CHAP, in.] THE PHASES OF LIFE. 411 



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 con- 

 stitution, 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 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 

 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 a part of the living substance of the tissue 

 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 struc- 

 tured 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 w^ant 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, 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 

 the retina ; the sensory and motor impulses pass with increasing 

 slowness to and from the central nervous system, and the brain 

 becomes a more and more rigid mass of nervous substance, the 

 molecular lines of which rather mark the history of past actions 

 than serve as indications of present potency. The epithelial 

 glandular elements seem to be those whose powers are the longest 



