OF NUTRITION. 257 



occur, there must have been an absorption or removal of the pre-existing 

 parts. The tinging by madder shows that growth is taking place wher- 

 ever the plasma of the blood can have access, and this not alone upon 

 the proper vascular surfaces, but also interstitially. 



It thus appears that bone, solid and dense as it is, is the seat of con- 

 tinual changes, which, though they may go on with more activity in the 

 growing state, take place also when the structure has reached maturity 

 or apparent perfection. From one portion a part is removed, on another 

 additions are made, the method by which this is accomplished being 

 through the access of the blood-plasma, which finds its way to every 

 part by reason of the pervious structure of the mass. 



As to the sources from which the phosphate of lime is derived, though 

 doubtless the food offers it in considerable quantity, there are gources from 

 many reasons for inferring that the identical portion which which material 

 has been removed from one part is used for the extension 1S denved - 

 of another ; and thus we may say that there is a plastic operation con- 

 tinually going forward, a remodeling, so as to adapt the structure to its 

 new conditions if in a growing animal, or to maintain it in good repair 

 if in an adult. 



Turning from the two cases with which we have been thus occupied, 

 the development and maintenance of the adipose and osseous tissues, to 

 the phenomena of nutrition generally, we may conclude that there are 

 several sources from which material for these purposes may be derived : 

 a part may be obtained by absorption directly from the food ; a part may 

 be manufactured or fabricated in the system itself, or may be taken from 

 some locality therein in which it has become redundant or useless, and 

 transferred elsewhere to the point at which it is required. 



The medium through which these additions and exchanges for the pur- 

 pose of development or remodeling are accomplished is of course the 

 blood. It bears with it, wherever it circulates, the substances that are 

 demanded fibrin for muscles, bone-earth for the skeleton, fat for the ad- 

 ipose tissues. 



It remains for us to inquire into the laws of deposit and development 

 involved in these processes, that is to say, why, for example, Partg are de _ 

 is phosphate of lime laid down at the points where the phos- veioped and 

 phate of lime has been, or, if growth be taking place, why Se^fluenL 

 are the accretions arranged in a definite way both as respects of physical 

 size and shape ? Upon this mquiry I do not propose at pres- 

 ent to enter, since it is closely connected with the general doctrine of de- 

 velopment, which will have to be considered in detail in the next book. 

 We shall then find that reasons may be assigned for the deposit of given 

 substances in places that have been vacated by others of the same kind, 

 as in the nutrition of muscles. We shall also then have to consider the 



