Lastly, in the insect tribe,—in the white-blooded animal,—we find the function of respiration effected altogether by the surface of the body; at least, so far as regards the reception of air, which passes into the body through apertures termed stigmata, the external terminations of tracheæ or air-tubes, whose office it is to convey the air to different parts of the system.

In all these cases we find precisely the same changes effected upon the inspired air, and especially, that oxygen has disappeared, and that carbonic acid is contained in nearly equal bulk in the residuary air.

CIRCULATION.

THE next function to be considered is that by which the products of the various absorptions, converted into arterial blood in the lungs, are distributed to every part of the body,—a function of the most important character to the physiologist and the pathologist, and without a knowledge of which, it is impossible for the latter to comprehend the doctrine of disease.

Assuming the heart as the great central organ of the function, every particle of the circulatory fluid must set out from it, be distributed through the lungs, undergo aeration there, be sent to the opposite side of the heart, whence it is distributed to every part of the system, and be thence returned, by the veins, to the right side, whence it set out, thus performing a complete circuit.

It is not easy to ascertain the total quantity of blood, circulating in both arteries and veins. Many attempts have been instituted for this purpose, but the statements are most diversified, partly owing to the erroneous direction followed by the experimenters, but, still more, to the variation that must be perpetually occurring in the amount of fluid, according to age, sex, temperament, activity of secretion, &c.

Harvey and the earlier experimenters formed their estimates, by opening the veins and arteries freely on a living animal, collecting the blood that flowed, and comparing this with the weight of the body. This method is, however, extremely objectionable, as the whole of the blood can never be obtained in this manner, and the proportion discharged varies in different animals and circumstances. By this method, Moulins found the proportion in a sheep to bed; King, in a lamb, th; in a duck, th; and in a rabbit, 3th. From these and other observations Harvey concluded, that the weight of the blood of an animal is to that of the whole animal as 1 to 20. Drelincourt, however, found the proportion in a dog to be nearlyth; and MOOR, th.

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An animal, according to Sir Astley Cooper, generally expires, as soon as blood, equal to about th of the weight of the body, is abstracted. Thus, if it weighs sixteen ounces, the loss of an ounce of blood will be sufficient to destroy it: ten pounds will destroy a man weighing one hundred and sixty pounds; and, on examining the body, blood will still be found—in the small vessels especially, even although every facility has been afforded for draining them.

The following table exhibits the computations of different physiologists, regarding the weight of the circulating fluid—arterial

Although the absolute estimate of Hofmann is below the truth, his proportion is probably nearly accurate. He conceives, that the weight of the blood is to that of the whole body as 1 to 5. Accordingly, an individual weighing one hundred and fifty pounds, will have about thirty pounds of blood; one of two hundred pounds, forty; and so on.

Of this, one-third is supposed to be contained in the arteries, and two-thirds in the veins. The estimate of Haller is, perhaps, near the truth; the arterial blood being, he conceives, to the venous as 4 to 9. If we assume, therefore, that the whole quantity of the blood is thirty pounds in a man weighing one hundred and fifty pounds, which is perhaps allowing too much,—nine pounds, at least, may be contained in the arteries, and the remainder in the veins.

The lower classes of animals differ essentially, as we shall find hereafter, in their organs of circulation: whilst, in some, the apparatus appears to be confounded with the digestive; in others, the blood is propelled without any great central organs; and in others, again, the heart is but a single organ. In man, however, and in the upper classes of animals, the heart is double;—consisting of two sides, or really of two hearts, separated from each other by a septum. all the blood of the body has to be emptied into this organ, and to be subsequently sent from it; and as its flow is continuous, two cavities are necessarily required in each heart, the one to receive the blood, and the other to propel it,—which contract and dilate alternately. The cavity or chamber of each heart, which receives the blood, is called auricle, and the vessels that transport it thither, are the veins; the cavity by which the blood is projected forwards, is called ventricle, and the vessels, along which the blood is sent, are called arteries.

One of these hearts is entirely appropriated to the circulation of venous blood, and has hence been called the venous heart, also the right or anterior heart, from its situation, and the pulmonary from the pulmonary artery arising from it. The other is for the circulation of arterial blood, and is hence called the arterial heart,

also the left or posterior, from its situation, and the aortic heart, from the aorta rising from it.

In the figure 109, the two hearts are separated from each

Fig. 109.

other, and shown to be distinct organs in the adult, although in the subject they seem to form but one organ. Between the two, after birth, there is not the slightest communication; and, consequently, every portion of blood, which has to attain the left side of the heart, must make the circuit through the lungs.

The whole of the vessels, communicating with the right heart, contain venous blood; whilst those of the left side contain only arterial.

If we consider the heart to be the Centre,

a, a. Venæ cavæ, ascendens, and descendens.-b. Right auricle. -c, c. Right ventricle.-d. Pulmonary artery.-e. Pulmonary two circulations are acveins.-f. Left auricle.-g. Left ventricle.-h, h. Aorta. The arrows indicate the course of the blood. complished, before the

blood, setting out from one side of the heart, performs the whole circuit to the other. One of these consists in the transmission of the blood from the right side of the heart, through the lungs, to the left; the other in its transmission from the left side, along the arteries, and by means of the veins, back to the right side.

The former of these is called the lesser or pulmonic, the latter the greater or systemic, circulation.

The organs, by which these are accomplished, will require a more detailed examination.

Anatomy of the Circulatory Organs.

The circulatory apparatus is composed of the organs, by which the blood is put in motion, and along which it passes during its

circuit.

To simplify the consideration of the subject, we shall consider the heart double; and that each system of circulation is composed of a heart; of arteries, through which the blood is sent from the heart; and of veins, by which the blood is conveyed to it. At the minute terminations of each of these, small vessels are situated, constituting the capillary system.

We shall first describe the central organ, as forming two distinct hearts; and afterwards regard these as united.

The pulmonic, right, or anterior heart,—called also the heart of black blood, is composed of an auricle and a ventricle. The auricle, so termed from some resemblance

Fig. 110.

to an ear, is situated at the base of the organ, and receives the whole of the blood returning from various parts of the body by three veins;the two venæ cavæ, and the coronary vein; the vena cava descendens,terminates in the auricle in the direction of the aperture by which the auricle communicates with the ventricle. The vena cava ascendens, the termination of which is directed more backwards, has the remains of a valve, which is much larger in the fœtus, called the valve of Eustachius. The third vein is the cardiac or coronary, which returns the blood from the heart, that has been carried thither by the coronary ar- artery. tery. In the septum, between the right and left auricle, there is a superficial depression, about the size of the point of the finger, which is the vestige of the foramen ovale, an important part of the circulatory apparatus of the foetus, as we shall see hereafter.

Pulmonic heart.

The opening, through which the auricle projects its blood into the ventricle, is situated downwards and forwards, and is seen in figure 111.

The inner surface of the proper auricle, or that which more particularly resembles the ear of a quadruped, the remainder being sometimes called the sinus venosus, or sinus venarum cavarum,—is distinguished by having a number of fleshy pillars in it, which, from their supposed resemblance to the teeth of a comb, are called musculi pectinati. They are mere varieties, however, of the columnæ carneæ of the ventricles.

Fig. 111.

B

The right ventricle or pulmo- A. Right auricle.-B. Right ventricle.-C. Pulnary ventricle is situated in the monary artery.

anterior part of the heart; the base aud apex corresponding to those