26 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 



those forces, are termed vital properties. Thus we say that the act of 

 contraction in a muscle is a vital phenomenon ; because its character 

 appears totally distinct from that of a Physical or Chemical action ; 

 and because it is dependent upon other vital changes in the muscular 

 substance. The act is the manifestation of a certain force, the posses- 

 sion of which is peculiar to the muscular structure, and which is named 

 the Contractile force. But that force is only exerted under certain con- 

 ditions, and these may only recur at long intervals, though the capacity 

 for exerting it be always present in the organized tissue ; this capacity 

 is termed a property ; and thus we regard it as the essential peculiarity 

 of living Muscular tissue, that it possesses the vital property of Con- 

 tractility. Or, to reverse the order, the muscle is said to possess the 

 property of Contractility ; the property, when the appropriate conditions 

 are supplied, gives rise to the Contractile Force ; and the force produces, 

 if its operation be unopposed, the act of Contraction. 



18. It may be said that the distinction here made is a verbal one ; 

 and that a very simple thing is thus made complex ; but it will be pre- 

 sently seen that it is necessary, in order to enable us to take correct 

 views of the nature of Vital phenomena, and to understand their rela- 

 tions to those of the Inorganic world. And, in fact, the distinction 

 between the property, the force, and the action, become apparent upon 

 a little consideration. Of the property we are altogether unconscious, 

 so long as it is not called into exercise ; we could not, for example, de- 

 termine by the simple exercise of any of our senses, whether a certain 

 piece of muscle retained, or had lost, its contractility. When the pro- 

 perty is called into action by its appropriate stimulus, we may convince 

 ourselves that a force is generated, even if no sensible action is pre- 

 sented ; thus, if we were to hold the two extremities of a muscle so 

 firmly, as to prevent them from approximating in the least degree when 

 its contractility was excited, we should be conscious of a powerful force 

 tending to draw our hands together ; and we might measure the amount 

 of that force, by mechanical means adapted to determine the weight it 

 would sustain. And lastly, if no obstacle be interposed to the act of 

 contraction, it then becomes obvious to our senses, by the change in the 

 shape of the muscle, and by the approximation of its two extremities, as 

 well as of the bodies to which they are attached. 



19. The advantage of this method of viewing the phenomena of Life 

 is best shown, by turning our attention for a moment to the mode of 

 investigation practised in Physics and Chemistry. Thus, when a stone 

 falls towards the earth, we say that this is an act or phenomenon of 

 Gravitation. The force with which the stone tends to fall to the ground, 

 whether it actually falls or not, is called the force of Gravitation ; and 

 we speak of the tendency which every substance has to act in this mode, 

 as the property of Gravitation. Now from observation of the Moon's 

 motion, it is shown that she too is drawn towards the Earth ; her ellip- 

 tical path around it being the result of the combined action of the cen- 

 trifugal or tangential, and of the centripetal or gravitative forces. And 

 it is further established, that not only does the Moon gravitate towards 

 the Earth, but the Earth gravitates towards the Moon ; so that, if the 

 two bodies were entirely free from the action of all other forces, they 



