Section I. OF THE CAPILLAKV VH88BL8.
TuE minute tubvs which coostitutc the ultimate terminations of the ftrtcries and the first origins of veinn, are distinguished by the name of zasa. Bichat regards them as forming a system by Uiemsolv which is distinct from the rest of the vascular &ystem, and in which the vital functions of secretion, nutritiot, caloritication, &c., are accomplished. Although this celebrated physiologist made too detined a separation between the capillaries and the larger blood-vessels, yet it is certain that these small canals arc the mechanical tentu,- by which the rariaus changes that occur in the cora- position of the body, whether healthy or morbid, are effected.
The capillary vessels are so minute that they cannot be detected by the naked eye. This circumstance gave to many errors before the art of magnifying objects with optical instruments, and that of filling the vessels with fine injection, were discovered. Thus anatomists for long lime believed, and the opinion is supported by some in the present day, that a peculiar substance or parenchyma intervened between the smallest visible branches of the arteries and the commencement of the veins.
<Callout type="important" title="Important">The truth of this theory is disproved by the fact that a fluid may be thrown from the arteries into the veins without any extravasation between them.</Callout>
A more direct source of information is obtained by carefully examining with a microscope the circulation in the transparent parts of animals, as the web of the frog's foot, or the mesentery of the rabbit. By this means we are able to trace the ramifications of the arteries until they become directly continuous with the branches of the veins.
It is impossible to state with precision where the capillary system begins, or where it ends. It will, however, be sufficient for all useful purposes, if we include under this denomination all arteries and veins which are invisible to the naked eye. I am aware that this definition is not free from objection, because, in the first place, it is arbitrary, for vessels which are seen by one individual may be imperceptible to another whose vision is less powerful; and in the second place, many writers restrict the term of capillaries to the minute ramifications of the arterial system.
The circulation of the blood being double in the human species, it follows that the capillary system must likewise be divided into two parts. One set of capillaries is diffused in all parts of the body, being placed between the terminal branches of the arteries and the roots of the veins; the other set, less extensive and capaciou.s, is situated between the minute ramifications of the pulmonary arteries and the origins of the pulmonary veins. In the former class the arterial blood is converted into venous; and in the latter, the venous blood is changed into arterial. The capillary vessels which are placed at the commencement and termination of the vena portse form a part of the general capillary system.
The capillary vessels are not all of the same eiu ; those which grow from the arteries become finer till at kniUh they only recct one red particle at a time. From this point they begin to enlarge, and continue to do so till they terminate in the primitive branches of the liver. It is a question of interest if there can be any capillaries so small that they cannot admit the coloured part of the blood. The great majority of anatomists think that such vessels do exist; indeed it is surprising that any doubts should have arisen on this subject, at least in modern times. Many parts of the body, such as the cartilages and the diaphanous membranes of the eye, are supplied by vessels which carry only the colourless portion of the blood, and which therefore escape observation; but we cannot for this reason suppose that these ports have no arteries or veins.
<Callout type="risk" title="Risk">Inflammation can cause transparent vessels to become apparent due to their dilation.</Callout>
Although the existence of these colourless vessels appears to be satisfiictorily proved, yet it must be confessed that little is known of their real nature. Haller, Bichat, and others have stated that in many parts of the body, as the skin, the serous and mucous membranes, &c., the arteries terminate in exhaling tubes, which themselves are supposed to end by little orifices or mouths: and in other organs, the pellucid capillaries are found to be continuous with excretory ducts. Other anatomists, as Mascagni and Richertind, contend, on the contrary, that the capillary vessels neither end in exhul&nt nor are continuous with excretory ducts; but that they possess lutfnil and organic pores, through which the various substances necessary for nutrition and secretion are separated from the blood. It is evident that both these theories are entirely conjectural, for it is impossible to perceive either exhtlant orifices or lateral pores.
The experiments of M. Fodera upon the imbibing power of the living solids have induced M. Magendie to renvc the old opinion, according to which exhulation depends on transudation. These investigations certainly shew that the dssues of a living animal are capable of being penetrated by substances which are in contact with them.* But this is only one step towards elucidating the wonderful and ever-varying process of secretion, by which the most dissimilar fluids are separated from the common mass of the blood. Notwithstanding the great progress that has been made in the study of the tvmimiJ economy, and the perfection which has been attained in the construction of the microscope, I believe we are still ignorant of the connexion which exists between the small blood-vessels and the solids of the body; or, in other words, of the process by which the particles required for nutrition, and those which form the secretions are separated from the circulating blood.
No direct connexion between the arteries and lymphatics can be perceived, even with the aid of magnifying glasses, and in consequence of this circumstance most anatomists deny that it exists. It is, however, found thut injections pass from the ortertca into the lymphatics; and therefore Mngendiv, adopting the ideas of Hartotine and others, considers that the latter has an immediate communication with the former.
The number of the capillaries, and their proportion with the solid substance, are extremely various in the different organic structures. In many parts, as in the epidermis, the arachnoid, and the cellular membrane, no arteries can be injected; but this depends on the imperfection of the method employed, and not on an actual deficiency of blood-vessels. The cartilages in the foetus can be injected, but not in the adult; the serous membranes lux- iimdo rather red by the injection of their vessels. The fibrous structures receive many capillary arteries, some of which admit the coloured part of the blood; the hones are more vascular, and are readily injected. The muscular and nervous system;* are abundiuntly supplied with ciipillories; but the most vascular parts of the body are the secretory organs, the skin, the mucous membrnneR, and the lungs.
<Callout type="important" title="Important">The organization of the capillary vessels cannot be ascertained by examination, in consequence of the minuteness of these tubes, and the difficulty of distinguishing them from the parts by which they are surrounded.</Callout>
We may, however, conclude that they are lined by the continuation of the internal membrane of the arteries; and it is also probable, as they exhibit distinct marks of contractility, that they are provided with a fibroufi cont.
Key Takeaways
- Capillaries play crucial roles in the body's secretion and nutrition processes.
- They are minute blood vessels that connect arteries to veins, facilitating exchange of substances between blood and tissues.
- The existence and nature of colorless capillary vessels remain largely unknown.
Practical Tips
- Use a microscope when examining small blood vessels for accurate identification and analysis.
- Understand the double circulation in humans to better appreciate the complexity of the circulatory system.
- Be aware that inflammation can make normally invisible vessels visible, which is important for diagnosing conditions like ophthamia.
Warnings & Risks
- Do not assume the absence of blood vessels based on their invisibility to the naked eye; they may be present but undetectable without magnification.
- Be cautious when interpreting anatomical theories that are based on limited understanding and technology, as modern research may contradict them.
- Avoid making assumptions about the nature of capillary structures without empirical evidence.
Modern Application
While the historical techniques described in this chapter provide foundational knowledge for understanding blood vessels, modern advancements in microscopy and imaging have greatly improved our ability to visualize and study these minute structures. However, the principles of how capillaries facilitate exchange between blood and tissues remain crucial for survival scenarios involving injury or disease diagnosis.
Frequently Asked Questions
Q: What is the significance of capillaries in the body's functions?
Capillaries are essential for the exchange of substances between the bloodstream and the surrounding tissues, playing key roles in secretion, nutrition, and other vital processes.
Q: How can one identify capillaries under a microscope?
Capillaries appear as very small blood vessels that connect arteries to veins. They are lined by the continuation of the internal membrane of the arteries and exhibit distinct marks of contractility.
Q: What does it mean if a part of the body can be injected with fluid but no capillaries are visible?
This indicates that while there may be blood vessels present, they are too small or their structure is such that they cannot be seen without magnification. It highlights the limitations of visual inspection in identifying all blood vessels.