Part 29 (1/2)

CHAPTER XXII.

NUTRITION.

438. NUTRITION is the vital act by which the different parts of the body renew the materials of which they are composed. Digestion, circulation, absorption, and respiration, are but separate links in the chain of nutrition, which would be destroyed by the absence of any one of them.

439. The nutritive process is also a kind of secretion, by which particles of matter are separated from the blood and conveyed with wonderful accuracy to the appropriate textures. The function of the nutrient vessels antagonizes those of absorption: while one system is constructing, with beautiful precision, the animal frame, the other is diligently employed in pulling down this complicated structure.

440. This ever-changing state of the body is shown by giving animals colored matter, mixed with their food, which in a short time tinges their bones with the same color as the matter introduced. Let it be withdrawn, and in a few days the bones will a.s.sume their former color--evidently from the effects of absorption. The changeful state of the body is further shown by the losses to which it is subjected; by the necessity of aliment; by the emaciation which follows abstinence from food.

438-454. _What remarks respecting nutrition?_ 438. What is nutrition?

439. What is said of the nutritive process? The function of the nutrient vessels? 440. Give a proof of the ever-changing state of the body. Give other instances ill.u.s.trative of the changeful state of the body.

441. Every part of the body is subject to this continual change of material, yet it is effected with such regularity, that the size, shape, and appearance, of every organ is preserved; and after an interval of a few years, there may not remain a particle of matter which existed in the system at a former period. Notwithstanding this entire change, the personal ident.i.ty is never lost.

442. Many calculations have been made to determine in what length of time the whole body is renewed. Some have supposed that it is accomplished in four years; others have fixed the period at seven years; but the time of the change is not definite, as was supposed by a genuine son of the Emerald Isle, who had been in America _seven years and three months_, and consequently maintained that he was a native American.

_Observation._ India ink, when introduced into the skin, is not removed; hence some a.s.sert that this tissue is an exception to the alternate deposition and removal of its atoms. The ink remains because its particles are too large to be absorbed, and when in the skin it is insoluble.

443. ”Those animals which are most complicated in their structure, and are distinguished by the greatest variety of vital manifestations, are subject to the most rapid changes of matter. Such animals require more frequent and more abundant supplies of food; and, in proportion as they are exposed to the greater number of external impressions, will be the rapidity of this change of matter.”

444. ”Animals may be situated so that they lose nothing by secretion; consequently, they will require no nutriment. Frogs have been taken from fissures in solid lime rock, which were imbedded many feet below the surface of the earth, and, on being exposed to the air, exhibited signs of life.”

441. Why is the personal ident.i.ty never lost in the change of materials, which is unceasing in the system? 442. Give the opinion of physiologists respecting the time required for the renewal of the whole body. What exception to the changing state Of the different textures? 443. What animals are subject to the most rapid changes of material? 444. May animals be situated so that they require no nutriment? What is related of frogs?

445. The renovation of the bone, muscle, ligament, tendon, cartilage, fat, nerve, hair, &c., is not perfected merely by the general circulation of the fluid which is expelled from the left side of the heart, but through the agency of a system of minute vessels, which, under ordinary circ.u.mstances, cannot be seen by the eye, even when aided by the microscope; still, minute as they are, the function of these agents is necessary to the continuance of life. They are the smallest capillary vessels.

446. ”As the blood goes the round of the circulation, the nutrient capillary vessels select and secrete those parts which are similar to the nature of the structure, and the other portions pa.s.s on; so that every tissue imbibes and converts to its own use the very principles which it requires for its growth; or, in other words, as the vital current approaches each organ, the particles appropriate to it feel its attractive force,--obey it,--quit the stream,--mingle with the substance of its tissue,--and are changed into its own true and proper nature.”

447. Thus, if a bone is broken, a muscle or a nerve wounded, and, if the system is in a proper state of health, the vital economy immediately sets about healing the rupture. The blood, which flows from the wounded vessels, coagulates in the incision, for the double purpose of stanching the wound, and of forming a matrix for the regeneration of the parts. Very soon, minute vessels shoot out from the living parts into the coagulum of the blood, and immediately commence their operations, and deposit bony matter, where it is required to unite fractured bones, and nervous substance to heal the wounded nerve, &c.

445. Show how the renovation of the bones, muscles, &c., is perfected.

446. What is said of the office of the nutrient capillary vessels?

447. When a bone is fractured, by what process is it healed?