IMMUNE SYSTEM
The lymphatic system is more highly developed than the venous system in reptiles. Although they lack lymph nodes, reptiles have vast plexiform lymphatic networks and large dilated reservoirs (cisternae) that occur at the sites of
Reptiles
mammalian lymph nodes.
These are pumped by lymph hearts, which are smooth muscle dilations in the lymphatic channels located in the caudal part of the trunk. The main connection with the venous system occurs at the base of the neck where a saccular precardiac sinus passes lymph to the venous system (Ottaviani & Tazzi 1977).The major lymphatic trunks are the jugular, subclavian, lumbar and thoracic. The jugular trunk drains the head and neck, the subclavian the forelimbs, the lumbar the hindlimbs, and the thoracic drains the trunk and celom. Both the lumbar and thoracic form a lymphatic dilation called the cisterna chyli.
The bone marrow, spleen, thymus and lymphatic system all play a part in immunoregulation. The paired thymus gland does not involute, although weight and size decrease with age (Bockman 1970). One or two yellow or white lobes are found on each side of the neck in lizards and chelonia and just cranial to the heart in snakes.
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CLINICAL NOTE
Lymphdilution is a common contaminant of blood samples as the lymphatic system is so intimately associated with the blood. Dilution of blood samples will mainly affect the packed cell volume (PCV) and white cell count so a very low PCV should always be carefully evaluated.
KEY POINTS
• Snakes and lizards have a kinetic skull, which allows a wide gape.
• Metabolic bone disease is common in lizards and chelonia but not in adult snakes.
• The single ventricle allows blood to be shunted away from the lungs while diving or breathholding.
• Reptile erythrocytes are nucleated.
• Reptiles have a highly developed lymphatic system so lymphdilution of blood samples is common.