It is from the twenty-fourth to the twenty-sixth year, that the organic muscles are completely developed. Then the thorax and the abdomen which contain them have their greatest capacity. These muscles are then as they are to remain through life; they have a density much greater than in youth; their power is increased and their colour is deeper. In general this is subject in the heart to frequent varieties, which coincide very nearly with the varieties of the preceding system. Acute and chronic diseases have nearly the same influence upon it. It is equally the index of the sanguineous, lymphatic temperaments, &c. by the different tinge it assumes. The colour of the gastric, intestinal and vesical fibres varies less; their whiteness, more uniform, is rarely influenced by diseases.
We cannot increase, by constant exercise, the nutrition of the organic muscles. Aliments taken beyond measure, making the stomach frequently contract, weaken it instead of developing its fibres more, as happens from constant exercise given to a superior or inferior extremity. The bladder incessantly in action in some cases of incontinence, is thus gradually weakened and loses its energy. We might say that these two systems were in this respect in an inverse order.
It appears that the nutrition of the organic muscles, like that of the others, is subject to frequent variations; that at some periods they are more developed and less so at others. Diseases have a great influence upon this phenomenon, which proves, like the softening of the bones and their return to the natural state, the constant composition and decomposition of which these organs are the seat. We find in the dissecting rooms many differences in different subjects, as it respects the colour, density and cohesion of the muscles. Now what many exhibit then at once, the same individual often experiences successively; the same man no doubt has, according to the different influences to which he is exposed, his heart red, dense, large and well nourished at one period of life, and feeble, pale and small at another; for the internal organs experience the same alterations as the exterior. Now we know that the external appearance often changes during life.
IV. State of the Organic Muscular System in Old Age.
As we advance in age, the muscular system of which we are treating becomes weak like all the others; yet its action is more durable; it survives, if we may so say, that of the other. When the old man, almost immoveable, crawls about but slowly and with pain, his pulse, digestion, &c. have vigour still. This difference of the two systems is so much the more remarkable, as the time of activity of the second is almost by half less than that of the first; sleep cuts off in fact almost half of the duration of the voluntary motions, whilst it leaves the involuntary wholly untouched. This phenomenon of the organic muscles as it were surviving the voluntary in the last periods of life, is derived on a great scale from the same principle from which arises on a small one the lassitude which follows the contraction in an insulated motion. A less durable motion is necessary to fatigue the voluntary muscles, than to fatigue the involuntary; the stomach empty remains for a long time contracted upon itself without producing any painful sensation, whilst if we hold a body strongly grasped between our fingers for a quarter of an hour, all the flexors are soon painfully affected. After a convulsion of half an hour, in which all the locomotive muscles have been stiff, the whole body is broken, as it is called; it cannot admit of any motion; whilst after a paroxysm of fever of six or eight hours in which the pulse has been violently agitated, the heart often preserves the natural type of its contractions; it requires repeated paroxysms to weaken it. All these phenomena of the two muscular systems evidently prove that that of animal life is fatigued much the soonest; it is this which occasions its intermission. Is it then astonishing that, although less often in exercise than the other, it exhausts sooner the quantum of force nature has given to it? is it astonishing that the other survives the longest? Life is a great exercise which gradually wears up the organs in motion, and which requires their rest; this rest is death; now each moveable organ arrives at it sooner or later, according to the different degree of the forces which it has to expend, and according to its greater or less disposition to be wearied by this great exercise.
Yet the organic muscles are gradually weakened. The pulse becomes slower, digestion is longer in old age; the bladder and rectum first cease to act; then the intestines remain inactive; the stomach and especially the heart die the last.
A long time before death, the muscular cohesion is weakened in this system as in the preceding; the fleshy texture becomes flaccid; the parietes of the heart support themselves in youth; they flatten in old age. The gastric system of a young animal suddenly killed during hunger is firm, dense and contracted upon itself; in an old one, under the same circumstance, it is but little contracted; the stomach and intestines remain much more dilated; they are loose and soft; it is the same phenomenon as in the preceding muscles, which vacillate under the skin, from want of cohesion. The bladder remains constantly large, though empty.
[MUCOUS SYSTEM.]
This system, the name of which I borrow from the fluid that constantly lubricates it, and which is furnished by small glands inherent in its structure, appears everywhere in a membranous form; that of fasciculi is wholly foreign to it. In speaking of the mucous organs we shall designate them almost always under the name of membranes. Their study is a new object of research. Pinel has been among the first, who has perceived the necessity of considering them in a general manner as it respects diseases. I believe that I am the first who has regarded them generally in an anatomical and physiological view. Few systems deserve more attention; upon it take place all the great phenomena of digestion, respiration, secretion, excretion, &c.; it is the seat of many diseases. It should alone, in a nosography in which diseases are distributed by systems, occupy a place equal to that of many.