422 OF NUTRITION. 



stances under which they are developed ; hence there is no a priori objection 

 to the belief (which other circumstances seem to favour), that Pus Corpuscles, 

 and Tubercular matter, are abnormal forms of the same elements, as those 

 which would otherwise produce a well-formed layer of Exudation Cells 

 ( 609, 610). 



561. From what has been stated, it appears evident that the process of 

 Nutrition mainly consists in the growth of the individual cells composing the 

 fabric ; and that these derive their support from the organic compounds with 

 which they are supplied by the blood, just as the cells composing the simplest 

 Plants derive theirs from the inorganic elements which surround them : and 

 as different species of the latter select and combine these, in such modes and 

 proportions, as to give rise to organisms of very diversified forms and proper- 

 ties, so it is easily intelligible that the different parts of the fabric of the highest 

 Animals should exercise a similar selective power, in regard to the materials 

 with which the blood supplies them. The structure composing every separate 

 portion of the body has (what may be termed) a special affinity for some par- 

 ticular constituents of the blood ; causing it to abstract from that fluid, and to 

 convert into its own substance certain of its elements. The conversion is 

 termed Assimilation. The property by which the cells of the Animal or 

 Vegetable structure are enabled to perform it, is one of which we are not likely 

 ever to know more. It will probably long remain an ultimate fact in Physio- 

 logy, that cells have the power of growing from germs, of undergoing certain 

 transformations, and of producing germs that will develop other cells similar 

 to themselves ; just as it is an ultimate fact in Physics, that masses of matter 

 attract each other ; or in Chemistry, that the molecules of different substances 

 have a tendency to unite so as to form a compound different from either of the 

 elements. It is of such ultimate facts as these that the science of Vitality 

 essentially consists ; since the Physical and Chemical phenomena which occur 

 in living bodies are not strictly removable from the laws of Inorganic Nature. 

 The conditions under which this Assimilating power operates, however, are 

 freely open to our investigation ; and it is a great step in the progress of the 

 inquiry, to become aware that these are so closely conformable throughout the 

 organized world, as they have been shown to be. It may be stated as a general 

 fact, that in assimilating, or converting into its own substance, matter which 

 was previously unable to exhibit any of the manifestations of life, every cell 

 thereby participates in the process of organization and vitalization ; for, by 

 the new circumstances in which the matter is placed, its properties undergo a 

 change, or, to speak more correctly, properties which were previously dor- 

 mant are caused to manifest themselves. No matter that is not in a state of 

 Organization can exhibit those properties which, from their being peculiar to 

 living bodies, and altogether different from Physical and Chemical, are termed 

 Vital; and it may also be asserted that no matter which exhibits perfect 

 organization, is destitute of the peculiar vital properties belonging to its kind 

 of structure.* As a corollary to this general fact, it may be stated that no 

 organism can be produced by any fortuitous combination of inorganic matter ; 

 since, even for the generation of the simplest cell, there is required a cell 

 previously existing, to furnish the germ. 



562. But this view also leads us to admit greater probability to the idea 

 that beings of the highest degree of organization may, by a perversion of their 

 assimilating processes, give origin to structures of a lower grade ; and it is 

 difficult to say that this is not the case in certain diseases with which the 



* For a fuller consideration of this question, and the grounds upon which this view is 

 supported, the reader is referred to the Article Life in the Cyclopaedia of Anatomy and 

 Physiology ; and to the Chapter on the Nature and Causes of Vital Actions," in his 

 Principles of General and Comparative Physiology. 



