FUNCTIONS. 



SECTION I. Of Vital Actions, and their mutual dependence. 



72. The idea of Life or Vital Action obviously involves that of change. 

 We do not consider any being as alive, which is not undergoing some continual 

 alteration perceptible to the senses. This alteration may be so trifling in its 

 amount, as not to be recognized except by frequent comparison. The slow- 

 growing Lichen, that forms the gray or yellow spots upon old walls, or the. 

 Oyster that is lying motionless in its massive bed, may appear to perform no 

 action ; and yet a sufficiently prolonged knowledge of the former would show, 

 that it is gradually though slowly extending itself, and that it is multiplying 

 its race by a humble yet effectual process of fructification ; whilst closer obser- 

 vation of the latter would enable us to perceive, that its surfaces are covered 

 with cilia which are in continual vibration, that food is being regularly taken 

 into its stomach, undergoes digestion, and is converted into materials fit for the 

 aliment of the body, that a constant circulation of blood is maintained, by 

 the action of a powerful heart, that this circulation is subservient to the 

 various processes of nutrition, secretion, and reproduction, that in due time a 

 number of young Oysters are produced, which swim forth from between the 

 valves of the parent shell, and locate themselves elsewhere, and lastly that, 

 apathetic as the creature seems, it may be excited by some kinds of stimuli to 

 a movement which seems to evince sensation, the closure of the shell being 

 produced by any mechanical irritation of the contained animal, or even, when 

 it lies undisturbed in its native haunts, by a shadow passing between it and 

 the sun. Thus, then, change of some kind is essential to our idea of Life. 

 It may be asked what is the condition of a seed, which remains unchanged 

 during a period of many centuries, and at last vegetates, when placed in 

 favourable circumstances, as if it had been ripened but the year before. The 

 seed is not alive, but it is possessed of the^property of vitality, or the power 

 of performing vital actions, when aroused to them by the necessary stimuli, 

 such as warmth, moisture, oxygen, &c. Its condition is analogous to that of 

 the human being in profound sleep ; he is not then a feeling thinking man ; 

 but he is capable of feeling and thinking, when he is aroused from his slum- 

 ber, and his mind is put into activity by the impressions of external objects. 



73. As the activity of a living being, then, is dependent upon two sets of 

 conditions, the organized structure which it possesses, and the stimuli to 

 which this is exposed, we can scarcely separate from our notion of an organ- 

 ized structure, that of the peculiar properties with which it is endowed; for 

 we never see an organized structure remaining as such, unless it possesses 

 some degree of vitality. It may be said that, when an Animal or Plant is 

 killed by a strong electric shock, its organization is unaffected, yet its vital 

 properties are destroyed. Yet no proof of such an assertion, which is con- 

 trary to all analogy, has ever been afforded. In no other circumstances do we 

 ever witness the departure of vitality, without some change of structure or of 



