ITS VITAL PROPERTIES, AND RELATIONS TO LIVING ORGANISM. 209 



servation of the organic elements of the blood in their normal condition, and for 

 the supply of the salt which is required as a component, not only of the solid 

 tissues, but also of all the secreted fluids ( 82). The presence of the Earthy 

 salts, on the other hand, would seem to have reference almost exclusively to the 

 composition of the tissues, into which some of them enter very largely. The 

 phosphate of lime in particular must be regarded almost in the light of a histo- 

 genetic substance, so constantly does it seem to be present in newly-forming 

 tissues ; whilst it is also in great demand as the principal consolidating material 

 of bone and tooth (86). Whether the carbonate of lime, the phosphate of mag- 

 nesia, the fluoride of calcium, and the silica of the blood, are of any other use 

 in it than to supply consolidating materials for the tissues, there is at present no 

 evidence whatever. Iron, like the alkaline salts, is an essential constituent of 

 the blood itself, forming a very large percentage of the haematin of its red cor- 

 puscles ; and it is supplied by the blood to various tissues, especially the muscles 

 and the hair, of which also it may be considered an essential component ( 87). 

 The normal proportions of all these substances appear to be chiefly maintained 

 by means of the excretory apparatus, which filters off (so to speak) any surplus; 

 it being through the urinary organs that they are chiefly eliminated. And it is 

 by them, too, that the normal proportion of Water in the blood is chiefly main- 

 tained; the Malpighian apparatus of the kidneys apparently acting as a kind of 

 safety-valve, through which any surplus that remains after the cutaneous, pul- 

 monary, and intestinal exhalants have performed their appropriate duties, is 

 allowed to make its escape. 



201. It is not alone by the proper Excretory apparatus, however, that the 

 fitness of the Blood for circulation through the body is maintained. Every 

 tissue draws from the circulating fluid some particular material, or combination 

 of materials, which constitutes its own special pabulum ; and as the "pabulum" 

 of each tissue is different, it follows that the normal composition of the blood 

 can only be preserved, without waste of substance, by the existence of such a 

 balance between the appropriative action of the several parts, as shall cause a 

 certain equivalent of blood to supply, without deficiency or surplus, the materials 

 which they collectively require. Such a balance is, in fact, ordinarily preserved; 

 and its maintenance is one of the most marvellous of those exemplifications of 

 Design, which the vital economy of the body presents in no less a degree than 

 its organized structure ; an exemplification, however, which becomes yet more 

 marvellous, when it is shown that not only every kind of tissue, but every spot 

 of every organ, has its own special " pabulum;" drawing something from the 

 blood, which is different from that appropriated by every other part of the body, 

 save by the corresponding spot on the opposite side. This position seems fully 

 established by the researches of Dr. W. Budd and of Mr. Paget on "Symmetrical 

 Diseases" 1 the phenomena of which are full of interest, as illustrating the ordi- 

 nary operations of Nutrition. Excluding the cases of congenital symmetrical 

 defects, and a few which seem to depend on morbid influence of the nervous 

 system, it may be stated as a general fact, that all symmetrical diseases depend 

 on the presence of some morbid material in the blood, which usually enters into 

 combination with the tissue that is diseased, or with the organized product of 

 the morbid process. Such a substance fastens upon certain spots or islands on 

 one side of the body, leaving the surrounding parts unaffected; and precisely 

 similar spots or islands are affected in like manner on the other side. The con- 

 clusion seems unavoidable, that, however closely one portion of skin or bone 

 may seem to resemble another, the only parts that are exactly alike are those 

 which repeat each other symmetrically on the opposite sides of the body; for, 

 although no power of artificial chemistry may determine the difference, the 



1 See their original Essays on this subject in the "Med.-Chir. Trans." vol. xxv. 

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