Comments & suggestions  should be directed to Visit my website Special Anatomical and Physiological Features Dental formula: 2 (I1/1,  C0/0,  P0/0,  M3/3).  The incisors grow continuously throughout life and are referred to as open-rooted incisors.  The molars are rooted.  Rats with malocclusion of the incisors require periodic trimming of the incisors so that the animals can eat.  Such animals should not be used as breeding stock as the condition is inherited. Rats lack taste receptors for water. Rats cannot vomit.  This is due to the entry of the esophagus into the stomach through a fold called the limiting ridge.  This fold separates the non-glandular and glandular portions of the stomach and prevents the rat from vomiting.  The inability to vomit is probably one of the reasons that rats are so cautious about ingesting foreign substances. The gastrointestinal tract of the rat contains a cecum, a large, thin-walled blind pouch.  A portion of the cecum contains lymphoid tissue and is thought to be homologous to the vermiform appendix found in humans.  The cecum has a rumen-like function in the digestion of cellulose.  The rat has no gall bladder. The inguinal canal remains open throughout life, and the testes can be withdrawn into the abdomen.  The scrotum lacks a covering of hair.  The male rat has an os penis (penile bone). The left lung consists of a single lobe, while the right lung consists of the cranial, middle, accessory, and caudal lobes.  The lung of the neonate is immature at birth and contains no alveoli, alveolar ducts, or respiratory bronchioles.  Gas exchange occurs in the smooth-walled saccules and channels until the lung undergoes maturation between 4 and 7 days after birth. Bone maturation is slower in rats than in other species and ossification is not complete until after one year of age.  The eyes of the rat normally have a bulged appearance.  Specialized glands known as Harderian glands are located behind the eye and are responsible for the secretion of porphyrin into the tears. When rats are stressed or ill the increase in porphyrin production may be seen as a dark secretion around the eyes. Rats have no sweat glands and cannot pant, therefore, they have trouble regulating their core body temperature when ambient temperature is increased.  Rats do not increase their water intake in response to high temperature conditions.  Instead, overheated rats burrow and seek shade, and demonstrate increased salivation in an attempt to keep cool.  Rats are better adapted to cold rather than heat.  The rat uses its tail to assist in thermoregulation.  The tail vasculature dilates in response to heat and constricts in response to cold.  Neonatal animals cannot regulate body temperature until they are about a week old.  Brown fat is found diffusely scattered throughout the ventral, lateral, and dorsal areas of the neck and plays an important role in thermogenesis when animals are exposed to cold temperatures. Hearing and smell are well developed in the rat but their eyesight is poor.  It is best to let the rat know you are nearby before picking it up to avoid being bitten. Rat urine normally contains a fair amount of protein (0.4 to 1.0 mg/ml).  Rats can concentrate urine to a level twice that of humans.  Normal urine specific gravity is 1.040 - 1.070. Female rats have 6 pairs of mammary glands: 3 thoracic, 1 abdominal, and 2 inguinal.  Male rats have no nipples. Approximately 80% of the leukocytes in the peripheral blood are lymphocytes.  Male rats generally have higher granulocyte (neutrophils and eosinophils) and lymphocyte counts than do females.  Basophils are rarely seen in peripheral circulation. Summary of physiological and behavioral parameters for rats and mice Table 5. Malocclusion - A frequently occurring problem in rodents.  The condition results when the incisors are misaligned and thus do not undergo normal wear.  An animal with malocclusion cannot eat and often appears runted after weaning. Euthanasia is suggested