GENERAL CONSIDERATIONS. 367 



We can hardly realize the vast extent of the problem of 

 nutrition from a review of the functions which we have al- 

 ready considered. We have seen that the blood contains all 

 the elements that enter into the composition of the tissues and 

 secretions, either identical with them in form and composition, 

 as is the case with the inorganic principles, or in a condition 

 which allows of their transformation into the characteristic 

 principles of the tissues, as we see in the organic substances 

 proper. These materials are supplied to the tissues, in the 

 required quantity, through the circulatory apparatus; and 

 the oxygen, which is immediately indispensable to all the 

 operations of life, is introduced by respiration. The great 

 nutritive fluid, being constantly drawn upon by the tissues 

 for materials for their regeneration, is kept at the proper 

 standard by the introduction of new matter into the system, 

 in alimentation, its elaborate preparation by digestion, and 

 its appropriation by the fluids by absorption. These pro- 

 cesses, many of them, require the action of certain secre- 

 tions. The introduction of new matter, so essential to the 

 continuance of the phenomena of life, is demanded, on ac- 

 count of the change of the substance of the tissues into what 

 we call effete matter ; and this is discharged from the animal 

 organism, to be appropriated by vegetables, and thus main- 

 tain the equilibrium between these two great kingdoms in 

 Nature. 



What is it that causes the parts of a living animal organ- 

 ism to undergo change into effete matter, incapable of any 

 further animal functions ; and what is it that gives to these 

 parts the power of self-regeneration, when new matter is 

 presented under proper conditions ? 



These questions are the physiological ignis fatuus, which, 

 it is to be feared, will forever elude the grasp of scientific in- 

 quiry. They constitute one of the great mysteries ever pres- 

 ent in the minds of the student of Nature, and one, the gran- 

 deur of which is so immense that it is a problem with which 

 our intelligence can scarcely grapple. Its greatness is com- 



