1152 OLD AGE. [Book iv. 



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 indi- 

 vidual decline of the several tissues, they owe many of their fea- 

 tures to the disarrangement of the whole organism produced by 

 the premature decay or disappearance 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 favourable nutritive conditions, is, in 

 the organism, further shortened by the similar life-terms of 

 other tissues ; the future decline of the brain is probably in- 

 volved in the early decay of the thymus. 



Two changes characteristic of old age are the so-called cal- 

 careous and fatty degenerations. These are seen in a com- 

 pletely typical form in cartilage, as, for instance, in the ribs ; 

 here the cell-substance of the cartilage corpuscle becomes hardly 

 more than an envelope of fat globules, and the supple matrix 

 is rendered rigid with amorphous deposits of calcic phosphates 

 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 cartilage is more or less evident in other tissues. 

 Everywhere we see a disposition on the part of the living sub- 

 stance of the tissue to fall back upon the easier task of forming 

 fat rather than to carry on the more arduous duty of manu- 

 facturing 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 proportionately hurried. 



Of the various tissues of the body the muscular and ner- 

 vous are however those in which functional decline, if not 

 structural decay, becomes 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 force, and the movements of the intestine, bladder, 

 and other organs, diminish in vigour. In the nervous system, 



