382 NUTRITION. 



of corpuscles will be proportionally postponed, and the 

 individual life of the corpuscles of the first set will be, by 

 the same time, prolonged. 



Such being the mode in which the necessity for the pro- 

 cess of nutritive maintenance is created, such the sources 

 of impairment and waste of the tissues, the next conside- 

 ration may be the manner in which the perfect state of a 

 part is maintained by the insertion of new particles in the 

 place of those that are absorbed or cast off. 



The process by which a new particle is formed in the place 

 of the old one is probably always a process of develop- 

 ment ; that is, the cell or fibre, or other element of tissue, 

 passes in its formation through the same stages of develop- 

 ment as those elements of the same tissue did which were 

 first formed in the embryo. This is probable from the 

 analogy of the hair, the teeth, the epidermis, and all the 

 tissues that can be observed : in all, the process of repair 

 or replacement is effected through development of the new 

 parts. The existence of nuclei or cytoblasts in nearly all 

 parts that are the seats of active nutrition makes the same 

 probable. For these nuclei, such as are seen so abundant 

 in strong, active muscles, are not remnants of the embryonic 

 tissue, but germs or organs of power for new formation, 

 and their abundance often appears directly proportionate 

 to the activity of growth. Thus, they are always abundant 

 in the foetal tissues, and those of the young animal ; and 

 they are peculiarly numerous in the muscles and the brain, 

 and their disappearance from a part in which they usually 

 exist is a sure accompaniment and sign of degeneration. 



A difference may be drawn between what may be called 

 nutritive reproduction and nutritive repetition. The former is 

 shown in the case of the human teeth. As the deciduous 

 tooth is being developed, a part of its productive capsule 

 is detached, and serves as a germ for the formation of the 

 second tooth ; in which second tooth, therefore, the first 

 may be said to be reproduced, in the same sense as that in 



