of such soft tubes as the veins. In the foetus, also, and animals which swallow air, circulation occurs, and yet no respiratory movements aid it. During expiration, arterial pressure is at its maximum, and then apoplexy, rupture of aneurisms, and increase of inflammatory pains are apt to take place. Carson supposed the right auricle had a "suction power" by its wall, or those of pericardium being pulled asunder by inspiratory movements, or by the resiliency of its walls after contraction, like the expansion of an india-rubber bottle. Muscular contraction assists the flow through deep veins, as seen by the greater constancy of the current when the limb is moved during venesection, but which is partly due to the greater quantity of blood a part receives while acting than while quiescent. All expenditure of force in a backward direction is prevented by the valves. The venous circulation is very much under the influence of gravitation, and if the tone of the vessels be deficient, great delay to its return may occur. The practical surgeon daily perceives this in the effect of long standing in producing varicose veins, and the beneficial effect of an elevated posture of the limb in treating ulcers upon it. Lower records a case where varicose veins were so dilated, and the circulating force so weak, that the patient fainted whenever he stood up, the blood gravitating to his extremities. It may be well if we recapitulate:

The Forces of the Circulation.

1. Contraction of the heart, or vis a tergo. 2. Elasticity of the arteries.

3. Muscularity of the arteries.

4. Capillary force, especially in the lungs.

5(?). Movements of the blood-cells themselves, as stated by Carus and Treviranus.

6. Pressure of the muscles on the veins.

7. Inspiration.

8(?). Expansion of right auricle, according to Carson.

QUESTIONS FOR EXAMINATION.

JUNIOR.

1. Sketch the course of the circulation, and say by whom its pulmonic and systemic portions were first demonstrated.

2. Why does the left ventricle rupture more frequently than the right?

3. Compare the cardiac sounds.

4. What instruments have been used to determine the force of the circulation?

5. Describe the structure of arteries and mention some experimental proofs of their elasticity and contractility.

6. In what vertebrates can the capillary circulation be demonstrated by the microscope?-Describe it.

SENIOR.

1. Enumerate the proofs of the circulation.

2. Describe the fibres of the heart, including the points in which they differ from those of voluntary and from other involuntary fibres, and from both.

3. Sketch the mechanism of the arterial valves, and mention what supports them underneath.

4. What chiefly produces the first sound of the heart, and with what other cardiac actions is it synchronous ?

5. With what circumstances in health may the frequency of the pulse vary?

6. What evidences are there of a circulatory force in the capillaries?

RESPIRATION AND ANIMAL HEAT.

THE physiological processes we have heretofore considered, are for the purpose of forming the tissues of the body from the blood. That fluid becomes loaded with results of the destructive combustion of the old tissues, and is freed from them by the means which this chapter and the next treat of.

Respiration, then, is a process of excretion by which carbonic acid and superfluous water are discharged, but

Fibrous Coat

is also the means by which oxygen is introduced. In plants the interchange is reversed, carbonic acid by them being absorbed as food and oxygen evolved. Animals are either airbreathing or waterbreathing, according to the medium in which the exchange takes place.

We shall first describe the structure of the respiratory organs. Air is admitted through both mouth and nostrils, and being raised to a temperature approaching that of the lungs, is conducted to the trachea. This tube extends from the 5th cervical to the 2nd dorsal vertebra, and is about 6 inches long and wide. It consists of about 18 or 20 rings, each forming of a circle, the tube being flat behind where they are deficient. They are set

in a strong fibrous

sheath, which being

Coats of Trachea.

Cartilage.

Submucous
Tissue.

Mucous

Gland.

Elastic

Layer.

Ciliated

Epithelium.

continued between them and behind them, forms the most essential part of the tube, and it is the first and most universally developed. The ends of the rings are connected across by unstriped muscular fibres-the

"trachealis," a striped muscle in birds. It can narrow the tube, which for that purpose is deficient in cartilage behind, and not merely to allow the oesophagus to distend into it. Within these structures the tube is lined by branching longitudinal, elastic fibres, which continue into the bronchi, and is finally coated by ciliated mucous membrane, one offset of the gastro-pulmonary tract. The cilia of the whole pulmonary tract produce a current upwards, excluding particles of dust and expelling mucus and air; but they must, in returning to the perpendicu

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Ciliated Epithelium of
Trachea of the Cat.

lar, introduce fresh air. There are from 10 to 22 on each epithelial cell in man, but less in those of other animals. The tracheal glands are tubular like the sweat glands, and dip into the elastic coat and trachealis at the back of the tube. They exhale an odorous, watery

vapour.

The two bronchi into which the trachea bifurcates divide in the lungs dichotomously, or in binary order, and doing so produce, according to geometrical progression, many thousand tubes whose diameter averages about. As the bronchi become smaller, the rings gradually disappear; but the muscular, elastic, and ciliated coats last to the terminal tubes.

The lungs are made up of the air cells in which the bronchi end, grouped into lobules, which can be recognized as polyhedral masses on the surface. These lobules, like those of a racemose gland which the lung much resembles, are joined by areolar tissue, the parenchyma, and constitute 2 lobes in the left, 3 in the right lung. The whole organ is covered by a smooth, serous envelope, the pleura, which overcomes friction, and an areolar capsule, which is continuous with the parenchyma, and which, by its elasticity, considerably aids expiration. The air-cells of one bronchus do not com

municate with those of another, but the blood-vessels lie between them and send branches to contiguous lobules. When the tube is near its termination it loses muscular tissue and the epithelium, unless the latter be so fine as to escape microscopic detection, as Rainey suggests. Radclyffe Hall maintains its existence. In some animals the air-cells are so small as to be incapable of containing a single epithelial cell. The tube ends by dilating into a conical form, the lobular passage or infundibulum of Rossignol, from the sides of which the air-cells bulge off, and which are thus sepa

rated by partitions containing the capillaries exposed to air on two surfaces. Two infundibula, the air-cells which project from them, and the areolar space which separates them, are here figured. Moleschott asserts there is muscular tissue in the walls of air-cells. The lobular passages measure from to do, the air-cells from 200 to 400. There are about 1700 cells from one tube, and thus about 600 millions in both lungs. The surface of the lung is enormously increased by this arrangement of cells, and has been calculated at 1,400 square feet for both lungs.

A Bronchial Tube bifurcating, and one of these branches again dividing. Aircells, interlobular tissue, and pleura, also depicted.

The septa are so thin that the capillaries project by nearly all their circumference into the cavity of the 2 cells. They may pierce the wall of the cell, and enter again on the opposite side. These minute vessels are figured on next page, and are best seen, by injecting size into the pulmonary artery when it passes into the capil