The parts of the lymphatic tube are so arranged that their composition is difficult to ascertain. The internal membrane, which many authorities consider as the only coat possessed by these vessels, is very thin, dense, and transparent. It distinguishes itself by forming an immense number of valves, more numerous and thinner than similar folds in veins; but in proportion to their thickness they have an equal or superior degree of strength. The disposition and form of lymphatic valves are very similar to those of the venous system; they are semicircular or parabolic in shape, with each having its convex edge attached to the side of the vessel and its straight margin loose in the interior. They are usually placed in pairs but sometimes, especially where branches terminate in trunks or into veins, there is only a single circular valve which frequently closes the canal imperfectly. The number of valves, extremely irregular, is greater in small lymphatics than those of larger calibre. In vessels of medium size, valves are found at intervals of 1/4 to 3/8 of an inch; for example, in spermatic absorbents, valves are regularly disposed about a line asunder. On the contrary, in some places a lymphatic is seen running several inches without a single valve; this is particularly remarked in the thoracic duct, which has, in many bodies, only three or four pairs of valves although in other instances it is crowded with them. The terminations of great trunks in subclavian veins are provided with double valves effectively preventing reflux of venous blood into their cavities. In consequence of the great number of valves and dilatations between them, vessels present a jointed appearance when injected.
The second or external coat can be demonstrated in the thoracic duct of horses; Cruikshank asserts that he has seen fibres in the duct of human subjects. The phenomena displayed by absorbent vessels seem to prove they possess a structure capable of contraction but no fibrous coat can be detected. The outer surface of these vessels is covered with loose cellular tissue connecting them with surrounding parts, this substance has been considered by some anatomists as forming the external coat.
It has been established by delicate injections that lymphatics are furnished with nutrient blood-vessels and it is also thought their walls receive minute absorbeats. No nerves have been traced except on the thoracic duct which is surrounded by branches from the par vagum and great sympathetic. Absorbents are much stronger in proportion to their size than veins, capable of sustaining a greater weight of mercury; they are also extensible and very elastic.
These vessels are endowed with contractile power which may be excited according to Cruikshank by the application of heat or exposure to cold air. This is illustrated by tying the thoracic duct or any other large lymphatic in a living animal, then making a puncture below the ligature; on doing this it is found that fluid contents escape with a degree of force never seen when a similar experiment is performed in a dead body.
It is probable that absorbents are provided with the same kind of sensibility possessed by other divisions of the vascular system.
Key Takeaways
- Lymphatic vessels have a complex structure including valves and internal membranes.
- Absorbent lymphatics are stronger than veins in proportion to their size.
- The contractile power of lymphatics can be demonstrated through experiments.
Practical Tips
- Understanding the structure of lymphatic vessels can aid in recognizing signs of swelling or blockages during medical assessments.
- Recognize that lymphatic valves function similarly to those in veins, ensuring fluid movement in one direction only.
- The presence and strength of lymphatic valves are crucial for preventing backflow.
Warnings & Risks
- Improper handling of lymphatic vessels can lead to complications such as infection or further injury.
- Experiments involving the manipulation of living animal tissues should be conducted with extreme caution and ethical consideration.
Modern Application
While this chapter focuses on historical anatomical descriptions, understanding the structure and function of lymphatic vessels remains crucial in modern medical contexts. Knowledge of these structures aids in diagnosing conditions such as lymphedema or infections affecting the lymph nodes. The principles described here form a foundational basis for contemporary vascular studies.
Frequently Asked Questions
Q: What are the key components of lymphatic valves?
Lymphatic valves are semicircular or parabolic in shape, with each valve having its convex edge attached to the side of the vessel and its straight margin loose in the interior. They are usually placed in pairs but sometimes occur singly where branches terminate.
Q: How does the contractile power of lymphatic vessels manifest?
The contractile power of lymphatic vessels can be demonstrated by experiments such as tying a thoracic duct or large lymphatic vessel and making a puncture below the ligature, showing fluid escape with force.
Q: What is the significance of the external coat in lymphatics?
The second or external coat of lymphatics can be demonstrated in the thoracic duct of horses. Some anatomists consider loose cellular tissue covering the outer surface as forming this external coat, though no fibrous coat has been detected.