DISSECTION OF THE INTERNAL EAR. The structures composing this organ lie in the projecting cartilage at the side of the medulla (auditory capsule). Remove the cartilage of the auditory capsule in thin slices and bit by bit, following the ductus endolymphaticus to the membraneous labyrinth. Dissect away the surrounding cartilage leaving the membraneous canals in place, until the entire labyrinth is exposed. The _membraneous labyrinth_ consists of a large central sac (_utriculo-saccular chamber_) into which the endolymphatic duct opens, and three membraneous tubes (_semicircular canals_) external to the chamber but communicating with it in various ways. Two, one anterior and the other posterior to the sacculus, lie in a nearly vertical plane (_anterior and posterior semicircular canals_); one is external to the sacculus and lies in a nearly horizontal plane (_horizontal or external semicircular canal_). At the ventral ends of the vertical canals are nearly spherical enlargements called _ampullae_. The ampulla of the horizontal canal is at its anterior end. The dorsal ends of the vertical canals open near each other into the upper part of the utriculo-sacculus. The ventral extremity of the anterior vertical canal and the anterior extremity of the horizontal canal open beside each other into an anterior projection of the sacculus. The ventral extremity of the posterior vertical canal opens into the posterior and lower part of the sacculus. The posterior extremity of the horizontal canal opens into the posterior side of the sacculus. During life the utriculo-sacculus and the semicircular canals are filled with a lymphatic fluid, and the sacculus contains a large calcareous ear-stone (_otolith_), which is usually dissolved by the formalin used in preserving the dogfish. Whitish patches of thickened sensory epithelium may be seen in the ampullae (_cristate acusticae_) and in the utriculo-saccular chamber (_maculae acusticae_). Branches of the eighth nerve can be followed to all these areas. A projection of the ventral wall of the utriculo-sacculus is the _lagena_, the rudiment from which the cochlea of higher animals developed. It also contains a macula acustica.