Fusion as an Evolutionary Principle of the Vertebrate Labyrinth

Abstract

This study was undertaken to demonstrate changes in the innervation of vestibular and auditory sense organs with the evolutionary ascent of the vertebrate labyrinth. Dissected labyrinths and their nerve supply prepared by the Sudan black B technique of Rasmussen were examined and photographed with a Canon A100 camera interfaced with a Zeiss operating microscope. In lizards and alligators, the utricular sense organ is represented by 2 small maculae, each with a separate nerve branching off the ampullary nerves to the anterior and lateral canal cristae. These 2 maculae fuse into a bilobed macula with ascent in frogs and pigeons, eventually becoming a single large macula with its nerve supply from the superior vestibular division in guinea pigs, cats, lions, monkeys, and humans. Along with these changes, there is a fusion of the lateral and anterior canal ampullary nerves and of the bifurcating branches of the vertical canal ampullary nerves. The saccular macula is single, but receives a dual innervation from the superior division (anterior ramus) and the inferior division (posterior ramus) of the eighth nerve in alligators, pigeons, guinea pigs, cats, lions, monkeys, and humans. The main innervation is usually from the inferior division; however, saccular innervation is from the inferior division in lizards and from the superior division in frogs. The auditory sense organ is represented by a curved tube with a low-frequency receptor (lagena) at its distal end in lizards, alligators, and pigeons. In mammals, in which there is a coiled cochlea with a variable number of turns, low frequencies are recorded at the apical turn. This configuration may represent fusion of the lagena into the apical end of the auditory sense organ. Fusion of sense organs and of their nerve supply appears to be an evolutionary principle in the vertebrate labyrinth.