690 OLD AGE. [BOOK iv. 



ment 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 dis- 

 turbances 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, 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. 



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

 careous and fatty degenerations. These are seen in a completely 

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

 protoplasm 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 dis- 

 position 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 proportionately hurried. 



Of the various tissues of the body the muscular and nervous 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 dimmish, 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, the lines of resistance, which, as we have seen, help 

 to map out the central organs into mechanisms, and so to produce 

 its multifarious actions, become at last hindrances to the passage of 

 nervous impulses in any direction, while at the same time the 

 molecular energy of the impulses themselves becomes less. The 

 eye becomes feeble, not only from cloudiness of the media and 

 presbyopic muscular inability, but also from the very bluntness of 



