ITS VITAL PROPERTIES, AND RELATIONS TO LIVING ORGANISM. 213 



the tendency to particular constitutional diseases, which at the same time mark 

 the advance of life, and indicate minute and otherwise inappreciable alterations 

 in the circulating fluid. For it is obvious that since the poison of smallpox, 

 for example, less readily produces its characteristic "zymosis" in the blood of 

 the adult than it does in that of the child, the latter must differ from the former, 

 either in composition or in vital endowments; and that since the tendency to 

 " fatty degeneration" of the tissues generally shows itself in a far stronger de- 

 gree in the aged person than in the adult, this is likely to be in part owing to 

 the condition of the blood, in which, according to the observations of Becquerel 

 and Rodier, there is a decided and progressive increase of cholesterin after the 

 age of 40 or 50 years. 



206. Thus, then, we seem justified in the belief that the Blood, like the solid 

 tissues, has a formative power of its own, which it exerts in the appropriation 

 of the new material supplied to it from the food ; and that, like all the other 

 parts descended from the component cells of the germinal mass, it goes through 

 a succession of phases, which are partly the cause and partly the effect, of de- 

 velopmental changes in the organism generally. So long as the operations of 

 Nutrition are normally carried on, the materials that are withdrawn by the seve- 

 ral parts of the body may be considered so far to balance one another, that no 

 waste is incurred from this source ; and if the amount of new matter introduced 

 be merely the equivalent of that which is required for the nutritive operations, 

 nothing else will occasion a demand for elimination, save the products of the 

 disintegration of the tissues, which are received back into the blood for this 

 purpose. But it must be very rarely that this balance is precisely maintained 

 for any length of time, since a multitude of circumstances are continually 

 occurring to derange it ; the most frequent, perhaps, being the ingestion of cer- 

 tain nutritive materials in greater quantity than they are required. And we 

 then find that the organs take upon themselves a supplemental action for the 

 removal of the superfluity ; the kidneys being especially charged with this duty 

 in the case of azotized and saline matters, and the liver and lungs in regard to 

 hydrocarbonaceous substances. It is obviously of importance, however, to over- 

 task these organs as little as possible ; and when such superfluity is becoming a 

 source of disease, the obvious treatment is rather to prevent it from being 

 thrown upon them for separation, by diminishing the supply of aliment gene- 

 rally, or of some particular article of diet, than to excite them to increased 

 activity by stimulating medicines. 



207. The self-maintaining power of the Blood is yet more shown in the phe- 

 nomena of Disease; and especially in its spontaneous recovery of its normal 

 condition, after the most serious perversions ; as we see more particularly in 

 febrile diseases of definite type (such, for example, as the Exanthemata, Typhoid, 

 Typhus, &c.), of whose origin in the introduction of specific poisons into the 

 blood, there is no reasonable ground for doubt. In studying the mode in which 

 these and other "morbid poisons" act upon the blood, and through it upon the 

 system at large, we may derive important assistance from a previous inquiry 

 into the history of the action of those poisonous agents, which, from their being 

 more readily traceable by chemical analysis, can be more satisfactorily made out. 

 Such an inquiry has a most important bearing also, on the modus operandi of 

 medicines. The operation of medicinal or poisonous substances for the most 

 part depends upon the power which they possess, when introduced into the cur- 

 rent of the circulation, of effecting some determinate change in the chemical 

 and thereby in the vital condition, either of the components of the blood, or of 

 some one or more of the tissues which it nourishes; and their determination to 

 some special part or organ must be attributed to the same kind of elective affinity, 

 as that by which the normal constituents of the blood are so determined ( 201). 

 Now of nearly all these substances it may be said, that the system, if left to 



