230

PECULIARITIES OF DISTRIBUTION OF ARTERIES.

originate the trunks which supply the brain in the usual manner. The object of this apparatus appears to be, to prevent the influence of gravitation from causing a too great rush of blood towards the brain, when the head is in a depending position; for the rapidity of its flow will be checked, as soon as it enters the network, and is distributed through its numerous canals. A similar conformation is found in the blood-vessels of the limbs of the Sloth, and of some other animals which resemble that animal in the sluggishness of their movements; and its object is probably to prevent the muscles from receiving too rapid a supply of blood, which would give them what (for these animals) would be an undue energy of action; whilst, by the very same delay, their power of acting is greatly prolonged, as we find it to be in Reptiles, whose circulation is languid (§ 284).

265. In the Whale tribe, and some other diving animals that breathe air, we find a curious distribution of the bloodvessels, which has reference to their peculiar habits. The intercostal arteries (which are sent-off from the aorta to the spaces between the ribs on each side) are enormously dilated, and are twisted into thousands of convolutions, which are bound together into a mass by elastic tissue. This mass, which is of considerable bulk, lies at the back of the chest, along both sides of the vertebral column; and it serves as a reservoir, in which a great quantity of arterial blood may be retained. The veins also have very large dilatations, which are capable of being distended, so as to hold a considerable amount of venous blood; and thus, while the animal is prevented from breathing by its submersion in the water, the circulation through the capillaries of the system is sustained, by the passage of the blood stored up (as it were) in the arterial system, into the venous reservoirs. If this provision did not exist, the whole circulation would come to a stand, in consequence of the obstruction it meets with in the lungs, when the breathing is stopped.

266. With regard to the Venous system, there is little to be added to what has been already stated (§§ 248-250) as to its general character and distribution. The large proportion which its capacity bears to that of the arterial system, is shown by the fact, that every main artery is accompanied by a vein (frequently by two) considerably larger than itself; and that the

DISTRIBUTION OF VEINS-PORTAL SYSTEM.

231

superficial veins, which lie just beneath the skin, are capable of conveying at least as much more. The veins of the body in general unite in two large trunks, the superior and inferior vena cava; which meet as they enter the right auricle of the heart (fig. 123). The superior vena cava is formed by the union of the veins which return the blood from the neck (the jugulars) with those which convey it from the arms (the subclavians), as shown in fig. 122; and the inferior cava (vc, fig. 122) receives the blood from the trunk, the organs contained in the abdomen, and the lower extremities.

267. There is, however, an important peculiarity in the distribution of the veins of the Intestines, which should not pass unnoticed. Instead of delivering their blood at once into the inferior vena cava, these veins unite into a trunk, called the Vena Porta (fig. 134), which enters the liver and subdivides into branches, whence a capillary network proceeds that permeates the whole of its mass. It is from the venous blood, as it traverses this network, that the secretion of bile is formed; and the blood which is brought by the hepatic artery serves chiefly to nourish the liver,-no bile being formed from it, until it has become venous. The blood is carried-off from this double set of capillaries by the hepatic vein, which conveys it into the inferior vena cava. In Fishes, not only the blood of the intestines, but that of the tail and posterior part of the body, enters this "portal" system, which is distributed to their kidneys as well as to their liver. Thus all the blood which flows through the portal system, has to go through two sets of capillaries, between each period of its leaving the heart by the aorta, and its return to it by the vena cava.

268. We have yet to notice the lesser circulation, which is confined to the Lungs only. The venous blood which is returned to the heart by the venæ cavæ, enters the right auricle, and thence passes into the right ventricle. By the contraction of this last cavity, it is expelled through the pulmonary artery (fig. 123), which soon divides into two main trunks that proceed to the right and left lungs respectively. The right trunk again subdivides into three principal branches, which are distributed to the three lobes or divisions of the right lung; whilst the left divides into two branches, which are in like manner distributed to the two lobes of the left lung. The capillaries, into which these branches ultimately

232 LESSER CIRCULATION-FORCES THAT MOVE THE BLOOD.

subdivide, are distributed upon the walls of the air-cells (fig. 162), and the character of the blood is in them converted, by exposure to the air, from the dark venous to the bright arterial. From this capillary network the pulmonary veins arise; and the branches of these unite into trunks, of which two proceed from each lung, to empty themselves into the left auricle (fig. 123). This auricle delivers the blood, now arterialized or aerated (§ 253), into the left ventricle, whence the aorta arises; and by the contraction of this cavity, it is delivered through that vessel to the system at large.—It will be observed that the vessel which proceeds from the heart to the lungs is called the pulmonary artery, although it carries dark or venous blood. This is because it conveys the blood from the heart towards the capillaries. And, for a similar reason, the vessels which return the blood from the capillaries to the heart are termed pulmonary veins, although they carry red or arterial blood.

Forces that move the Blood.

269. The mechanical action, by which the blood is caused to circulate in the vessels, is easily comprehended. The cavities of the heart, as already explained (§ 245), contract and dilate alternately, by the alternate shortening and relaxation of the muscular fibres that form their walls (Chap. XII.); and the force of their contraction is sufficient to propel the blood through the vessels which proceed from them. The two ventricles contract at the same moment; the auricles contract during the relaxation of the ventricles, and relax whilst the ventricles are contracting. The series of movements is therefore as follows:-The auricles being full of the blood which they have received from the vena cava and pulmonary veins, discharge it by their contraction into the ventricles, which have just before emptied themselves into the aorta and pulmonary artery, and which now dilate to receive it. When filled by the contraction of the auricles, the ventricles contract in their turn, so as to propel their blood into the great vessels proceeding from them; and whilst they are doing this, the auricles again dilate to receive the blood from the venous system, after which the whole process goes-on as before. It is when the ventricles contract, that we feel the beat of the heart, which is caused by the striking of its lower extremity

MECHANISM OF THE HEART.

233

against the walls of the chest; and it is by the same action that the pulse in the arteries is produced (§ 276).

270. The combined actions

of each auricle and its ventricle, may be illustrated by an apparatus like that represented in fig. 126. It consists of two pumps, a and b, of which the pistons move up and down alternately; and these are connected with a pipe cf, in which there are two valves d and e, opening in the direction of the arrow. The portion c of the pipe represents the venous trunk by which the blood enters the heart; the pump a represents the auricle, and the raising of its piston enables the fluid to enter and fill it. When its piston is lowered, its fluid is forced through the valve d into the pump 6 (which represents the ventricle), whose piston rises at the same time to receive it; and when this piston is lowered in its turn, the fluid (being prevented from returning into a by the closure of the valve d) is propelled through the valve e into the pipe f, which may represent an arterial tube; whilst at the same time a fresh supply of blood is received into the pump a by the raising of its piston.

Fig. 126.

271. The number of contractions of the heart ordinarily taking place in an adult man, is from 60 to 70 per minute. It is usually rather greater in women; and in children it is far higher, being from 130 to 140 in the new-born infant, and gradually diminishing during the period of infancy and childhood. It is rather greater in the standing than in the sitting posture, and in sitting than in lying down: it is increased by exercise, especially by ascending a steep hill or going upstairs, and also by any mental emotion. It is important to remember these facts, in reference to the management of those who are suffering under diseases of the heart or of the lungs, which prevent the ready passage of the blood through these organs; for if more blood be brought to the heart by the great veins, than it can propel through the pulmonary arteries, a feeling of

234

VALVES OF THE HEART.

very great distress is experienced; and there may be even danger of rupture of the heart or large vessels, or of sudden cessation of the heart's action, causing instant death. Such persons ought, therefore, carefully to refrain from any violent muscular movement, and also to avoid giving way to strong mental emotions.-In syncope or fainting, the heart's action is so weakened as to be scarcely perceptible, though it does not entirely cease; and this state may be brought on by several causes which make a strong impression on the nervous system, such as violent mental emotion (whether joy, or grief, or terror), sudden loss of blood, and the like.

272. The blood which has been received by each ventricle from its auricle, is prevented from being driven back into the latter, on the contraction of the former, by a valve that guards the aperture through which it entered. This valve consists of a membranous fold, surrounding the borders of the aperture, and so connected with the neighbouring parts, as to yield when the blood passes from the auricle into the ventricle, but to be tightened so as completely to close the aperture when the blood presses in the contrary direction. The manner in which these valves act will be seen from fig. 127, which is a section of the right auricle with its ventricle. The auricle, a, receives its blood from the two venæ cavæ, e, e; and transmits it into the ventricle, b, by the orifice, c. On either side of this orifice are seen the membranous folds, which are kept in their places by the tendinous cords, d. Now when the blood is passing from

e

a

f

b

a to b, these folds yield to the current; but when the cavity b is filled and begins to contract, the blood presses against g their under sides, so as to make them close against each other, as far as they are permitted to do by the tendinous cords. In this manner the aperture is completely shut, and no blood can flow back. A valve of this kind exists on each side of the heart; but there is a slight difference between the forms of the two, whence they have received different names. That on the right side has three pointed divisions, to which the tendinous cords are attached, and it is hence called the tricuspid valve;