Communication between cells of cellular tissue. Shown in dropsy.

FIG..2.

also the fibres of the same tissue. It varies much in its compactness in different parts. It is very fine and compact where it is necessary that it should be so; while in other cases it is loose and delicate, allowing a free motion of the parts which it envelops and connects together. It is abundant and loose among the muscles, and between them and the skin. Fig. 2 represents a portion of cellular tissue, inflated and dried, exhibiting the arrangement of its larger meshes. This is magnified twenty diameters. The free communication which exists between the interstices or cells in this tissue is exemplified in dropsy. These cells are bathed, in the healthy state, with a small amount of a watery fluid; and when this increases largely, forming the disease termed dropsy, it obeys the force of gravity in the cells, and accumulates most in the lowest parts of the lower extremities. This tissue is very elastic in health, so that if you press on the

CELLULAR TISSUE.

skin, there is no indentation left when the pressure is taken away, for the elastic cellular tissue at once rises from its state of compression, pushing the skin before it. But in dropsy it loses its elasticity by over distension; therefore there is pitting, as it is termed, after removing the pressure. We sometimes have an opportunity of seeing the communication between the cells manifested by the introduction of air into them. This has. occurred in some cases in which an opening has been made, by disease or accident, from the air tubes in the lungs into the cellular tissue in the walls of the chest. The whole body has been seen largely swollen, from the air which has from this cause accumulated in this tissue directly under the skin. Among the many tricks of impostors, the inflation of the cellular tissue of the head has been practised; and as it produces a frightful appearance, and therefore excites pity, the trick is a very successful one.

Deposits of fat. Its uses. How kept in its cells. Mucous tissue.

65. There are here and there in the cellular tissue deposits of fat. Various purposes are answered by these deposits. They are sometimes of use in promoting a free motion of the adjacent parts. The eye has, intervening between it and the bony socket, a cushion of fat, on which it rests, and against which it is pressed when any violence is offered to it. Fat, as a nonconductor, is a protection against the cold, and it is therefore deposited largely in the cellular tissue under the skin, in animals that inhabit cold climates. Fat, also, sometimes serves as nourishment to the system when its necessities require it. In diseases in which food cannot be taken in any amount, the fat is absorbed into the system. The heat of the body is maintained also, in part, by this process. This occurs in the torpid condition of hybernating animals. They commonly become very fat in autumn, as a preparation for the winter, and in the spring they come forth very lean, their store of fat having been used up for the purposes of nutrition and heat during their confinement. The fat thus deposited in the cellular membrane or tissue is not diffused merely in the interstices, but it is confined in cells of its own, which lie in these interstices. Minute blood-vessels pass from the fibres of the cellular tissue to these fat cells. The fat, which is an oily fluid, is kept from oozing through the pores of the cells that hold it by the blood, which is very nearly four-fifths water, and by the watery fluid which I have spoken of as bathing all the interstices of the cellular tissue; for oil, you know, will not pass through any porous substance that is wet with any watery fluid. If a portion of cellular membrane containing fat be dried, the fat, which in the moist state is wholly confined to its cells, now oozes through their pores. This is the reason that a lump of fat, as it is called, feels so oily after it has been exposed for a while to the air.

66. The mucous tissue is that which lines all the cavities of the body that have outlets. It lines the mouth and the cavities of the nose, and descends into the lungs, the stomach, &c. It takes its name from the fluid called mucus, which is constantly secreted by innumerable minute glands, that are situated in the substance of this membrane. The chief object of this viscid fluid is to protect the membrane from the substances which come in contact with it, which would otherwise produce some irritation. This membrane is continuous with the skin, shading off into it insensibly, as you may. observe on the lips.

67. The serous tissue or membrane forms the outer coat or

Serous membranes. Compound character of the organs.

lining of nearly all those organs the inner coat of which is mucous membrane. This is the case with the lungs, the stomach, and the intestines. The serous membranes are white, smooth, and shining, and are lubricated with a watery fluid, called serum. Every serous membrane forms a cavity or sac without an outlet, differing in this respect entirely from the mucous membranes. Thus, in the case of the lungs, the serous membrane lining the outside of each of these organs passes from the lungs to the walls of the chest, lining the inside of them, and thus makes a sac without an outlet for each lung. This sac could be dissected off, and taken out whole. When the fluid which lubricates the inside of this sac increases to any extent, the disease called dropsy in the chest is produced. The membrane which thus lines the outside of the lungs and the inside of the walls of the chest is called the pleura, and it is the seat of the disease termed pleurisy.

68. I have thus described some of the principal of the tissues which make up the human structure. The other tissues will be spoken of in the proper connection as we proceed. Before dismissing this subject, I will call your attention to the fact, that the organs of the body are generally composed of several tissues united together. Thus, the stomach has three coats, as they are termed, the mucous as the inner coat, the serous as the outer, and the muscular between them. And then we have the cellular tissue uniting these together. Besides these, there are arteries, and veins, and nerves, so that the stomach, which looks like a simple pouch, is really quite a composite thing. And the same can be said of the other organs.

69. Before entering upon the particular consideration of the functions by which the system is built up and kept in repair, it will be well to take a general view of them, that you may see them in their connection and mutual dependence. Each of these functions has its special and appropriate part to play, in effecting the formation and repair of the structure. The material from which all parts of the body are formed and repaired is the blood. There are organs whose special duty it is to make this material; organs which distribute it; and organs which use it after it is distributed. There are also organs whose duty it is to receive the blood after it has been used, and fit it to be used again. This common building material of the body is made out of the food; and the succession of processes by which it is done I will describe in the next chapter. After it is made, it is distributed by the complicated apparatus of the

Summary of nutritive functions. Processes of digestion.

circulation. This apparatus is therefore the common carrier of the building material of the system. It is the numberless little formative vessels, so small as to be invisible to the naked eye, that use the blood thus brought to them, and make and keep in repair all the various structures that we see in the body. When the blood has been used by these formative vessels, it is not fit to be used again until it is submitted to a purifying process by exposure to air; and to this particular object the lungs are devoted. And besides all this, as there are continually some particles which, in the wear and tear of the machinery, become useless, and even injurious, they must be got rid of in some way; and so there are organs for this purpose-organs of waste, as they are termed. It is also to be remembered, that the processes to which I have now alluded are so carried on, that the heat of the body, as will be fully explained hereafter, is steadily maintained. In the following chapters of this part, I proceed to a particular examination of the functions of which I have now given a brief summary.

CHAPTER V.

DIGESTION.

70. I SHALL include, under the term digestion, all those processes which are necessary to effect the separation from the food of its nutritious portion, and the introduction of it into the circulation. A summary of these processes may be thus given. The food is broken up and ground in the mouth, and it is at the same time mixed with the saliva. It is then taken into the stomach, where it is kept in constant motion, and is under the solvent action of a fluid of a peculiar character. When it is brought into the right condition, it is passed from the stomach into the intestines. Here it is acted upon by two fluids, the bile, the secretion of the liver, and the secretion of another gland, the pancreas or sweet-bread. These secretions have some agency in separating from the mass its nutritious portion, and this is taken up or absorbed, in the form of a milky fluid, by little vessels lying on the surface of the inner membrane of the intestine. These vessels unite together to form a large tube, and through this the milky fluid is poured into the circulation, to replenish the blood.

Mastication. Teeth various, according to different kinds of food.

Having given this summary of the processes which make up digestion, I proceed to speak of them more particularly in the order of their succession. In doing so, I shall notice some of the points in which other animals differ from man, in regard to these processes and the arrangements of the apparatus of digestion.

71. Mastication is an important part in the process of digestion. The teeth, which perform this act, are very hard bodies. The body of a tooth is composed of two substances. The inner part is called the ivory, and the outer is called the enamel, which is exceedingly hard. The teeth are of different shapes for different modes of action. There are long and pointed teeth, for tearing; others, for cutting, which have a sharp edge; and others, for grinding, having for this purpose a broad and irregular surface. The teeth are differently shaped in animals, according to the kinds of food which they eat. Thus, the herbivorous, or vegetable-eating animals, have grinding teeth to bruise their food; while the carnivorous, or flesh-eating animals, have sharp-edged teeth and long-pointed teeth, by which their food is torn and cut in pieces. And it is to be observed, that the movement of the jaws always corresponds with the character of the teeth. In the carnivorous animals, the motion of the lower jaw upon the upper is a mere up-and-down, or hingelike motion. As they have no grinding teeth, there is no need of any lateral or grinding motion. But in the animals

that have grinding teeth, there is a lateral motion, to enable them to grind. You see this difference very plainly, if you observe the dog and the horse while they are eating. In Fig. 3, you see represented the teeth of a carnivorous animal, The front teeth are long and pointed, for rending, while the back

FIG. 3.

TEETH OF CARNIVOROUS ANIMAL.

FIG. 4.

TEETH OF HERBIVOROUS

ANIMAL.