PROCESS OF NUTRITION. 305 



part. The less a part is exercised the longer do its component 

 particles appear to live : the more active its functions are, the 

 less prolonged is the existence of its individual particles. So 

 in the case of single cells ; if the general development of the 

 tadpole be retarded by keeping it in a cold, dark place, and if 

 hereby the function of the blood-corpuscles be slowly and im- 

 perfectly discharged, they will maintain their embryonic state 

 for even several weeks later than usual, the development of 

 the second set 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 process 

 of nutritive maintenance is created, such the sources of impair- 

 ment and waste of the tissues, the next consideration 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 development ; that 

 is, the cell or fibre, or other element of tissue, passes in its 

 formation through the same stages of development 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 cyto- 

 blasts 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 forma- 

 tion, 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 re? 

 produced, in the same sonse as that in which we speak of the 

 organs by which new individuals are formed, as the reproduc- 

 tive organs. But in the shark's jaws, and others, in which we 

 see row after row of teeth succeeding each other ? the row be- 



