Lymph Nodes of Cattle, Hogs, Sheep, and Horses
Lymp Nodes of Cattle, Hogs, Sheep, and Horses – The lymphatic system is widely used as an evaluation tool in meat inspection. In many cases the decision whether a carcass is wholesome or not is determined by what is found upon examination of the lymph nodes and associated structures.
It is important, therefore, that meat inspection personnel know the anatomical location of lymph nodes in the various species of animals presented for slaughter.
Also, when making carcass dispositions, the veterinarian must frequently rely on his or her knowledge of the afferent and efferent drainage of lymph nodes so that he or she can determine the pathogenesis of a disease process.
The purpose of this revised guideline is to update the original version and to expand it somewhat to include the equine. Since the original Guideline 5 was written, many of the names of the lymph nodes have been changed.
However, to make it less confusing, both the new and old names of the nodes will be listed. The old name will be in parentheses.
The lymphatic system consists of the lymph vessels qwqycHIM and nodes that are engaged in conveying the lymph. It’s CIV! The basic function is to drain extracellular fluid from the tissue interstices back into the blood stream and thus prevent the accumulation of extracellular materials.
It also acts as a defense mechanism against bacteria and other noxious materials by filtering them out of tissue fluid and phagocytizing them.
The Lymph – Lymph is a clear, colorless fluid except in the intestinal vessels, in which, after digestion, it is milky in color and is termed “chyle.” Lymph is generally considered to be identical with extracellular fluid, but in fact the fluid contained within lymphatics may differ markedly from that contained within the extracellular space of tissues. Therefore, it is more correct to consider lymph as only that fluid contained within lymphatic vessels and not to include the entire extracellular space outside of blood vessels.
The composition of lymph from various tissues varies with the relationship of the lymph capillaries to blood capillaries and with the amount of material produced by parenchymal cells of the tissues that finds its way into the lymphatics.
Lymph has a specific gravity of about 1.015. It contains lymphocytes, and normally a few red cells are present.
Neutrophiles are normally absent; however, they may be present in great numbers in infections. Platelets are absent. Lymph does contain fibrinogen and prothrombin, and all lymph will clot slowly.
Lymph contains water, gases, proteins, nonprotein nitrogenous substances, glucose, inorganic substances, hormones, enzymes, vitamins, and immune substances.
The Lymph Vessels – The lymph vessels start as blindending, finger-like, thin-walled capillaries in the connective tissue.They form three-dimensional capillary networks that drain into the larger and thicker-walled lymph vessels, the lymph ducts, and the trunks.
The content of these large trunks and ducts then empties into the cranial vena cava at the thoracic inlet. Lymph vessels are absent from regions lacking in connective tissue, such as the parenchymatous portions of the liver, spleen, tonsils, and lymph nodes.
Nor are lymphvessels found in the bone marrow, central nervous system (except the meninges), umbilical cord, embryonic membranes, hyaline cartilage, epithelial layer of the skin, cornea, lens, or vitreous humor of the eye.
The lymph capillaries consist of an endothelial tube embedded in connective tissue, ending blindly in rounded or slightly enlarged ends. Compared to blood capillaries, the lymph capillaries lack a surrounding basement membrane, a layer of pericytes. This fact probably accounts for their ability to absorb macromolecules from tissue fluid and inflammatory exudates more readily than the blood capillaries.
All lymph vessels with the exception of capillaries contain valves. Smaller vessels often have only one foldlike valve while the rest have two leaflets. Some of the larger valves may have muscular fibers which help move the lymph. The lymph moves very sluggishly in the lymph vessels. The movement from the periphery to the center is affected by pressure gradients brought about mainly by forces outside of the lymphatic system.
Factors influencing the lymph flow are: (1) respiratory movements, (2) abdominal pressure, (3) intestinal peristaltic movements, (4) increase in blood pressure, (5) venous congestion, (6) pathological conditions, and (7) increase of body temperature. Lymph flow may also be affected by anesthesia and various drugs.
The Lymph Nodes – On the course of the lymph vessels are situated discrete nodular structures called lymph nodes (Figure 1). They consist of an accumulation of lymphatic tissue enclosed by elastic fibers and smooth muscle fibers containing a connective tissue capsule, from which trabeculae and septi pass into the parenchyma of the node.
Under the capsule and around the trabeculae and septi extends a complex system of lymph sinuses. These sinuses are lined with endothelium. The sinus under the capsule is called the “marginal sinus” and those around the septi and trabeculae are called the “intermedial sinuses.” The sinuses in the medullary substance (parenchyma) are the “medullary sinuses,” which join and form the “terminal sinus” at the hilus of the lymph node.
The lymph vessels that carry lymph to the lymph node are called afferent lymph vessels. They enter the marginal sinus. The efferent lymph vessels carry the lymph away. They start at the terminal sinus. In swine, the situation is reversed—the afferent vessels enter the center and the efferent vessels start at the periphery of the lymph node.
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