These are liable to be absorbed ; at least the more fluid parts of them, by which they are again carried into the mass of the circulating fluids, and such parts selected as are fit for the uses of the animal economy, such is the case with the fluids exhaled by the skin; the mucous membranes; the saliva; the bile; the gastric and pancreatic fluids; the milk, &c. Thus, has it been remarked, does nature choose to subject the materials of decomposition to a careful revision, before rejecting them finally from the body.

16. All these varieties of absorption are constantly going on from the moment of birth to that of dissolution, and all the fluids which are absorbed, are changed in their character, and fitted to repair the wastes of the system. Thus, by absorption in the lungs, oxygen is converted into one of the elements of the blood; in the intestines, chyme is changed into chyle; in the tissues of the organs, solid particles are converted into fluid lymph; and from the cavities secreted and excreted fluids are again taken up and thrown into the circulation, to be once more revised and elaborated.

17. Frogs, and several other amphibious animals, are furnished with large receptacles for the lymph, situated immediately under the skin, which exhibit distinct and regular pulsations like those of the heart. The use of these lymphatic hearts is evidently to propel the lymph along the lymphatic vessels. The frog has four of these organs; the two posterior being situated behind the joint of the hip; and the two anterior ones on each side of the third vertebra. These organs have also been discovered in the toad, salamander, and lizard.

Questions. What is meant by absorption? What composes the absorbent system? Where do the lymphatic vessels arise? Describe their structure. Are they found in every part of the body? What are the agents of digestive absorption? Describe the lacteals. What is chyle? Does it exist ready formed in chyme What changes does it undergo in its passage to the heart? Where are the chyliferous

vessels chiefly found? Are alimentary substances absorbed without be. ing previously changed into chyle Is alcohol digested? Are medicinal substances absorbed dy How is this proved? Does absorption take place from the skin? What is interstitial absorption? What is emaciation owing to ?-plethora? Are foreign bodies absorbed when introduced into the substance of organs Are the bones ever absorbed ? What is respiratory absorption? What is recrementitial absorption ? What excrementitial? Are these processes always going on? What peculiarity is there in the lymphatic system of frogs?

CHAPTER XXIII.

NUTRITION.

1. NUTRITION may be considered as the completion of the functions of digestion. It is that process by which the waste of the organs is repaired, and by which their development and growth are maintained. Respiration, digestion, circulation, absorption, and secretion, are but separate links in the chain of nutrition; which would be instantly destroyed by the absence of any one of them.

2. In the construction of a machine, or an instrument, designed to last for many years, the mechanist seeks for the most durable materials. In making a watch, for instance, he forms the wheels of brass, the spring and barrel-chain of steel, and for the pivot, which is subject to incessant friction, he employs the hardest of all materials—the diamond. The necessity for this arises from the fact, that such instruments do not contain, within themselves, the power of repairing their own losses

3. But far different is the case with the animal machine. In order to qualify it for exercising the functions of life, it must be so constructed as to render it capable of continual alterations, displacements and adjustments; and these subject to continual variation, according to the stage of growth, and also to the different circumstances in which it may be placed Instead, therefore, of a few elementary bodies, or their simpler combinations, nature has employed such compounds as admit of greater change, and a more variable proportion of ingredients, and greater diversity in the mode of combination. It is nutrition that moulds these plastic materials, and forms these ever-changing compounds; and so preserves the animal machine, amid all the varying changes of condition to which it is subject.

4. No one can doubt that the system is continually undergoing changes. This is proved by the losses to which it is subjected; by the necessity of frequent supplies of aliment; by the rapid wasting of flesh on the withdrawal of food; and by the emaciation caused by sickness and old age. It is also shewn by an experiment, which has often been made, viz. of giving madder to animals mixed with their food; which in a short time tinges their bones of a red colour. If the madder be withdrawn, the red colour in a few days disappears from the bones; evidently from the effects of absorption

5. Every part of the body is subject to this constant change of matter. While one set of vessels, the lympha. tics, are taking to pieces and carrying away the various parts of which the machine is composed, another set, viz., the capillaries, are constantly at work, repairing the loss, depositing bone, muscle, cartilage, nerve, tendon, fat, membrane, ligament, hair, nails, &c. where each is wanted, and this with such regularity and order, as to preserve the shape, size, and appearance of every organ, so that, though after an interval of a few years there may not remain in the body a single particle of which, at the former period, it was made up; still, the individual preserves the same form and features; his personal identity is never lost.

6. Those animals, which are the 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 a greater number of external impressions, so will be the rapidity in this change of matter. The frog, for in stance, has been dug from the earth many feet below its surface; and even taken from cavities in solid limestone, where he had been shut up probably for centuries, and still exhibited signs of life when exposed to the open air. As he was so situated as to lose nothing, by secretion or evaporation, of

course he required nothing to supply any loss, but how the vital principle was preserved for such a length of time, is a mystery not easy to be explained.

7. The blood contains all the materials of nutrition. The process by which the food is changed into blood has been already explained. As it goes the round of circulation, the nutrient capillary vessels select and secrete those parts which are similar to the nature of the structure, and the other portions pass on; so that every tissue takes up 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.

8. (Before the body has attained its full growth, the func tion of nutrition is very active; a large amount of food is taken, being not only sufficient to supply the place of what is lost by the action of the absorbents, but also to contribute to the growth of the body. In middle age, nutrition and absorption are more equal; but (in old age the absorbents are more active than the nutrient vessels); the size consequently diminishes; the parts grow weaker; the bones more brittle; the body bends forward; and every function exhibits marks of decay and dissolution,

9. A few years ago, a man by the name of Calvin Edson, of Vermont, commonly called the living skeleton, exhibited himself through the country for money. From having been a large man, he had wasted away by degrees, so that instead of his usual weight, he weighed but sixty pounds. He had been gradually losing flesh for eighteen years; and he attri buted it to having taken cold from sleeping on the ground. This emaciation was owing to the absorbent vessels being more active than those of nutrition; whatever may have been the cause of the loss of balance.