the sanguineous

Palpitation of the heart.

Arteries

and veins.

As Arterial

structure.

surrounding sac will allow it. And hence again one cause I. Maof the violent palpitations to which this organ is subject, chinery of as we shall hereafter have to explain more at large. The general structure of the arteries and veins has, system. till of late years, been considered as alike, both being supposed to consist of two separate tunics, an elastic or outer, and a muscular or inner, independently of the soft and common covering which lines them within. Yet nothing can differ more widely than the relative spissitude and power ascribed to these tunics compared with each other in different parts of the circulating course. the heart is the salient point of the circulation, and pours fourth about two ounces of blood at every jet, the greatest force is exerted against the arteries that immediately issue from the heart. Here, therefore, we find the greatest resisting power; for in the aorta and pulmonary artery, the elastic tunic is stronger than the muscular, by which contrivance the arterial canal is never too much dilated in either by the action of the heart in its contraction, or, as the Greeks call it, systole. In like manner this tunic becomes stronger at the bending of the joints, and continues so through the whole length of the curve; and the same provision takes place at the sharp angles made by a trunk and its branch, or at an angle formed by the division of one trunk into two. As the arteries, how ever, recede from the heart, the blood, resisted at every the arterial step by the elastic tunic of the canal it flows through, progressively loses its impetus, and a less elastic power becomes necessary and is actually provided. At a considerable distance, therefore, from the heart, in whatever direction the arteries ramify, their muscular tunic soon balances their elastic, and gradually becomes superior; till at length, in the capillary arteries, it is nearly, if not altogether, the only tunic of which the canal consists: whence the ease with which these vessels collapse on some occasions, as loss of blood, or the exercise of terror, or any other depressing passion; and the equal facility with which they open in other cases, as in the sud- blushing. den blush of shame or modesty.

Skilful ad

justment of

and muscu

lar tunics.

Cause of

collapse on

loss of

blood.

Cause of

1. Machinery of

the sanguineous system. Venous structure.

Why furnished with

numerous

valves,

Whether muscular

fibres really exist in ar

teries and veins.

Causes of gradual diminution

of projectile force.

In the veins the elastic and muscular tunics are considerably weaker than in the arteries; they have, nevertheless, a more difficult task to perform than arteries; for, with a few exceptions, they have uniformly to force the current of blood upwards to the heart against the power of gravitation. They are hence far more numerously furnished with valves than the arteries, by which the ascending columns of blood are prevented from retrograding; and have by many physiologists been supposed to possess some degree of contractile, and consequently of propulsive power by the joint pressure of the sides of the arteries or muscles that accompany them, and that of the external atmosphere; to which subject, however, we shall have occasion to return presently.

I have thus far adverted to the commonly received opinion, and that taught by the most celebrated physiologists of our own country, and especially by Mr. John Hunter. Nevertheless it has long been a disputed point, whether, not merely the veins, but even the arteries, possess muscular fibres. The physiological arguments of Bichat, and the chemical researches of Berzelius, militate so strongly against the affirmative to this proposition, that the existence of such fibres in both classes of vessels has of late been doubted by many, and the contractility of the arteries been ascribed to their elasticity of texture alone; while the veins are conjectured to be altogether passive in the change of diameter they sustain. Yet whatever doubts may be entertained upon this subject in veins and arteries, the existence of muscular fibres cannot be questioned in the minute vessels termed capillaries.

I have observed that the force with which the blood is at first projected from the heart, is progressively diminished by the resistance it encounters in the thick and powerfully elastic tunic of the trunks or large arteries into which it is immediately propelled. There are two other causes which co-operate in producing a progressively diminishing force. The first is the short angles against which the blood has to strike at the origin of all

chinery of

the san

of the arte

rial system,

a cone.

the different branches: and the next, and most import- I. Maant, is the larger diameter of the general mass of the arteries, compared with that of the heart or the arteries from guineous which they immediately proceed; the range of the dia- system. meter augmenting in proportion to the increase of the ramifications. From experiments, indeed, made by Mr. John Hunter on the carotids of camels and swans*, the very same arteries appear gradually to widen from the upper end or that nearest the heart to the lower or that most remote. From all which he concludes that the Diameter aggregate diameter of the arterial system forms a cone whose apex is at the heart. And he concludes, also, and most correctly, that this conic proportion is most portion obvious, increases most rapidly, and spreads with its varies in broadest base in infants, or rather in the fetus; for here different the main trunks of the arteries are extremely short, while the capillaries are very large, and, from the obliteration of many vessels in subsequent life, more numerous than at any other period. It is highly probable indeed that while the aorta in childhood is not a fourth part of the size of the same vessel in an adult, the aggregate of the capillaries of the former possesses a diameter more than four times as large as the aorta in the latter.

Conic pro

ages.

Why the

We may hence, in some degree, account for the difpulse differference in the quickness of the pulse at different periods ent in difof life. In early infancy it beats as much as 140 strokes ferent ages. in a minute; towards the end of the second year it is reduced to 100; at puberty it is only 80; about virility 75; and after sixty years of age seldom more than 60 in a minute. For reasons connected with the preceding, it is more frequent in persons of short stature, those of strong passions of mind, those of great muscular exertion, and in females. From the increasing diameter of the blood-vessels as they diverge from the heart, the blood has a greater space for moving forward, and is Why the able to move with more freedom: and hence one reason arteries are

On Blood, Inflammation, &c. Part. 1. Sect. viii. p. 170.

found empty after death.

I. Ma

for the empty state in which the arteries are found imchinery of mediately after death: a second reason is that the tunics

the san

guineous system.

Why blood

is accumu

lated in the

chest after death.

The above

facts urged

against the doctrine of

of the veins possessing little or no elasticity, readily dilate to the distentive power of the blood as it moves forward: a third, and indeed the principle reason, as sufficiently proved by Dr. Carson of Liverpool, is the natural elasticity or resilience of the lungs, which, by keeping them after death in a state of dilatation, allows the blood to accumulate here as in the vacuum. And hence, again, the reason of the accumulation of blood which is usually found in the chest after death, as well as the empty state of the vessels.

This vacuity of the arteries upon death, was one of the objections urged very forcibly by the ancients against the circulation of the blood, or even its following at all circulation. the course of the arteries; and which Dr. Harvey very unsatisfactorily replied to, by asserting, contrary indeed to fact, that the heart continues to contract for some time after death, and even after it has received blood :-for the heart is generally found loaded with blood. And it is this objection, together with some others, that has Circulation induced Mr. Ker of Aberdeen once more to revive the still denied. doctrine of the ancients, and deny that of a circulating system altogether, resigning to the arteries the uses the ancients allotted them.

It still, however, remains to be ascertained by what means the ultimate branches of the arteries terminate in those of the veins, and how this communication is conducted.

The pulmonary artery, which receives from the heart. the blood returned into it from the veins, bears a very close proportion to the diameter of the aortat, which sends the blood from the heart over the whole of the larger circulation. The aorta possesses more strength, but their elasticity is nearly equal, and the measure of each,

* See Dr. Carson "On the Vacuity of the Arteries after Death." Medico-Chir. Trans. Vol. xI. Part 1.

+ See Hunter on Blood, p. 133.

chinery of the san

Balance of

maintained.

on being slit, is about 33 inches: and hence there can I. Mabe little doubt that the quantity of blood sent back the heart, is on an exact balance with that which flows guineous from it. It is not, however, at any time the identical system. blood which is thus returned to the heart; for every arterial and organ takes from the general current, as it visits it, venous such parts and such principles as it stands in need of blood, how to support the wear and tear of its own action; while another considerable portion is thrown off, as we have already observed, in the form of secretions or exhalations from various emunctories that open externally or into internal cavities. Bnt the drain which is hereby produced on the arterial blood is compensated by the various fluids collected from every part of the absorbent vessels, and by the flow of the chyle from the digestive organs; both which are poured into the thoracic duct, and finally intermixed with the returning current of venous blood a short time before it reaches the heart; and in this manner the balance of arterial and venous blood is maintained.

the blood es

With respect to the actual quantity of blood contained Sum total of in the entire system, our means of determination are so timated very imprecise, and consequently the calculations, or rather differently the conjectures that have been offered upon the subject, are so strikingly discrepant, that it is not easy to reach a satisfactory conclusion. It is only necessary to state a few of the different opinions that have been offered to show the absurdity of several of them. Muller and Abeildgaard estimate the weight even in an adult at very little more than eight pounds *; Borelli at 20; Planch at 28; Haller at 30; Dr. Young at 40+; Hamberger at 80; and Keil at 100. Blumenbach states the proportion in an adult healthy man to be as 1 to 5 of the entire weight of the body. Yet little reliance can be placed on this last mode of determination, on account of the great diversity in point of bulk and weight of adults, whose aggregate quantity of blood is in all probability nearly alike. The mean numbers, as those of Baron Haller and Dr. Young, + Phil. Trans. 1809. p.5.

• Blumenb. Elem. Phys. p. 4. § 6.