Page images
PDF
EPUB

220

CAPILLARY BLOOD-VESSELS.

nuous with the commencing twigs of the Venous system. The communication is established by means of a set of extremely minute vessels, which are termed Capillaries. These capillaries form a network, which is to be found in almost every

part of the body (fig. 121). It is in them alone that the blood ministers to the operations of nutrition and secretion. Even the walls of the larger blood-vessels are incapable of directly imbibing -o nourishment from the blood which passes through them; but are supplied with minute branches, which proceed from neighbouring trunks, and form a capillary network in their substance. The diameter of the capillaries must of course bear a certain proportion to that of the blood-discs which have to pass through them : in Man they are commonly from about 1-2500th to 1-1600th of an inch in diameter. In the true capillaries, it would seem that only one row or file of these particles can pass at a time; but we frequently see vessels passing across from the arteries to the veins, which will admit several rows. There seems, however, to be a considerable difference in the diameter of the same capillary at different times; a change sometimes taking place from causes which are not yet understood. The rate at which the blood moves

[graphic]

Fig. 121.-PORTION OF THE MEMBRANE
BETWEEN THE TOES OF THE HIND
THE FROG, more highly

FOOT OF

magnified than in fig. 120, showing the

network of Capillaries that traverses it;

a, small venous trunk; bb, branches communicating with the capillaries; c, intervening tissue covered with epi

thelium cells.

*From the Latin capilla, hair; so named on account of their being, like hairs, of very minute size. Their diameter is really, however, far less than that of ordinary hairs.

The circulation of the blood in the Frog's foot, the tail of the Tadpole, the gilis of the larva of the Water-Newt, the yolk-bag of embryo Fish, and other appropriate subjects for the observation, is one of the most beautiful and interesting spectacles that the Microscope can open to us. Details of the various modes of exhibiting it will be found in the Author's treatise on "The Microscope and its Revelations," Chap. xviii.

RESPIRATORY CIRCULATION.

221

through the capillaries of a warm-blooded animal, has been determined by microscopic examination to be about 3-100ths of an inch per second. From the comparison of this rate with that of the flow of blood through the larger arteries, which has been found by experiment to be nearly 12 inches per second, it appears that the area of the capillary system must be nearly four hundred times as great as that of the vessels which supply it with blood.

252. Thus the Arterial and Venous systems communicate with each other at their opposite extremities; their large trunks through the medium of the heart; and their ultimate subdivisions through the capillaries. Hence we may consider this double apparatus of vessels as forming a complete circle, through which the blood flows in an uninterrupted stream, returning continually to its point of departure; and the term circulation is therefore strictly applicable.

253. But the conveyance of the nutritive fluid to the several organs of the body, for their support and maintenance, is not the only object to which its circulation has to minister. It is requisite that the blood should be continually exposed to the influence of the air, by which it may get rid of the carbonic acid with which it has become charged during its circulation in the system, and may take-in a fresh supply of oxygen to replace that which has been withdrawn from it. In order to effect this exposure, the blood is conveyed to a particular organ, in which it is made to pass through a special set of capillary vessels, that bring it into almost immediate contact with air. In the lower tribes, in which this aeration is (from various causes hereafter to be explained) much less constantly necessary than in the higher, we find the respiratory organ supplied by a branch from the general circulation; and the blood which has passed through it, and which has been subjected to the invigorating influence of the air, is mingled in the heart with that which has been deteriorated by circulating through the system, which is again supplied with this mixed, half-aerated blood. But in the highest classes, there is a distinct circle of vessels subservient to the respiratory function: namely, an arterial trunk issuing directly from the heart, and subdividing into branches which terminate in the capillary system of the respiratory organ; a set of capillaries, in which the aeration of the blood takes place; and a system of veins

222

OTHER USES OF THE CIRCULATION.

which collects the blood from these, and returns it to the heart. This circuit of the blood is sometimes called the lesser circulation; to distinguish it from that which it makes through the general system, which is called the greater circulation.

254. Although carbonic acid is one of the chief impurities with which the blood becomes charged during its circulation, in consequence of the changes of composition which are continually taking place in the living body, it is by no means the only one; and other organs are provided, besides the lungs, for removing the noxious matters from the current of the circulation as fast as they are introduced into it. Thus, in the course of its movement through the general system, the blood is made to pass through the liver, the kidneys, and the skin, each of which has its special purifying office; these organs, however, have no such special circulation of their own as the respiratory apparatus of higher animals possesses, though the liver, as we shall hereafter see (§ 267), is peculiarly supplied by a sort of offset from the general circulation, so that the blood from which its secretion is formed is venous instead of arterial, like that transmitted to the lungs.

255. The course which the blood takes, and the structure of the apparatus which is subservient to its movement, differ very greatly in the several classes of animals. The chief of these differences will be pointed out hereafter; and it will be preferable to commence with the highest and most complex form of the circulating system, such as we find in Man, that it may serve as a standard of comparison with which the rest may be contrasted.

Circulating Apparatus of the Higher Animals.

256. In Man, and those animals which approach him most nearly in structure, the heart is situated between the lungs in the cavity of the chest, which is termed by anatomists the thorax. Its form is somewhat conical; the lower extremity tapering almost to a point, and the upper part being much larger. The lower end is quite unattached, and points rather forwards and to the left; during the contraction of the heart, it is tilted forwards, and strikes against the walls of the chest, between (in Man) the fifth and sixth ribs. It is from the large or upper extremity that the great vessels arise; and

CIRCULATING APPARATUS IN MAN.

223

these, being attached to the neighbouring parts, serve to suspend the heart, as it were, in a cavity in which its movements may take place freely. This cavity is lined by a smooth serous membrane (§ 43), which, near its top, is

[merged small][graphic][merged small][ocr errors][merged small][merged small][merged small][ocr errors]

Fig. 122.-LUNGS, HEART, AND PRINCIPAL VESSELS OF MAN.

ar, right auricle; vr, right ventricle; vl, left ventricle; a, aorta; vc, vena cava; ac, carotid arteries; vj, jugular veins; as, subclavian artery; vs, subclavian veins; t, trachea.

reflected downwards over the vessels, and covers the whole outer surface of the heart. Hence as the surface of the heart, and the lining of the cavity in which it works, are alike smooth, and are kept moist (in health) with a fluid secreted for the purpose, there is as little interruption as possible from friction in the working of this important machine.

257. The heart may be described as a hollow muscle, which, in Birds and Mammalia, as in Man, is divided into four distinct chambers. This division is effected by a strong vertical partition, that divides the entire heart into two halves, which are almost exactly similar to each other, excepting in the greater thickness of the walls on the left side; and each of these halves (which do not communicate with one another) is again subdivided by a transverse partition, into two cavities,

[ocr errors]

as

224

STRUCTURE OF THE HEART.

of which the upper one is termed the auricle, and the lower the ventricle. Thus we have the right and left auricles, and the right and left ventricles. Each auricle communicates with its corresponding ventricle, by an aperture in the

[merged small][merged small][graphic][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

Fig. 123.-IDEAL SECTION OF THE HUMAN HEART.

transverse partition, which is guarded by a valve. The walls of the ventricles are much thicker than those of the auricles; and for this evident reason,-that the ventricles have to propel the blood, by their contraction, through a system of remote vessels; whilst the auricles have only to transmit the fluid that has been poured into them by the veins, into the ventricles, which dilate themselves to receive it. The difference in the thickness of the walls of the left and the right ventricles is explainable on the same principle; for the left ventricle has to send the blood, by its contractile power, through the remotest parts of the body; whilst the right has only to transmit it through the lungs, which, being much nearer, require a far less amount of force for the circulation of the blood through them.

258. The arterial system of the greater circulation entirely springs from one large trunk, which is called the aorta (see figs. 122-124); this originates in the left ventricle, and is the only vessel which passes forth from that cavity. It first ascends towards the bottom of the neck; then forms what is termed the arch, a sudden curve, which gives it a downward

« PreviousContinue »