Concerning the lessened quantity of fluid we have already made some remarks, and hinted at its relation with impaired digestion and slackened circulation. Here it is sufficient to observe that the fact is a sign of diminished vitality, by which we mean merely a diminu tion of vital actions, especially of those of nu trition. The abundance of fluid in the young succulent body is adapted to the constant accu mulation of new particles, and to the increasing complexity of the organization of the tissues, as well as to the reparation of waste, or to the counteraction of decomposition ;—by the still abundant though diminished quantity in the adult the composition is maintained and ren dered more exquisite;—in the old man there is only enough required to keep up that degree of renovation, which is necessary to the integrity of the structure, and even this action is less than in former periods, because the organiza tion, from its chemical nature, is less prone to decomposition. This brings us to the con sideration of the third general fact, or the condensation of tissue, which will require more particular notice, because great impor tance has been assigned to it by some writers. The condensation is a result of the deficient humidity just spoken .of; but this is not all, otherwise the condensation would be merely that of dryness ; the tissue itself is of firmer materials. Thus membrane becomes ligament, ligament cartilage, cartilage bone, and bone increases in its earthy proportions. This har dening of the whole body is spoken of by many writers as the cause of decay, and ulti mately of death, by the gradual closure of all the small vessels, and the obstruction to vital motions ; while the methods of averting old age, proposed by the same authors, turned chiefly upon an artificial supply of moisture to the body. Galen constantly alludes to this condition when treating of old age, and the means of resisting its tendencies if' Lord Bacon, in his curious and highly interesting treatise, entitled Historia Vitae et Mortis, has much to say upon desiccation and the methods of pre venting it, such as bathing and inunction. The fable of the restitution of old /Esop by the cauldron of Medea, he considers typical of the utility of the warm bath in softening the substance of the body. So much stress does Haller lay on the effect of the universal tendency to induration, that he tells us that one of the reasons why fishes are so long-lived is because their bones are never hardened to the same degree as in the higher animals—" Inter animalia ayes longzeviores Bunt, longzevissimi pisces, quibus cor minimum, et leutissimum incrementum, et ossa nunquam indurantur." Primw Linea, § 972. There is, however, we think, but little foundation for the supposition that induration stands in the relation of cause to the general failure of the functions of the body. It is rather a symptom of decline, or one of the phenomena in which decline con sists, and is therefore itself the effect of the failure or alteration of some of the functions, more especially of the assimilative. It is a deterioration of interstitial secretion, partly promoted by the changes in circulation, in di gestion, and probably in innervation, and partly itself contributing to these changes, but pri marily owing its origin, like the latter, to the ultimate law, which determines that at a certain period decay shall transpire. It is in one re spect a descent in the scale of organization. This indeed is indicated by the paucity of fluids and by the slow nutritive motions, which conditions are always sufficient to warrant our application of the terms, diminished vitality or less vitalized structure ; but the substance itself, indepen dently of these deficiencies of action, belongs to a more simple organization. We examine a bloodvessel, and instead of finding its coats of that complex texture which enables it to ac commodate itself by a property, known only in living bodies, similar but superior to elasti city, we mean tonicity, we observe a plate of osseous matter, unyielding, insensible, immo bile, possessing no other vital character than bare assimilation or molecular growth. We search for those admirably constructed sub stances which are interposed between the ribs and the sternum, and by their elasticity give extent and facility to the respiratory move ments, and we discover them converted into the same matter as the contiguous bones, with the coarse property of cohesion, and, as in the former instance, with nothing but its growth to redeem it from the character of mere inorganic matter. We untangle the muscle, and instead of the irritable fibre, soft in texture but firm in contraction, we find a torpid substance, scarcely fibrous in form, firm in mere physical cohesion, weak in vital contraction, and consequently of a degraded organization. The processes of induration about the joints, the glands, and the integuments, will all, when examined, be found to approximate more than the former conditions of these parts to the qualities of the inanimate world. Homogeneousness of sub stance is alone an indication of a low organi zation, and a body which possesses both this property and hardness, may be considered on the very outskirts of the region of vitality. Such are the properties of osseous deposits. May we not here perceive an analogy with the animals of the inferior classes? In many of the mollusca how trifling a degree of vitality seems adequate to the formation, growth, and reparation of their calcareous coverings and appendages; or. to go down to the coralines,
madrepores, and porifera, we observe that the very lowest structure that can be considered animal is sufficient to secrete or assimilate those vast collections of earthy matter which pave the ocean, and rise into islands, moun tains, and mighty continents. In this har dened constitution, this simplified but dege nerate structure, we see that the frame of man, in its natural decay, loses the characters that once distinguished it from the dust, and that not less literally than truly it has become more and more " of the earth earthy." We have now traversed as far and as mi nutely as our space would allow, the organs and .tissues, with their various alterations. It remains for us to inquire whether any one of them may be considered to stand in the rela tion of cause to the others. We have already dismissed the supposition, that rigidity and con cretion are productive of the other alterations, and we also partly entertained the question, when treating of the relations between assimi lation, the fluids, and the organs subservient to circulation and digestion. But there are one or two additional points which must be alluded to in this place.
The decay of all the organs, concerned in the life of relations, has been shewn to depend on a failure in the actions which are necessary to their generation and maintenance ; these organs may, therefore, be dismissed at once from our inquiry into the causation or priority of the processes of degeneration. Yet the observation of the marked declension of the function of the nervous system throughout the body, has led to the hypothesis, that the failure in this power is the ultimate fact in the history of our decline, the fact to which all the others may be traced. This view is suggested by Dr. Roget in his justly-admired article on Age, in the Cyclop'edia of Practical Medicine.
He considers the general condensation of tissue throughout the system, to be occasioned by a diminished force of circulation, which allows the capillaries to collapse and become obli terated; the weakened circulation this distin guished author is inclined to attribute to a diminution of nervous power in the muscular fibres of the heart ; whence he infers that the declension of nervous power bears the priority in the chain of events. We do not feel pre pared to adopt the inference ; for if we admit this failure in the innervation of the heart, (and whether its fibres are dependent on nerves for their contractility, is still an unsettled ques tion,) are we to pass over the condition of the blood ? Might we not say that the enfeebled contractions of the heart are referable to an alteration in the prOperties of its appropriate stimulus ? It is known that this vital fluid has been less affected by respiration than in former periods of our existence ; we might when searching for the earliest antecedent in decay, stop at the imperfect arterialization oi the blood. But this 'would be, in our humblE opinion, to pause too soon. The deficieni oxygenation of the circulating fluid is sufficient) well known to be the effect of certain change in the apparatus of respiration. And to whai do these changes belong ? To a variety 0: structural, functional, and nervous phenomena which, if pursued, would lead us into a maz( of events, from which it would be impossibli to select that which was earliest in its occur. rence. Or if we leave the respiratory system and follow the blood backward to the process of chylification, and ultimately to digestion, w( shall, as was shewn above, • be equally unsuc• cessful in obtaining satisfaction. Or finally, if we return to the heart, and investigate the dimi nished nervous power, admitting this diminu tion to be alone sufficient for the debility of circulation, is it possible to stop at this pheno menon ? Nervous power is nothing but the function of nervous substance, and whether the latter belongs to the ganglionic system, or to the cerebro-spinal, it may have undergone some change, or have been stimulated differently from usual. We know that the sensibility of the nervous system is most intimately connected with the quality of the blood,and with the force of its impulse ; so that if it be true that diminished circulation is the effect of diminished innerva tion, it is no less true that the latter is also the result of the former. Thus it appears that in this inquiry we are constantly arguing in a circle, and it can scarcely be otherwise ; the principal structures and functions of the organic life commenced simultaneously ; they must.de cline simultaneously : they assisted one another to grow ; they accelerate each other in the way to dissolution. If, however, we are disposed in some measure to qualify this remark, and still hold that there must be some organic changes primary in the work of decay, all ana logies must, we think, conduct us to the simple processes of assimilation and secretion, into which all the more complicated functions must be ultimately resolved ; but we can go no farther, for we know not what determines or modifies the play of those subtle affinities, motions, and contractions, in which such changes consist.