The deficiency of animal matter renders the bones of the aged fragile ; they are broken by the most trivial accidents. It is also the cause of their slowness to unite; for the activity of assimilative, and consequently of reparative processes, is dependent on the vascularity and fluidity of a tissue. The lightness, however, of these organs produced by the same cause is beneficial, or at all events in harmony with the state of the muscular system.
If we next turn our attention to the ar ticulations, we shall find that similar pro cesses of disqualification for former functions have ensued. The spinal column, which once adapted itself with such ease and flexi bility to the motions and curves of the body, has become almost as rigid as a single bone by the drying up of the intervertebral cartilages, and sometimes by the encroach ments of ossification." Scarcely any traces of cartilages between the ribs and the sternum can now be found ; one of the causes to which we alluded above, in connection with dimi nished respiration. The same deficiency of cartilage is observable in the bones of the wrist and of the tarsus. A change, the opposite of mobility, may also be detected in the liga ments which embrace the joints ; they are dense, dry, and inelastic. The gelatine which enters so largely into their composition has become altered in its chemical properties ; it is less easily soluble in water, and has all the characters of glue rather than of jelly. Ill-adapted as this state of the articulations is to the purposes of motion, it is, we think, not altogether difficult to discern its appropriateness to the human being at this advanced period. Were the joints supple and flexible, while the muscles have so little power, how much greater would be the risks of accidents to the aged man in the slight motions which he achieves. In order to preserve their frames from falling, those whose joints move easily upon each other are compelled to exercise those muscles which keep the limbs in the requisite degrees of extension and stability, during cer tain attitudes and motions; but this end is accomplished in the feeble old subject by the very stiffness of his articulations.
The muscles are subject to changes no less decided than those in the organs just men tioned. They are pale, flabby, atrophied, and indisposed to contract on the application of stimuli; but the fibre itself is tough and not easily torn, and the true muscular substance seems to have given way in some places to a sort of dense cellular membrane, or a yellow ish degeneration of tissue particularly de scribed by Bichat. Their tendons are often studded with calcareous matter, and the sheaths in which they play are rigid and unmoistened with synovia. They obey the stimulus of the
will tardily and irregularly ; the uncertain tremulous movements, the tottering gait, the stooping posture, the unsteady grasp of the aged, are familiar to every one.
The organ of voice comes next to be con sidered. The larynx, once composed of seve ral cartilages that moved freely on each other, is now a cavity capable of much less variation in its dimensions, owing to the rigidity of its parietes; the extent of the cavity gives in early old age that depth of tone, which by its gravity and solemnity excites our homage. In more advanced age, however, the tone becomes hoarse, shrill, and piping ; this in all pro bability is produced by the contraction and stiffness of the rima glottidis, but still more by the want of vigour in the muscles of the mouth and throat. The incapability of ma naging the tone, and the tremulous articu lation, are also results of changes in the muscles of the larynx, pharynx, and tongue, similar to those which transpire in other parts of the muscular system. Many senile impediments of speech are also produced by the loss of teeth, by the falling in of the cheeks, and by the disproportion of the lips to the space which they occupy.
In our investigation of the signs of decay in the parts that are subservient to sensation and thought, we shall be met by the same difficulties which formerly opposed our way, when inquiring into the phenomena of their development. We traced the progress of the nervous substance both in the nerves and in the cerebro - spinal centre from the almost pulpy state recognized in the infant, to its firm consistence in the adult. If we now inves tigate the anatomical quality presented by the tissue in advanced life, we shall find that it has shared the alteration of nearly all the other tissues,—that in short it has increased in density. This fact viewed in connection with another, namely, that ramollissement and induration produce very nearly the same lesion of func tion, will account for the failure in the sensific powers of old age. Besides this alteration in the substance of the nerves, they are found to be diminished in diameter ; their neurilemmes are become, like other membranous parts, much harder and stronger. Moreover, Bichat has remarked that the nervous tissue of old am mals is much less easily affected by reagents than that of younger ones ; so that there would appear to be an alteration in the chemical composition as well as in the mechanical con sistence.