THE PHASES OF LIFE. 859 



bulk, do the turning-points of the several tissues and organs coincide either 

 with each other or with that of the body at large. We have already seen 

 that the life of such an organ as the thymus is far shorter than that of its 

 possessor. The eye is in its dioptric prime in childhood, when its media are 

 clearest and its muscular mechanisms most mobile, and then it for the most 

 part serves as a toy ; in later years, when it could be of the greatest service 

 to a still active brain, it has already fallen into a clouded and rigid old age. 

 The skeleton reaches its limit very nearly at the same time as the whole 

 frame reaches its maximum of height, the coalescence of the various epiphyses 

 being pretty well completed by about the twenty-fifth year. Similarly the 

 muscular system in its increase tallies with the weight of the whole body. 

 The brain, in spite of the increasing complexity of structure and function to 

 which it continues to attain even in middle life, early reaches its limit of 

 bulk and weight. , At about seven years of age it attains what may be con- 

 sidered as its first limit, for though it may increase somewhat up to twenty, 

 thirty, or even later years, its progress is much more slow after than before 

 seven. The vascular and digestive organs as a whole may continue to 

 increase even to a very late period. From these facts it is obvious that 

 though the phenomena of old age are, at bottom, the result of the individual 

 decline of the several tissues, they owe many of their features to the dis- 

 arrangement of the whole organism produced by the premature decay or dis- 

 appearance of one or other of the constituent bodily factors. Thus, for 

 instance, it is clear that were there no natural intrinsic limit to the life of 

 the muscular and nervous systems, they would nevertheless come to an end 

 in consequence of the nutritive disturbances caused by the loss of the teeth. 

 And what is true of the teeth is probably true of many other organs, with 

 the addition that these cannot, like the teeth, be replaced by mechanical 

 contrivances. Thus the term of life which is allotted to a muscle by virtue 

 of its molecular constitution, and which it could not exceed were it always 

 placed under the most favorable nutritive conditions, is, in the organism, 

 determined by the similar life-terms of other tissues ; the future decline of 

 the brain is probably involved in the early decay of the thymus. 



813. Two changes characteristic of old age are the so-called calcare- 

 ous and fatty degenerations. These are seen in a completely typical form 

 in cartilage, as for instance, in the ribs ; here the protoplasm of the carti- 

 lage-corpuscle becomes hardly more than an envelope of fat-globules, and 

 the supple matrix is rendered rigid with amorphous deposits of calcic phos- 

 phates and carbonates, which are at the same time the signs of past and the 

 cause of future nutritive decline. And what is obvious in the case of carti- 

 lage is more or less evident in other tissues. Everywhere we see a disposi- 

 tion on the part of protoplasm to fall back upon the easier task of forming 

 fat rather than to carry on the more arduous duty of manufacturing new 

 material like itself; everywhere almost we see a tendency to the replacement 

 of a structured matrix by a deposit of amorphous material. In no part of 

 the system is this more evident than in the arteries ; one common feature of 

 old age is the conversion by such a change of the supple elastic tubes into 

 rigid channels, whereby the supply to the various tissues of nutritive material 

 is rendered increasingly more difficult, and their intrinsic decay proportion- 

 ately hurried. 



814. Of the various tissues of the body the muscular and nervous are, 

 however, those in which the functional decline, if not structural decay, be- 

 comes soonest apparent. The dynamic coefficient of the skeletal muscles 

 diminishes rapidly after thirty or forty years of life, and a similar want of 

 power comes over the plain muscular fibres also ; the heart, though it may 

 not diminish, or even may still increase in weight, possesses less and less 



