42 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 



and will consequently lose its vitality. The animal tissues are more 

 liable, as already mentioned, to spontaneous decomposition; and the 

 only instances in which they can retain their vitality for a lengthened 

 period, without any nutritive actions, are those in which all decomposi- 

 tion is prevented, either by the action of cold, or by the complete depri- 

 vation of air or of moisture, as when Frogs, Snakes, &c., have been 

 preserved for years in an ice-house, or Wheel-Animalcules have been 

 dried upon a slip of glass. 



47. The class of phenomena last brought under notice, serves to ex- 

 hibit in a very remarkable manner the dependence of all Vital Action 

 upon certain other conditions, than those furnished by the organized 

 structure alone. Thus a seed does not germinate of itself ; it requires 

 the influence of certain external agencies, namely, warmth, air and 

 moisture ; and it can no more produce a plant without the operation of 

 these, than warmth, air, and moisture could produce it without a germ 

 prepared by a pre-existing organism. Now when we come to study 

 these conditions, we find that they may be arranged under two catego- 

 ries, the material and the dynamical. Thus, a seed cannot germinate 

 without sufficient water to bring the contents of its cells into a state in 

 which their chemico-vital reactions can take place ; and it must be sur- 

 rounded with an atmosphere containing oxygen, since without the pre- 

 sence of this element the necessary chemical transformations cannot go 

 on. Thus oxygen and water are the material conditions required by 

 the germinating seed ; in almost every other case, alimentary matters 

 are required in addition ; but these the seed possesses within itself. 

 Even if supplied, however, with an unlimited amount of water and oxy- 

 gen, the seed cannot germinate, unless it be acted on by Heat ; and 

 this, in fact, may be considered with great probability, as supplying the 

 force, of which not merely the chemical transformations, but the growth 

 and development of the tissues of the Plant, are the manifestation. This 

 view will be more fully developed hereafter (Sect. 3). 



48. This dependence of Vital actions upon certain external Agencies, 

 as well as upon the properties of the organism which manifest them, is 

 no greater than the dependence of any of the phenomena exhibited by 

 an Inorganic substance, upon some other agency external to itself. In 

 fact, no change whatever can be said to be truly spontaneous; that is, 

 no property can manifest itself, unless it be called into action by some 

 stimulus fitted to excite it. Thus, when spontaneous decomposition (as 

 it is commonly termed) occurs in an organized or an inorganic substance, 

 it is due to the forces generated by the mutual attraction between cer- 

 tain elements of the substance, and the oxygen of the atmosphere ; and 

 this attraction is sufficient to overcome that which tends to hold toge- 

 ther the particles in their original state. If the air be totally excluded, 

 decay will not take place ;* because no new force comes into operation, 

 to cause a separation of the components from their original modes of 

 union. The influence of the Dynamical conditions which are essential 



* On this principle meats, vegetables, and even liquid soups, are now largely pre- 

 served, for the use of persons undertaking long voyages ; by enclosing them in tin cases, 

 carefully soldered up. There is no limit to the time, during which decomposition may 

 thus be prevented. 



