In the blood we find two varieties of cells or corpuscles. The red blood corpuscle owes its color to a secretion within its walls of a peculiar chemical nature. Notwithstanding, judging from analogy we should suppose that the red corpuscles in man and the other mammalia would contain a nucleus, as they do in all other animals, yet they have never been discovered by the most careful microscopic examination. Their principal office seems to be the introduction of oxygen into the blood that circulates through the systemic capillaries, and the removal of the carbonic acid set free there, serving as a medium for bringing the tissues in relation with air, which is necessary for the maintenance of their vital activity. The colorless corpuscles seem to have a different office to perform. While the red corpuscles are only found in the vertebrated classes and the higher invertebrata, the colorless are found throughout the whole animal scale. Hence we might suppose that the colorless corpuscles are closely connected with nutrition, while the functions of the red corpuscles must be of a more limited character. In animals, in a state of starvation, we find very few colorless corpuscles, while in those that are well fed, they exist in great abundance. In inflammation, colorless corpuscles rapidly increase in the inflamed part. There is then scarcely a reasonable doubt, that elaboration of fibrin is a consequence of this form of cell-life, and is one of its express objects.

A class of cells next in independence to the cells in the animal fluid we find on the membranous surface of the body, forming the epidermis and epithelium. The epidermis is a thin semi-transparent pellicle, covering the entire body in close connection with the true skin. It consists of a series of flattened scale-like cells, the outer layers of which are constantly thrown off by desquamation , new ones having formed below. The epidermis is not traversed by nerves and blood-vessels, but is pierced by the excretory ducts of the sebaceous, sweat-glands and the hairs. These perspiratory tubes passing from the sweat-glands, which are only a short distance beneath the skin, and pouring out an enormous quantity of perspiration are almost innumerable. Each of the perspiratory pores is the aperture of one of these tubes, about a quarter of an inch long. Wilson calculates the average number of these pores to the square inch to be 2800, making in an ordinary sized man 7,000,000. The number of inches of perspiratory tube then would be 1,750,000 or nearly twenty-eight miles. What an argument for cleanliness! The office of the epidermis is, to protect the true skin from pain occasioned by slight abrasion, and the irritating effect of air and the changes of temperature.

Other cells mingle with the epidermic and secrete coloring matter instead of horn, these are termed pigment-cells. They are not readily distinguished in the epidermis of the fair races of mankind, but in the colored races they are very marked.

The layer of cells covering the internal free surface of the body is called the ephithelium. The principal forms in which we see them are the tesselated and cylindrical.

The tesselated epithelium covers the serous and synovial membranes, the lining membrane of the bloodvessels, and the tubuli of most glandular structures connected with the skin or mucous membrane. The cylindrical covers the mucous membrane of the alimentary canal, and is also found in the larger ducts of glands which open into it. Both forms of epithelial cells are frequently fringed at their free margin with delicate filaments called cilia. These, although of extreme minuteness, are organs of vast importance in the animal economy. Their functions are to propel the secretions, which would otherwise accumulate on the membranes, towards the exterior orifices, whence they may be carried off. The secretions, both in the serous and mucous membrane are carried on through the epithelial cells, and the difference between the two membranes consists principally in the different arrangement of these cells.

Let us now glance at the process of nutrition as it is carried on in the human system, and the various phenomena it produces.

Every hour of our lives a change is going on in our system. Every motion of the hand, every movement of the body, is occasioned by a transformation of the structure or its substance; every mental affection is followed by changes in the secreting fluid; every thought and every sensation is accompanied by a change in the composition of the substance of the brain. We can neither move, nor think without a corresponding change in some portion of the system. The matter which composes the body to-day is constantly passing off in the form of perspiration, urine, or the carbonic acid of the lungs, its place being supplied by new material taken into the system in the air we breathe and the food we eat. In this constant, never-ceasing change not only is nutrition supplied, but force produced. In the closed galvanic circuit, certain changes which a metal undergoes when brought in contact with an acid, produces a current of what we call electricity; so in the human system, in consequence of certain changes undergone by matter which previously constituted a part of the organism, phenomena of motion and activity are produced, which we call life or vitality. Man from his birth to his death, by night and by day, is constantly taking into his system through the organs of respiration, a vast amount of oxygen. Notwithstanding during the year this amounts to nearly eight hundred pounds, yet his weight is not materially increased, but it is oftentimes considerably diminished. The question now arises what becomes of this vast amount of oxygen?

It does not remain in the system, but passes off in the form of a compound of carbon or of hydrogen. These which once formed a part of the tissue entering into combination with the oxygen, are given off in the form of carbonic acid and water. We find that blood yields 20 per cent. of dry residue; the remaining 80 per cent. is water In 100 parts of dry residue we find carbon 51.96, hydrogen 7.25, nitrogen 15.07, oxygen 21.30, ashes 4.42. If we take the statement of Lavoisier and Seguin, man receives into his system daily 32i ounces of oxygen, (46.037 cubic inches = 15.661 grains French weight,) and the weight of the whole mass of blood, of which as we have stated 80 per cent. is water, is 24 pounds. In order to convert the whole of its carbon and hydrogen into carbonic-acid and water, 64,103 grains of oxygen are required. This quantity will be taken into the system of the adult in four days and five hours. We cannot then escape the conclusion, that if man takes into his system 32 1/2 ounces of oxygen in a day, he must also receive daily, in the form of nourishment, as much carbon and hydrogen as would make 24 pounds of blood, that is, if the weight of the body remain unchanged, and is in the same condition as it regards health. Experiments have been tried by which the actual amount of carbon consumed in the system during the day has been ascertained with great accuracy. The food of 30 soldiers in barracks was accurately weighed every day for a month, even to the most minute articles, such as pepper and salt, and each article of food was separately subjected to analysis. The faeces and urine were also weighed, and the amount of unburnt carbon ascertained and deducted. An adult taking moderate exercise consumes 13 9/10 ounces of carbon daily. This carbon escapes through the skin and lungs in the form of carbonic-acid gas, and requires to convert it into that state 37 ounces of oxygen.