from venous to arterial, returning by the pulmonary veins to the left auricle. This sends it past the mitral valves into the left ventricle, which drives it past the semilunar valves into the aorta, and thence, by its ramifying arteries and capillaries, into all parts of the body except the lungs. From the systemic capillaries, the blood, now changed from arterial to venous, is gathered by the veins, and conveyed back to the heart.

The Rate of the Blood Current generally increases with the activity of the animal, being most rapid in birds.117

In insects, however, it is comparatively slow; but this is because the air is taken to the blood the whole body being bathed in air, so that the blood has no need to hasten to a special organ. However, activity nearly doubles the rate of pulsation in a bee. The motion

the distance from the heart increases. In the carotid of the horse, the blood moves 12 inches per second; in that of man, 16; in the capillaries of man, I to 2 inches per minute; in those of a frog, I.

The Cause of the Blood Current may be cilia, or the contractions of the body, or pulsating tubes or hearts. In the higher animals, the impulse of the heart is not the

:

sole means it is aided by the contractions of the elastic walls of the arteries themselves, the movements of the chest in respiration, and the attraction of the tissues for the arterial blood in the capillaries. In the chick, the blood moves before the heart begins to beat; and if the heart of an animal be suddenly taken out, the motion in the capillaries will continue as before. It has been estimated that the force which the human heart expends in twenty-four hours is about equivalent to lifting 217 tons one foot.

CHAPTER XVII*

HOW ANIMALS BREATHE

Arterial Blood, in passing through the system, both loses and gains certain substances. It loses constructive material and oxygen to the tissues. These losses are made good from the digestive tract and breathing organ. It gains also certain waste materials from the tissues, which must be got rid of. Of these waste products, one, carbon dioxide, is gaseous, and is passed off from the same organ as that where the oxygen is taken in. This exchange of gases between the animal and its surroundings is called respiration.

The First Object of Respiration is to convert venous into arterial blood. It is done by bringing it to the surface, so that carbon dioxide may be exhaled and oxygen absorbed. The apparatus for this purpose is analogous to the one used for circulation. In the lowest animals, the two are combined. But in the highest, each is essentially a pump, distributing a fluid (in one case air, in the other blood) through a series of tubes to a system of cells or capillaries. They are also closely related to each other: the more perfect the circulation, the more careful the provision made for respiration,

Respiration is performed either in air or in water. So that all animals may be classed as air breathers or water breathers. The latter are, of course, aquatic, and seek the air which is dissolved in the water. Land snails, myriapods, spiders, insects, reptiles, birds, and mam* See Appendix.

mals breathe air directly; the rest, with few exceptions, receive it through the medium of water. In the former case, the organ is internal; in the latter, it is more or less on the outside. But however varied the organs-tubes, gills, or lungs — they are all constructed on the same principle a thin membrane separating the blood from the atmosphere. (1) Protozoa, Sponges, and Polyps have no separate respiratory apparatus, but absorb air, as well as food, from the currents of water passing through them or bathing the surface of their bodies.

In the starfish, sea urchin, and the like, we find the first distinct respiratory organs, although none are exclusively devoted to respiration. There are two

sets of canals
one carrying the nutrient
fluid, and the other, radiating from a ring
around the mouth, distributing aërated
water, used for locomotion as well as
respiration. This may be called the
"water-pipe system." Besides this, there
are sometimes numerous gill-like fringes,

which cover the surface of the body and FIG. 274.- - Lobworm probably aid in respiration.

(Arenicola piscatorum), a dorsibranchiate, showing the tufts of capillaries, external gills. The large head is without eyes or

or

jaws.

Freshwater worms, like the leech and earthworm, breathe by the skin. The body is always covered by a viscid fluid, which has the property of absorbing air. The air is therefore brought into immediate contact with the soft skin, underneath which lies a dense network of blood vessels.

But most water-breathing animals have gills. The simplest form is seen in marine worms: delicate veins projecting through the skin make a series of arborescent

tufts along the side of the body; as these float in the water, the blood is purified.118 Bivalve mollusks have four flat gills, consisting of delicate membranes filled with blood vessels and covered with cilia. In the oyster, these ribbonlike folds are exposed to the water when the shell opens; but in the clam, the mantle incloses them, forming a tube, called siphon, through which the water is driven by the cilia. The aquatic

B

a

k

a

FIG. 275.- Diagrammatic Section of a Lamellibranch (Anodonta): a, lobes of mantle; b, gills, showing transverse partitions; c, ventricle of heart; d, auricles; e, pericardium; f, g, kid

neys; k, venous sinus; k, foot; A, branchial, or pallial, chamber; B, epibranchial chamber.

gastropods (univalves) have
either tufts, like the worms,
or comblike ciliated gills in
a cavity behind the head, to
which the water is admitted
through an opening. In
others the breathing organ
is the vascular lining of this
cavity. The cuttlefish has
flat gills covered by the man-
tle; but the water is drawn
in by muscular contractions
of the mantle instead of by

cilia.
cilia. The end of the siphon
through which it is ejected is
called the funnel.
The gills
of lobsters and crabs are

placed in cavities covered by the sides of the shell (carapace); and the water is brought in from behind by the action of a scoop-shaped process attached to one of the jaws, which constantly bails the water out at the front.

The perfection of apparatus for aquatic respiration is seen in fishes. The gills are comblike fringes supported on four or five bony or cartilaginous arches, and contain myriads of microscopic capillaries, the object being to expose the venous blood in a state of minute subdivision