CONDITIONS OF VITAL ACTIONS. 219 



always results, either from some change of structure or composition in the 

 tissue itself, or from some corresponding change in the external conditions, 

 under which the properties of the organ are called into action. Thus, when 

 a Muscle has been long disused, it can scarcely be excited to contraction by 

 the usual stimulus, or may even be altogether powerless ; and minute exami- 

 nation of its structure shows it to have undergone a change, which is obvious 

 to the Microscope (the fibres being as it were shrunken, and the fibrillae in- 

 distinct), though it may not be perceptible to the naked eye, and which results 

 from imperfect nutrition. Or, again, convulsive or irregular actions of the 

 Nervous System may be produced, not by any change in its own structure 

 or composition, but by the presence of various stimulating substances in the 

 blood (such as urea or strychnine), although their quantity may be so small, 

 that they cannot be detected without great difficulty. Further, whenever the 

 peculiar properties of an Organized structure can no longer be excited by the 

 requisite stimuli, we find that it has undergone some incipient change of com- 

 position, or that some of the other conditions are wanting. Thus, the depar- 

 ture of the contractility from the muscles of warm-blooded animals, at no long 

 period after the cessation of the circulation, is due in part to the lowering of 

 their temperature, and in part to the cessation of the supply of oxygen to the 

 elementary parts of their substance ; either of which would alone suffice to 

 prevent their respondence to the stimuli, that would ordinarily produce ener- 

 getic contractions. Lastly, we find special properties constantly associated 

 witli distinct forms of organized tissue ; thus we never find contractility exist- 

 ing in the fibres of Nerve ; nor do we ever find the power of conducting 

 impressions to exist in the fibres of Muscle. The details given in the pre- 

 ceding Chapter make it evident that each tissue, distinguished from others by 

 its peculiar composition, and by the form of its elementary parts, has some- 

 thing peculiar in its properties ; and this is true, as well of properties that are 

 simply physical, as of those that belong to a different category: thus the 

 Yellow Fibrous tissue is distinguished from the White as much by its elas- 

 ticity, as by its peculiar composition ; and it does not lose its elasticity, until 

 it is in a state of evident decay. 



259. By the study of the various forms of Elementary Tissue, of which 

 the Human fabric (or any other of similar complexity) is made up, we are led 

 to the very same conclusion, with that which we should derive from the 

 observation of the simplest forms of organized being, or from the scrutiny 

 into the earliest condition of the most complex ; namely, that the simple 

 Cell may be regarded as the type of Organization; and that its actions 

 constitute the simplest idea of Life. Between the humblest Confervoid Plant, 

 and the highest Animal, there is originally no perceptible difference ; they may 

 be said to have a common starting-point ; and the subsequent difference of 

 their course consists essentially in this, that the successive generations of 

 cells, which are the descendants of the former, are all similar to it, each cell 

 being capable of existing by itself, and therefore ranking as an independent 

 individual ; whilst the subsequent generations, which originate from the 

 latter, undergo various departures from the primary type, and lose the power 

 of independent existence, their several actions being mutually dependent upon 

 each other, so that the integrity of the whole fabric is essential to the con- 

 tinued life of any individual cell. Every individual part, however, even in 

 the most complex and highly-organized fabric, has its own power of develop- 

 ment ; and the properties which it possesses are the result of the exercise of 

 that power. But instead of the power of cell-growth being exerted, as in the 

 Plant, upon the inorganic elements around, it can only be put in action, in 

 the Animal, upon certain peculiar compounds, having the same chemical com- 

 position with its own substance ; and it is for the reception of these, for their 



