of the Vertebrate Skeleton. 267 



§ 2. The Mechanism of Growth. 



Sir James Paget happily interprets the coexistence of growth 

 of different tissues in the same organism by adopting a doc- 

 trine advanced by C. F. Wolff and Treviranus, that each single 

 part of the body, such as fat, muscle, bone, &c., in respect of 

 its nutrition, stands to the whole body in the relation of an 

 excreted substance. Modern chemistry may be considered to 

 have demonstrated that this organized excrement which con- 

 stitutes the animal, continually excretes itself in other struc- 

 tures, which are capable ot passing naturally out of the body. 

 Even while remaining as constituent in the body, the tissues 

 change from a more live to a less live kind ; so that muscle is 

 degraded into urea and fat before the fat is got rid of in car- 

 bonic acid and water ; and cartilage must first be degraded 

 into bone, the most feebly organic of structures, before it is 

 removed from the body by the natural processes of secretion. 



Here, then, the question arises. By the operation of what 

 law are the assimilated parts of our food converted into the 

 tissues which manifest this complemental interrelation of 

 organs? for it would seem probable that there is but one 

 general law governing them all, since, when a bone elongates, 

 almost invariably the muscles, nerves, and vessels which are 

 related to it undergo a corresponding growth. 



And the reply to this question will recognize that growth 

 consists of two seemingly different processes : — first, simple 

 increase of substance; and, secondly, differentiation of sub- 

 stance. The increase of bulk is well studied in the individual, 

 while the differentiation of parts can only be observed in the 

 aggregate of individuals which constitute a tribe or order. 

 The tribe-growth has two totally different aspects — in embryo- 

 logy on the one hand, and in morphology on the other ; while 

 the chief means for experimental investigation are offered by 

 the mechanical and pathological aspects of growth. 



With the comparative anatomist, nerves, muscles, bones, 

 and the other tissues are ultimate facts, as much so as are the 

 different mineral species to the mineralogist ; and their es- 

 sential difference from each other is in chemical composition. 

 They are only to be spoken of, as to origin, as organic colloids 

 separated from each other by continuous organic dialysis. 

 What is named nutrition is no more than dialysis of the nu- 

 triment which has been elaborated into blood — a process which 

 is made possible by the disintegrating function of the capillaries 

 of the veins and the repairing function of the arteries. And 

 it comes about by the covering membrane of nerves dialyzing 

 nerve-substance, by the covering of muscular fibres dialyzing 



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