ENDOCRINE SYSTEM
Thyroid gland
Like mammals, thyroid hormones maintain and stimulate metabolism under pituitary control. However, it is only effective in increasing metabolism if temperatures are suitable for that species.
It also plays an important role in shedding and growth. In chelonia and snakes, this gland is unpaired and spherical and lies ventral to the trachea just cranial to the heart. In lizards the thyroid varies between species and can be paired, bilobed or unpaired. The commonest is the bilobed organ with an isthmus over the trachea, as in mammals.Parathyroid glands
The parathyroids have a similar structure to those found in mammals but, unlike them, in reptiles they are found near the thymus or ultimobranchial bodies and not with the thyroids. Chelonia have two pairs; the rostral pair are hard to visualize because they lie within the thymus gland but the caudal pair can be clearly seen near the aortic arch (Clark 1970).
In snakes the rostral pair of parathyroid glands lie near the angle of the jaw, with the second pair lying more caudally, near the thymus and heart. Lizards may have 1-2 pairs depending on the species. A rostral pair lies in the neck near the bifurcation of the carotid arteries and the caudal pair (when present) lie just behind them near the aortic arch (Clark 1970).
The parathyroids control calcium and phosphorous levels. Reptiles on a low calcium diet develop hypocalcemia, which stimulates increased production of PTH. This acts to mobilize calcium from the bones to increase serum calcium. Nutritional secondary hyperparathyroidism and osteopenia eventually results and can be seen clearly on radiographs (Figs. 2.5 and 2.6).
Adrenal glands
These are yellow/red in color and lie retroperitoneally in crocodiles and chelonia and closely adherent to the gonads in lizards and snakes. However, they always lie dorsal to the gonads and, except in chelonia, lie asymmetrically with the right cranial to the left (Gabe 1970).
Chelonia have dorsoventrally flattened glands which lie against the kidney. Ventrally they are covered by peritoneum that extends forward to form the mesorchium or mesovarium of the adjacent gonads. Snakes and lizards have adrenal glands actually incorporated into the mesorchium or mesovarium, close to their respective gonads. They are elongated in snakes and usually globular in lizards. The right gland is attached to the caudal vena cava (Gabe 1970).
Pancreas
This forms a c-shaped loop in chelonia, attached to the mesenteric border of the duodenal loop. In lizards it has three parts: one extending toward the gall bladder, one toward the duodenum, and one toward the spleen.
In snakes the pancreas is often pyramidal in shape and lies caudal to the spleen in the first part of the duodenum. Its location can vary with species but it is often intimately associated with the spleen (Miller & Lagios 1970).
Pineal gland
The pineal gland is closely associated with the parietal eye. It is a saccular organ lined by epithelial cells containing photoreceptor and secretory-like cells. It converts photic stimuli into neuroendocrine messages and may play a role in thermoregulation. Some lizards have a more superficial parietal gland or third eye, which has a lens, cornea, and retina and is located just beneath the skin in the parietal foramen at the junction between the parietal and frontal bone. Although it does not form images, it is thought to sense changes in the intensity and wavelength of light and may aid thermoregulatory shuttling (Bellairs 1969e; Firth & Turner 1982). Crocodiles lack both parietal and pineal glands.