Imágenes de página
PDF
ePub

structure.

surrounding sac will allow it. And hence again one cause I. Maof the violent palpitations to which this organ is subject, chinery of

the sanas we shall hereafter have to explain more at large. guineous The general structure of the arteries and veins has, system.

Palpitation till of late years, been considered as alike, both being of the heart supposed to consist of two separate tunics, an elastic or Arteries outer, and a muscular or inner, independently of the soft and veins. 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. As Arterial 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

Skilful addivision of one trunk into two. As the arteries, how- justinen ever, recede from the heart, the blood, resisted at every the arterial

and muscustep by the elastic tunic of the canal it flows through, lar tunics. 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

Cause of altogether, the only tunic of which the canal consists :

BUS. collapse on whence the ease with which these vessels collapse on loss of some occasions, as loss of blood, or the exercise of ter- o ror, 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.

blood.

1. Ma

Venous structure.

nished with numerous

In the veins the elastic and muscular tunics are consichinery of

hi the sana

derably weaker than in the arteries; they have, neverguineous theless, a more difficult task to perform than arteries; system.

for, with a few exceptions, they have uniformly to force

the current of blood upwards to the heart against the Why fur- power of gravitation. They are hence far more nume

rously furnished with valves than the arteries, by which valyes, the ascending columns of blood are prevented from re

trograding; 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. Whether I have thus far adverted to the commonly received muscular fibres really opinion, and that taught by the most celebrated physioexist in ar- logists of our own country, and especially by Mr. John teries and

Hunter. Nevertheless it has long been a disputed point, veins.

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. Causes of I have observed that the force with which the blood gradual di. minution

• is at first projected from the heart, is progressively dimiof projectile nished by the resistance it encounters in the thick and force,

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 progres, sively diminishing force. The first is the short angles against which the blood has to strike at the origin of all

system.

stem,

varies in

the different branches : and the next, and most import- I. Maant, is the larger diameter of the general mass of the ar- ;

the santeries, compared with that of the heart or the arteries from guineous which they immediately proceed; the range of the diameter 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

of the arteaggregate diameter of the arterial system forms a cone whose apex is at the heart. And he concludes, also, a cone. and most correctly, that this conic proportion is most Conic pro

portion obvious, increases most rapidly, and spreads with its broadest base in infants, or rather in the fetus; for here different

ages. 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. We may hence, in some degree, account for the dif- Why the

pulse 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

found

empty after * On Blood, Inflammation, &c. Part. I. Sect. viii. p. 170. death.

[ocr errors]

guineous system.

I. Ma- for the empty state in which the arteries are found imchinery of

of mediately after death: a second reason is that the tunics the san

of the veins possessing little or no elasticity, readily di-
late to the distentive power of the blood as it moves for-
ward: a third, and indeed the principle reason, as suffi-
ciently proved by Dr. Carson of Liverpool, is the natu-
ral elasticity or resilience of the lungs, which, by keep-

ing them after death in a state of dilatation, allows the
Why blood blood to accumulate here as in the vacuum. And hence,
is accumu-
lated in the again, the reason of the accumulation of blood which
chest after is usually found in the chest after death, as well as the
death.

empty state of the vessels. The above This vacuity of the arteries upon death, was one of facts urged

the objections urged very forcibly by the ancients against
against the
doctrine of 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. D ins of the onein

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 con-

ducted. Diameter of The pulmonary artery, which receives from the heart the aorta

pulmo- the blood returned into it from the veins, bears a very nary artery close proportion to the diameter of the aortat, which alike.

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." Medi. co-Chir. 'Trans. Vol. xi. Part 1.

+ See Hunter on Blood, p. 133.

maintained.

on being slit, is about 3 inches : and hence there can I. Mabe little doubt that the quantity of blood sent back toch

the santhe heart, is on an exact balance with that which flows guineous from it. It is not, however, at any time the identical system.

Balance of blood which is thus returned to the heart; for every ,

y arterial and organ takes from the general current, as it visits it, venous

blood, how such parts and such principles as it stands in need of main 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. With respect to the actual quantity of blood contained Sum total of

the blood esin 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 40t; 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,

[ocr errors][merged small]
« AnteriorContinuar »