shown by their becoming elongated and tortuous. 79

be tied, it does not become more capacious on the side next the heart.

The opinion, that the flow of blood in increased quantity to a part results from the relaxation of its small arteries, is remarkably confirmed by what is noticed respecting the larger vessels, wherever local action frequently occurs, or happens to exist for a considerable period. The arteries of such parts become elongated and tortuous. This is the character of the arteries of the testis, of the uterus, of the mammæ towards the latter period of and after uterogestation, of the face and temples. The latter instance, perhaps, requires an explanation to show its coincidence with the three former. In a child the temporal arteries are straight; in proportion as life advances they become more and more tortuous; but as life has advanced, the sources of passion and excitement have multiplied, and the face has flushed and burnt, and the temples have throbbed with an increased flow of blood on countless occasions. It remains to show in what manner the tortuous form of arteries is consistent with the explanation of local action which I have advanced.

We may presume that an artery at the average tone of arteries would be affected in the same manner by an unusually forcible contraction of the left ventricle, as a relaxed artery under the ordinary pressure of the blood. The former case is easily obtained. It has been already mentioned that the carotid artery laid bare in the neck of an ass lies without apparent change, when the animal becomes composed. But if the animal be alarmed, as by holding its nostrils for a few seconds, the heart acts violently, and the carotid artery leaps from its place and

80 Cause of the Tortuousness of certain Veins.

becomes elongated and tortuous at each stroke of the ventricle. It follows that if the coats of the same vessel were specially relaxed, a like phenomenon would ensue during the ordinary action of the heart. But if an artery were frequently lengthened and rendered tortuous, it is analogically certain that it would grow to this shape, and become permanently of the figure thus accidentally given to it.

It appears, therefore, that the phenomena of local action whether in large or in small arteries are equally referable to one cause, the spontaneous relaxation of the coats of these vessels. But where local action exists, the veins likewise become tortuous. Let us inquire whether this circumstance may result from the same cause.

It is not likely that veins are irritable; the effect of their valves, which act by their mechanical adjustment to a given area, would be defeated were this area readily capable of enlargement.

What are termed varicose veins are tortuous and dilated veins. They are frequently observed below the integuments of the thigh and leg. No doubt is entertained that the veins of the leg often become varicose through the pressure of the column of blood in the descending cava, which by a gradual process of dilatation renders each pair of valves in succession useless. The same pressure, which gradually dilates the veins, naturally tends to elongate them. Pressure, then, upon the inner surface of a vein, tends to enlarge and elongate it.

Varicose veins of the legs are again produced by ligatures tied below the knee; the superficial veins are in this instance observed to be continually swollen, and gradually to become tortuous as if knotted. The swollen

Influence of Gravity on the Circulation.

81

state of the veins shows the internal pressure to which they are subjected: but this internal pressure is the force of the blood propelled from the left ventricle.

Now by our hypothesis, the blood during local action would arrive in the veins through larger channels than before; its force therefore would be less broken; its pressure would be increased upon the veins. But increased pressure upon the inner surface of the veins has just been shown to enlarge and elongate them; and thus the state of the veins in parts subject to local action tends to support the theory which I have advanced.

Blood is not returned to the heart so readily from a dependent part, as from parts whence it has to descend. The circulation in the lower extremities always appears more sluggish than in the upper part of the body. If the hand be held up, it becomes whiter and less in bulk; if it hang down, it becomes swollen and darker. In the one case the weight of the blood favours its return by the veins to the heart; in the other case its weight is opposed to its ascent along the veins. The veins of the lower extremities have coats as thick as those of arteries: the arteries are perfectly straight, in order that there may be no unnecessary waste of the impulse derived from the heart.

The arteries distributed to the human brain are four in number, the two internal carotids and the two vertebrals. The brain is an organ of so slight and delicate a texture, as to suffer more readily than any other from an unusual force of the blood in the arteries, or from its accumulation in the veins. Accordingly in some animals, as for instance in the common ox, the carotid artery, upon entering the skull, divides into many branches,

G

82

Of the Circulation in the Brain.

which subsequently re-unite and form a trunk, in which the force of the blood must be greatly diminished. This contrivance is termed the rete mirabile. In human beings another provision is employed: each of the four arteries of the brain is bent twice at an abrupt curve just before or after entering the cranium. As less pressure is necessarily made upon the vessels beyond these curves, the arteries of the brain are formed consistently with the œconomy of Nature of much thinner coats than arteries of the same size elsewhere.

The veins of the brain, instead of collecting into large trunks continually varying in their degree of distension, open into cylindrical or triangular canals in the dura mater, which are termed sinuses, and terminate after circuitous routes in the internal jugular vein of either side. The sinuses of the dura mater are formed of materials so dense and so strictly adherent to the cranial bones, that they can at no time materially alter in dimension. The oblique entrance of the veins of the brain into the sinuses, the undilatable nature of the latter, their long and winding course, tend greatly to prevent the reflux of venous blood upon the brain, when its entrance into the chest is impeded.

CHAPTER V.

'

OF THE PULMONARY CIRCULATION.

THE blood probably suffers some alteration at every instant in every part of the vascular system: but the principal changes which it undergoes appear to take place in the capillary vessels. The heart and arteries and veins are machinery for propelling the blood to every organ, but in the capillaries the ends of the circulation appear to be attained.

In the human body there essentially exist two sets of capillary vessels, the one interposed between the pulmonary artery and the pulmonary veins, the other between the branches of the aorta, and the veins which return blood to the right side of the heart.

Each lung is a tissue of air-cells, with which the windpipe communicates in a manner already described, and upon which the capillaries of the pulmonary artery ramify.

.

If a lung be inflated and dried, its substance upon a section independently of the arteries and veins cut through appears uniformly porous. The larger pores appear sections of tubes, the lesser are shallow cups, being segments of air-cells. The air-cells are smaller, as M. Majendie observed, in infants than in adults, in adults than in persons advanced in age. In the lungs from a subject about five years of age, I found the aircells vary in size, but on an average to be To of an inch in diameter, and to be nearly circular. In the lungs