Chapter 3 THE LYMPHATIC SYSTEM

3.1 Objectives

• 1. Describe the functions of the lymphatic system.
• 2. Give the purpose for examining the lymphatic system at the time of slaughter.
• 3. Name the major parts of the lymphatic system.
• 4. Name the lymph nodes that may be inspected (with species variations) in the head, viscera, and carcass 

3.2 REVIEW OF THE LYMPHATIC SYSTEM 

The examination of the lymphatic system of the animal or carcass in meat inspection is very important in detecting abnormal conditions. The function of the lymphatic system related to the evidence found during the examination aids in determining the stage to which the disease had progressed. Standard inspection procedures require the examination of specific lymph nodes and organs in order to detect abnormal and disease conditions. Animals and carcasses found with abnormalities -- including abnormalities of the lymphatic system -- must be segregated and examined by the veterinarian.

The supplement contains references on the lymphatic system, MPI guideline, lymph nodes of cattle, hogs and sheep.

Standard texts may be reviewed for more detail on anatomy, physiology, and pathology of the lymphatic system.

This will be a condensed review of the lymphatic system.

The lymphatic system consists of the following:

• 1. The lymph
• 2. The lymph vessels
• 3. The lymph nodes
• 4. The accessory lymphatic structures.

We will review some of the more important points of each of the four components of the lymphatic system.

The lymph is derived mainly from the blood and its composition is similar to blood plasma. It passes from the thin wall of the blood capillaries into the tissue. It carries nutritious substances from the blood to the body tissues and waste material from the tissues to the blood.

The lymphatic system is the first line of defense in protecting the body against disease. Many disease causing microorganisms (pathogens) are retained and destroyed in lymph tissue. Some pathogens are retained and produce disease in the lymph node. Some pathogens pass through a node without leaving any trace of their passage. Particles of carbon may be filtered out and remain in the node. Blood and tissue pigments may be present due to tissue destruction somewhere upstream.

The lymph vessels consist of a system of tubes beginning in the tissue spaces as blind lymph capillaries and converging to form smaller, then larger vessels. The system of vessels terminates into the large vessels, the thoracic duct and the right lymphatic duct that empties into the heart. The flow of lymph is sluggish and in one direction from the tissues to the heart due to:

•  1. There is a difference in pressure at the two ends of the system.There is higher pressure in the small vessels and the lower pressure in large vessels.
• 2. The massaging effect of muscular movements.
• 3. The valves in the larger vessels prevent backward flow of lymph. The thoracic duct receives lymph from all parts of the body except the right forelegs and the right half of the head, neck, and thorax which is received by the right lymphatic duct. An area drained by a particular node is referred to as a drainage area.

A lymph node is a collection of specialized cells held together and separated by a tissue framework and surrounded by a thick capsule. An inspector often depends on the lymph nodes in making a decision of health of disease and in deciding if a disease condition is localized or generalized. The size and shape of lymph nodes varies. In young animals, they are yellowish white, larger, and contain more fluid than in old mature animals in which they are firm, compact and fibrous. The nodes vary in color and consistency. Mesenteric nodes may be from white to dark gray, brown, red, or even black in color. Mottled, black, or white nodes are usually not diseased. Hemorrhage in a lymph node does not always indicate disease.

Other lymphatic structures include the spleen and lymph follicles. The system is rich in lymphatic tissues. It is the largest concentration of lymph type tissue in the body. It differs because the flow goes into the blood stream and not into the lymph. It filters the blood the same as the lymph nodes filter the lymph. Lymph follicles are present in the intestines, soft palate, base of the tongue, tonsils, and posterior nares.

The last topic for discussion is the lymph nodes that require routine inspection for the MPI manual. Inspection is classified as head, viscera or carcass inspection. The head nodes that may be required to be inspected, depending on species requirements include the mandibular, parotid, suprapharyngeal (medial retropharyngeal), and atlantal (lateral retropharyngeal).

The lymph nodes of the viscera that may require routine inspection include the mesenteric, hepatic, right and left bronchial (right and left tracheobronchial), posterior, middle, and anterior mediastinal (cranial, middle, caudal). The body nodes that may be required to be inspected (depending on the species inspected) are the prescapular (superficial cervical), popliteal (deep popliteal), prefemoral (subiliac), superficial inguinal (scrotal), supramammary (mammary), internal iliac (medial iliac), lumbar (lumbar aortic) and renal lymph nodes.