Section II. Oroanizatiok of the veins. The coats of these vessels are much thinner than those of the arteries, and they present other peculiarities which require to be noticed. The intfnuil membrane lines all the veins with the exception of the pubnonary; it also covers the inner surface of the right cavity of the heart and of the pulmonary arteries. Many anatomists suppose that the funuses of the dura mater do not possess a lining membrane; but the fallacy of this opinion can be shown by a careful dissection of these vessels. This tunic is more thin and delicate than the inner coat of the arteries, and it is also more extensible and less brittle. The internal membrano forms within most of the veins numerous folds or valves, which contain condensed cellular substance, and occasionally distinct fibres. Each of these folds has an adherent margin, which is firmly attached to the circumference of the vessel; and a free edge, which is straight or slightly concave, and turned in the direction of the heart; both these borders are rather thicker than the other part of the valve. When the valve is put in action by the pressure of the blood, the surface towards the heart becomes concave, and the other side convex; it may also be noticed at this time, that the vein, which is rather dilated opposite to the valve, is rendered turgid and somewhat prominent. In most of the vessels the valves are placed in pairs; in the veins whose diameter is less than a line, the valves are single, and a similar disposition exists at the termination of the vena coronaria; some of the large trunks possess triple valves; and in some rare instances the folds are quadruple, and even quintuple. The number of valves is subject to variation in the different regions of the body; but it is generally in an inverse ratio to the size of vessels; although in the very small veins they entirely disappear. In the superficial vessels, in those exposed to the compression of the muscles, in the veins of the lower extremity, the valves are numerous; they are also usually observed in the places where the branches empty themselves into the trunks. On the contrary, they are not found in the right side of the vena cava, in the veins of the uterus, of the heart, of the lungs, of the brain, or of the spinal cord; they are also absent in the renal cavie, in the umbilical vein; and, usually, in the renal veins; the vena uterina seldom possesses any valves. The mechanism of these valvular bodies, which is so simple and yet so admirably adapted to its purpose, is such that, although it allows the free passage of the blood towards the heart, it effectively prevents any reflux in the opposite direction, and in this manner it powerfully assists in the venous circulation. The middle membrane is much thinner than the corresponding structure of the arterial system; it is formed of reddish, soft, and yielding fibres, which may be observed in the larger veins running longitudinally. It was denied by Bichat and Meckel that there are any transverse fibres, and it is true that none can be satisfactorily demonstrated passing in that direction. This coat is proportionally thicker in the small than in the large vessels; it is also more developed in the system of the ascending cava, than it is in that of the descending; and, lastly, the veins on the superficial part of the body are more distinctly fibrous than those which are deep-seated. These facts, as Hunter remarked, show that the contractile power is increased wherever there are obstacles to the return of the blood. A kyer of fibres resembling those of the auricle, may generally be seen in the two vena cava for about an inch before they terminate. The middle coat is deficient in the sinuses of the cranium, in which vessels its place is supplied by the dura mater. The external membrane is composed like the same clinic of the arteries, of elastic cellular substance; it is however, so thick nor so much condensed. It is very firmly connected with the middle and intenud coats through means of fibrous processes. The veins are supplied with vasa vasonima, and also with nerves, which may be traced in some of the large trunks.<Callout type="important" title="Critical Valve Function">The valves effectively prevent any reflux in the opposite direction, ensuring efficient blood flow towards the heart.</Callout> Section III. Professions and Uses. It has been generally admitted since the experiments of Wintringham that the density of the veins exceeds that of the arteries; but there is great reason to doubt the correctness of this opinion, which is supposed to be based on observations of several excellent modern anatomists. The strength of the venous parietes is also stated to surpass that of the arterial; and, if the inquiries of Wintringham be considered conclusive, the truth of this statement must be allowed. My own experience, however, induces me to agree with Dr. Gordon in thinking that the veins are much weaker than the arteries. The veins are capable of being considerably extended, particularly in the circular direction, without rupturing; this property is displayed in the great dilatation which the vessels undergo when the return of the blood is obstructed; also in cases of aneurismal varix, and in forcible injection after death. The parietes of the venous tubes are very elastic, but less so, according to the best modern writers, than the arteries; it was however, contended by Haller and Bichat that the former vessels possess more elasticity in the transverse direction than the latter.<Callout type="risk" title="Vein Rupture Risk">Be cautious when applying force to veins as they can dilate significantly without rupturing.</Callout> It is generally supposed, that the veins are not endowed with an active power of contraction, or, as it is more usually called, with irritability; but many facts may be mentioned which prove the existence of such a property. It is admitted by Haller and most other physiologists that the trunks placed near the heart possess irritability; and the direct experiments of Verschmr and Hastings show that this power extends to the whole venous system. Again, if a vein in a living animal be tied with two ligatures, and then punctured between them, the blood is rapidly ejected; whilst, if the same experiment be performed after death, the blood oozes out slowly and imperfectly. Although the contractile power of these vessels seems to be well established, it is necessary to add that it is inconsiderable and not to be compared to that of the arteries.<Callout type="tip" title="Living vs Dead Experiment">The living animal experiment demonstrates the active power of contraction in veins.</Callout> The remarks which were offered at page 279, concerning the sthnssthility of the vessels in general, may be applied to the veins. There has been much discussion among physiologists concerning the powers which accomplish the venous circulation. The illustrious Hales supposed that the propulsive action of the heart, extending its influence through the arteries and capillaries to the veins, was the efficient cause of this process. It has however, been determined by subsequent investigations, the the arterial and capillary vessels themselves, exert important influence on the movement of the blood. Bichat indeed, thought that the venous circulation depended solely on the action of the capillary system; but it is easy to disprove this hypothesis by direct observation. I shall satisfy myself with mentioning two facts to show that the systole of the left ventricle has a decided influence on the flow of the blood within the veins. The most sinking illustration is afforded by the experiment of M. Mugendie, who passed a ligature around the thigh of a dog, so as to include every part of it, except the femoral artery and vein. He punctured the latter vessel, and then made a small opening in it, and having done so, he found that the blood immediately escaped, and formed a considerable jet; but that, on compressing the artery, the flow was stopped, although the whole length of the vein was full; on removing the pressure from the artery the stream from the vein was re-established. The second instance which I shall adduce to demonstrate the influence of the heart is the distinct pulsation that a occasionally perceptible in the veins, and which is synchronous with the stroke of the left ventricle. We may then conclude that the matter which propels the blood along the veins is deprived principally from the heart, and from the contraction of the arteries and capillaries.<Callout type="important" title="Heart's Influence on Veins">The systole of the left ventricle significantly influences venous flow.</Callout> Physiologists have enumerated some other causes which have been thought to assist in the venous circulation, such as the contraction of the veins themselves; the pulsation of the neighbouring arteries; the compression of the surrounding parts, particularly of the muscles; and, in a more especial manner, atmospheric pressure. The facts which I stated when describing the structure and properties of the veins render it evident that these vessels possess an active contractile power, which assists in propelling their contents.<Callout type="warning" title="Atmospheric Pressure Risks">Be cautious with applying excessive external pressure as it can affect venous circulation.</Callout> With respect to the pulsation of the arteries, it is very doubtful if it has any influence on the venous circulation. This opinion, which is supported by so many high authorities, was founded on the erroneous idea that the arteries alternately dilated and contracted; but, as it is now admitted, that there is no perceptible motion in the artery during the ordinary action of the heart, it is difficult to conceive how any power of the above nature can exist. The pressure of the muscles and of the skin appears to favour the return of the venous fluid; and it is well known to surgeons that artificial compression, when properly applied, is a most powerful means of removing congestion of different parts by aiding the powers of the circulating vessels.<Callout type="tip" title="Compression Techniques">Applying proper pressure can aid in venous circulation.</Callout> The most important auxiliary to the vis tergo is, according to a commonly received opinion, the pressure exerted by the atmosphere on the surface of the body. Various attempts have been made to explain the mode in which this pressure acts on the veins. Some physiologists contend that a tendency to a vacuum is produced, in consequence of the active dilatation of the cavities of the heart; others think that the elasticity of the lungs powerfully assists in dilating the chambers of the heart; it has been applied that, as in inspiration the expanding parietes of the chest tend to leave a vacuum, the pressure of the atmosphere must necessarily force the blood into the thorax. In the ordinary and quiet state of the circulation, believe that the influence of inspiration in determining the flow of the venous blood is very insensible; but when the breathing is excited by any cause, the contents of the veins are forcibly drawn towards the heart during inspiration. With respect to the dilatation of the cavities of the heart, it appears that the auricle during its diastole spontaneously expands, and it is probable that the mechanism of the chest also assists in separating the walls of the auricle. I conceive, however, that the effect of the thoracic vacuum, whether it occurs in the lungs or in the heart, is not indispensably essential to the venous circulation. This opinion is supported by the fact, that if the vena cava, or any other great vein, is tied so as to cutoff the communication with the heart and thorax, it is distended with blood coming up towards the heart; and if wounded on the distal side of the ligature, the blood streams out till death ensues. Again, in the numerous animals which have neither heart nor lungs, and in the imperfect human foetus, heart is wanting; it is evident in these cases that the tendency to a thoracic vacuum can take place, and yet the circulation of the venous blood is perfectly accomplished. The instance of the circulation in the vessels of the vena portae might be adduced as an additional proof, that the occurrence of the above phenomenon is not necessarily required.<Callout type="important" title="Thoracic Vacuum Not Essential">The thoracic vacuum is not a necessary condition for venous circulation.</Callout> The blood moves more slowly in the veins than does in the arteries, partly in consequence of the larger capacity of the venous system, and partly owing to the difficulty of the propulsive powers. But the current in the veins is more rapid than it is in the capillaries, because as these small vessels unite to form the veins, the capacity of the vascular channel diminishes; and for the same reason, the blood flows with greater velocity as it pasties from the branches towards the trunks of the venous cslvx.<Callout type="risk" title="Vein Capacity Changes">Be aware that changes in vein capacity can affect blood flow.</Callout> The contraction of the ventricle, which causes the blood in the arteries to flow per saltum, does not produce a similar effect in the veins; so that if one of the latter vessels is opened, the stream that issues from it is perfectly uniform. It occasionally happens, however, when the action of the heart is violently increased, or when there is some obstacle in the circulation, that a distinct pulsation is observed in the veins, which is synchronous with and dependent on the stroke of the ventricle. There is also another kind of venous pulse in the vessels placed near the heart, which is produced by a very different cause, viz. the contraction of the auricle. It has long been remarked that when this cavity contracts, a certain quantity of the blood which it contains, flows back into the vena cava; and in some cases, the undulation extending to the hepatic and jugular veins, may be seen in the latter vessels, particularly in a thin individual. The distention thus produced must be distinguished from the fulness that is caused by the act of expiration, during which period, the blood being forced from the chest towards the veins, is accumulated about the head and neck, and also in the viscera of the abdomen. It is evident from the mechanism of the venous system that the reflux cannot in either case extend beyond the first pair of valves.
Key Takeaways
- Veins have thinner walls and more flexible structure compared to arteries.
- Valves within veins prevent blood from flowing backward, ensuring efficient circulation towards the heart.
- The contraction of the left ventricle significantly influences venous flow.
Practical Tips
- Understand that valves in veins are crucial for preventing backflow and maintaining proper blood direction.
- Be cautious when applying force to veins as they can dilate significantly without rupturing, but this also means they need careful handling during medical procedures.
- Recognize the importance of the heart's systole in driving venous circulation.
Warnings & Risks
- Avoid excessive external pressure on veins as it can affect their natural function and potentially cause issues like varicose veins or blood clots.
- Be aware that changes in vein capacity can significantly impact blood flow, especially during physical activity or injury.
- Do not assume the thoracic vacuum is a necessary condition for venous circulation; other factors play significant roles.
Modern Application
While the anatomical descriptions and physiological explanations provided in this chapter are rooted in historical understanding, they still hold relevance today. The principles of vein function, such as valve mechanisms and the role of the heart, remain foundational to modern medical practices. Understanding these concepts can help in recognizing issues like varicose veins or deep vein thrombosis, ensuring proper care and treatment.
Frequently Asked Questions
Q: What are the main functions of valves in veins according to this chapter?
Valves in veins prevent blood from flowing backward, ensuring efficient circulation towards the heart. They effectively stop any reflux in the opposite direction.
Q: How does the contraction of the left ventricle influence venous circulation?
The contraction of the left ventricle has a significant influence on venous flow. Experiments by Mugendie and others demonstrate that it can stop or start blood flow in veins, depending on arterial pressure.
Q: What are some factors that assist in the venous circulation according to this chapter?
Factors include the contraction of the veins themselves, pulsation of neighboring arteries, compression of surrounding parts (especially muscles), and atmospheric pressure. These can all aid in propelling blood through the venous system.