scope, the soft mucous membrane of the human stomach presents a honeycomb appearance, caused by numerous depressions or cells. At the bottom

of these depressions are clusters of spots, which are the orifices of the tubular follicles. The follicles are about of an inch in diameter, and number millions.

50

3. The pancreas, or "sweetbread," so important in the process of digestion, exists in all but the lowest animals. In its structure it closely resembles the salivary glands. In the cuttlefish, it is represented by a sac; in fishes, by a group of follicles. It is proportionally largest in birds whose salivary glands are deficient. The pancreatic juice enters the duodenum. 4. A so-called "liver" is found in all animals having a distinct digestive cavity. In the lower animals been shown to be that of a pancreas.

FIG. 287. Follicles from the Stomach of a Dog, x 150; near the mouth, a, there is a lining of columnar epithelium.

its function has Thus, in polyps

it is represented by yellowish cells lining the stomach; in insects, by cells in the wall of the stomach; in mollusks, by a cluster of sacs, or follicles, forming a loose compound gland. In vertebrates, a true liver, the largest gland in the body, is well defined, and composed of a multitude of lobules (which give it a granular appearance) arranged on the capillary veins, like

FIG. 288.-Pancreas of Man; o, pancreas; g, grapes on a stem, and

gall bladder; s, cystic duct; c, duct from the liver;, pyloric valve; e, i, duodenum.

containing nucleated

secreting cells. It is of variable shape, but usually two, three, or five lobed, and is centrally situated-in mammals, just below the diaphragm. In most vertebrates, there is an appendage to the liver called the gall bladder, which is simply a reservoir for the bile.

FIG. 289. Liver of the Dog: F, F, liver; D, duodenum and intestines; P, pancreas; r, spleen; e, stomach, f, rectum; R, right kidney; B, gall bladder; ch, cystic duct; F', lobe of liver dissected to show distribution of portal vein, VP, and hepatic vein, vh; d, diaphragm; VC, vena cava; C, heart.

The so-called liver of invertebrates is more like the pancreas of vertebrates in function, as its secretion digests starches and albuminoids. The liver of vertebrates is both a secretory and an excretory organ. The bile performs an important, although ill-understood, function in digestion, and also contains some waste products.

The gland also serves to form sugar (glycogen) from part of the digested food, and may well be called a chemical workshop for the body. In animals of slow respiration, as crustaceans, mollusks, fishes, and reptiles, fat accumulates in the liver. 'Cod-liver oil" is an example.

The Great Excreting Organs are the lungs, the kidneys, and the skin; and the substances which they remove from the system carbonic acid, water, and urea

are the

All,

products of decomposition, or organic matter on its way back to the mineral kingdom. 124 Different as these organs appear, they are constructed upon the same principle: each consisting of a very thin sheet of tissue separating the blood to be purified from the atmosphere, and straining out, as it were, the noxious matters. moreover, excrete the same substances, but in very different proportions: the lungs exhale carbon dioxide and water, with a trace of urea; the kidneys expel water, urea, and a little carbon dioxide; while the skin partakes of the nature of both, for it is not only respiratory, especially among the lower animals, but it performs part of the work of the kidneys in case they are diseased.

1. The lungs (and likewise gills) are mainly excretory organs. The oxygen they impart sweeps with the blood through every part of the body, and unites with the tissues and with some elements of the blood. Thus are produced heat and work, whether muscular, nervous, secretory, etc. As a result of this oxidation, carbon dioxide, water, and urea, or a similar substance, are poured into the blood. The carbon dioxide and part of the water are passed off from the respiratory organs. This process is more immediately necessary to life than any other; the arrest of respiration is fatal.

2. While the lungs (and skin also, to a slight degree) are sources of gain as well as loss to the blood, the kid

neys are purely excretory organs. Their main function is to eliminate the solid products of decay which can not pass out by the lungs. In mammals, they are discharged in solution; but from other animals which drink little the excretion is more or less solid. In insects, the kidneys are groups of tubes (Figs. 239, 240); in the higher mollusks, they are represented by spongy masses of follicles (Fig. 244); in vertebrates, they are well-developed glands, two in number, and consist of closely packed tubes.

FIG. 290.

secretion.

3. The skin of the soft-skinned

7, the ureter, or outlet for the animals, particularly of amphibians and mammals, is covered with minute pores, which are the ends of as many delicate tubes that lie coiled up into a knot within the true skin. These are the sweat glands, which excrete water, and with it certain salts and gases.

Besides these secretions and excretions, there are others, confined to particular animals, and designed for special purposes: such are the oily matters secreted from the skin of quadrupeds for lubricating the hair and keeping the skin flexible; the tears of reptiles, birds, and mammals; the milk of mammals; the ink of the cuttlefish; the poison of jellyfishes, insects, and snakes; and the silk of spiders and caterpillars.

CHAPTER XIX*

THE SKIN AND SKELETON

The Skin, or Integument, is that layer of tissue which covers the outer surface of the body. The term Skeleton is applied to the hard parts of the body, whether external or internal, which serve as a framework or protection to the softer organs, and afford points of attachment to muscles. If external, as the crust of the lobster, it is called exoskeleton; if internal, as the bones of man, it is called endoskeleton. The former is a modification of the skin; the latter, a hardening of the deeper tissues.

1. The Skin. In the lowest forms of life, as amoeba, there is no skin. The protoplasm of which they are composed is firmer outside than inside, but no membrane is present. In Infusoria, there is a very thin "cuticle" covering the animal (Fig. 9). They have thus a definite form, while the amoebas continually change. Sponges and hydras also have no true skin. But in polyps, the outside layer of the animal is separated into two portions- ectoderm and endoderm 125. which may be regarded as partly equivalent to epidermis and dermis in the higher animals. These two layers are, then, generally present. The outer is cellular, the latter fibrous, and may contain muscular fibers, blood vessels, nerves, touch organs, and glands. It thus becomes very complicated in some animals.

* See Appendix.