In the rat, as in all tetrapods, there is a sharing of the anterior structures of the digestive and respiratory system. A special gate-like structure, the epiglottis, is present at the entrance to the respiratory structures leading to the lungs, which makes possible a time-sharing arrangement between the two systems. Usually much of the oral cavity and the nasopharyngeal region perform a respiratory function, passing air in and out of the lungs, past the raised epiglottis. When food is taken into the mouth the oral cavity becomes, functionally, a part of the digestive system. The act of swallowing, resulting in the passage of food back through the pharynx to the esophagus, simultaneously causes the epiglottis to close off the respiratory system, thereby ensuring that food does not enter the passages leading to the lungs.
The vertebrate digestive system consists of the alimentary canal and its associated glands. The alimentary canal is essentially a tube running from the mouth to the anus. Specialization in both the gross anatomy and the fine structure is found along the tube to cope with the demands of the animal's food habits and with the various functions performed, (ingestion, digestion, absorption and elimination).
Pry the jaws apart to view the buccal cavity. Upon doing so, you will notice that the roof of the mouth is divided into a posterior soft palate and a bony secondary (hard) palate anteriorly. The buccal cavity contains the long, narrow, bony jaw, with teeth set in sockets in the jaw bones. The teeth of the rat, like those of other mammals, are heterodont (i.e. teeth are not all alike). Primitively, mammals teeth consist of incisors, canines, premolars and molars.
