Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/82



The digestive system consists of the mouth cavity, pharynx, esophagus, stomach, small and large intestines, salivary glands, liver and pancreas.

The mouth opening is triangular. It is located on the anteroventral surface of the head some distance back from the tip of the snout. The lower lip conceals the greater part of the two long lower incisor teeth. Externally it is hairy; internally it is covered with smooth mucous membrane. A fleshy ridge on its inner surface fits into the space between the incisor teeth. This ridge joins ventrally a short vertical septum, the frenulum, which passes between the incisor teeth, connecting the lip with the mandible. From the angle of the mouth a lobe of integument, covered with hair, turns upward along the inner surface of the cheek.

The lateral halves of the upper lip meet in front at an acute angle pointed toward the nasal septum. The apex of the angle is connected with the septum by a groove. The upper lip is therefore cleft, exposing the incisor teeth of the upper jaw. A frenulum is lacking. The edge of each upper lip immediately behind the incisor teeth is turned inward, forming a lobe whose lower hairy surface touches the dorsal surface of the tongue. This lobe is hairless dorsally, and is in contact here with the roof of the mouth. The lip lobes, tongue, and a fleshy tubercle on the middorsal surface of the mouth's roof just behind the lobes, are all in contact with one another when the jaw is closed, practically separating the mouth opening from the mouth cavity.