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begins in the back part of the mouth, opening into the pharynx by a slit, called the glottis, which, in mammals, is protected by the valvelike epiglottis. The trachea passes along the neck in front of the esophagus, and divides into two branches, or bronchi, one for each lung. In birds and mammals, the bronchial tubes, after entering the lungs, subdivide again into minute ramifications.

Vertebrates are the only animals that breathe through the mouth or nostrils. Frogs, having no ribs, and turtles, whose ribs are soldered together into a shield,

FIG. 284. Skeleton of a Frog.

are compelled to swallow the air. Snakes, lizards, and crocodiles draw it into the lungs by the play of the ribs. 121 Birds, unlike other animals, do not inhale the air by an active effort; for that is done by the springing back

of the breastbone and ribs to their natural position. To expel the air, the breastbone is drawn down toward the backbone by muscles, which movement compresses the lungs.

Mammals alone have a perfect thorax-i.e., a closed cavity for the heart and lungs, with movable walls (breastbone and ribs) and the diaphragm, or muscular partition, separating it from the abdomen.122 Inspiration (or filling the lungs) and expiration (or emptying the lungs) are both accomplished by muscular exertion; the former, by raising the ribs and lowering the diaphragm, thus enlarging the capacity of the chest, in

consequence of which the air rushes in to prevent a vacuum; the latter, by the ascent of the diaphragm and the descent of the ribs.

As a rule, the more active and more muscular an animal, the greater the demand for oxygen. Thus, warm-blooded animals live fast, and their rapidly decaying tissues call for rapid respiration; while in the cold-blooded creatures the waste is comparatively slow. Respiration is most active in birds, and least in water-breathing animals. The sluggish toad respires more slowly than the busy bee, the mollusk more slowly than the fish. eBut respirations, like beats of the heart, are fewer in large mammals

[graphic]

An

than in small ones.
average man inhales about
300-400 cubic feet of air
per day of rest, and much FIG. 285.
more when at work.

Another result of respiration, besides the purification of the blood, is

Human Thorax: a, vertebral column; b, b', ribs, the lower ones false; c, clavicle; e, intercostal muscles, removed on the left side to show the diaphragm, d; f, pillars of the diaphragm attached to the lumbar vertebræ; g, muscles for elevating the ribs; h, sternum.

the production of heat. The chemical combination of the oxygen in the air with the carbon in the tissues is a true combustion; and, therefore, the more active the animal and its breathing, the higher its temperature. Birds and mammals have a constant temperature, which is usually higher than that of the atmosphere (108° and 100° F. respectively). They are therefore

called constant temperatured or warm blooded. Other animals do not vary greatly in temperature from that of their surroundings, and are called changeable temperatured or cold blooded. Still, their temperature does not agree exactly with that of the air or water. The bee is from 3° to 10°, and the earthworm and snail from 11 to 2°, higher than the air. The mean temperature of the carp and toad is 51°; of man, 98.5°; dog, 99°; cat, 101°; squirrel, 105°; swallow, 111o, all according to the Fahrenheit scale.

CHAPTER XVIII*

SECRETION AND EXCRETION

IN the circulation of the blood, not only are the nutrient materials taken around through the body to be used in the construction of various tissues, but certain special fluids are taken up and conveyed to the external or internal surfaces in the body, where, in glandular structures, further elaboration takes place. The resulting products are of two kinds: some, like saliva, gastric juice, bile, milk, etc., are for useful purposes; others, like sweat and urine, are expelled from the system as useless or injurious. The separation of the former is called secretion; the removal of the latter is excretion. Both processes are substantially alike.

In the lower forms, there are no special organs, but secretion and excretion take place from the general surface. The simplest form of a secreting organ closely resembles that of a respiratory organ, a thin membrane separating the blood from the cavity into which the secretion is to be poured. Usually, however, the cells of the membrane manufacture the secretion from ma

terials furnished by the blood. Even in the higher animals, there are such secreting membranes. The membranes lining the nose and alimentary canal and inclosing the lungs, heart, and joints, secrete lubricating fluids.

The infolding of such a membrane into little sacs or short tubes (follicles), each having its own outlet, is the

* See Appendix.

type of all secreting and excreting organs. The lower animals have nothing more complex, and the apparatus for preparing the gastric fluid attains no further development even in man. When a cluster of these follicles, or sacs, discharge their contents by one common duct,

FIG. 286. Three plans of secreting Membranes. The heavy line represents the areolar-vascular layer; the next line is the basement, or limiting membrane; and the

dotted line the epithelial layer: a, shows

increase of surface by simple plaited or fringed projections; b, five modes of increase by recesses, forming simple glands, or follicles: c, two forms of compound glands.

food.

we have a gland. But whether membrane, follicle, or gland, the organ is covered with a network of blood vessels, and lined with epithelial cells, which are the real agents in the process.

The Chief Secreting Organs are the salivary

glands, gastric follicles, pancreas, and liver, all situated along the digestive tract.

1. The salivary glands, which open into the mouth, secrete saliva. They exist in nearly all vertebrates, higher mollusks, and insects, and are most largely developed in such as live on vegetable

[graphic]

The saliva serves to lubricate or dissolve the food for swallowing, and, in some mammals, aids also in digestion of starch.123

2. The gastric follicles are minute tubes in the walls of the stomach secreting gastric juice. They are found in all vertebrates, and in the higher mollusks and arthropods. In the lower forms, a simple membrane lined with cells serves the same purpose. Under the micro

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