44 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 



by any difference in its aspect or composition ; yet neither can be deve- 

 loped into any other form than that of the parent species, and they must 

 be regarded, therefore, as essentially different in properties. In the 

 same manner we shall find that, in the same organized fabric, there are 

 very great varieties in the actions of its component cells, which indicate 

 a similar variety in their properties ; and yet they are to all appearance 

 identical. But there can be no reasonable doubt, that differences really 

 exist in such cases ; though our means of observation are not such as 

 to enable us to take cognizance of these, by the direct impressions they 

 make upon our senses. Analogous instances are not wanting in the 

 Mineral world ; for the Chemist is familiar with a class of compounds 

 designated isomorphous, in which, with perfect similarity in external 

 form and physical properties, there is a difference, more or less com- 

 plete, in chemical composition, and consequently in the effects of re- 

 agents. 



51. Whatever may be the peculiar vital properties possessed by an 

 organized tissue, we find that they are always dependent upon the main- 

 tenance of its characteristic structure and composition, by the nutritive 

 operations of which we have spoken ; and that their existence forms a 

 part, as it were, of the more general phenomena of its Life. They 

 manifest themselves with the first complete development of the tissue ; 

 they are retained and exhibited so long as active nutritive changes are 

 taking place in it ; their manifestation is weakened or suspended if the 

 nutritive operations be feebly exerted; and they depart altogether, 

 whenever, by the cessation of those actions, and the uncompensated 

 influence of ordinary Chemical forces, the structure begins to lose that 

 normal composition and arrangement of parts, which constitutes its state 

 of organization. Hence we may regard these peculiar properties as con- 

 formable, in all the essential conditions of their existence, with those 

 more general properties, which have been previously dwelt upon as 

 characterizing a living organized structure. 



3. Of the Forces concerned in the Production of Vital Phenomena. 



52. In prosecuting his inquiry into the causes of those phenomena 

 of Living organisms, which, being of a totally different order from those 

 of Inorganic matter, are distinguished as Vital, the Physiologist must 

 take as his guide those methods of investigation, which have proved 

 successful in other departments of scientific research. If he turn, 

 then, to the sciences of Mechanics, Optics, Thermotics, Electricity, 

 Magnetism, or Chemistry, he finds that the phenomena which they 

 respectively comprise are referable to the operation of certain forces, 

 and that what are termed the laws of those sciences, are nothing else 

 than expressions of the conditions of action of those forces. Thus in 

 Mechanics we have principally to do with the motion of masses of mat- 

 ter, and our idea of force is chiefly derived from our own experience of 

 the exertion of a power in producing or resisting motion ; whilst the 

 ' laws' of Mechanics are nothing else than expressions of the conditions, 

 under which the forces or powers that produce motion ope^te upon 

 matter. So in Optics, we have to do with the force which we term light ; 



