Observations on the comparative anatomy of the VIIIth nerve complex

Abstract

1. Measurements of each of the inner ear sensory areas are reported and compared for various vertebrates. These indicate: a. 5:5 and 6:4 ratios are most common in comparing the total pars superior sensory area of the inner ear with the total pars inferior area; b. 7:3 to 2:8 ratios exist, in comparing the canal type with the macular type areas, which appear to be related to the equilibratory demands made upon the organism; c. that, in comparing cochlear type with saccular type areas, the tailless amphibians exhibit a surprisingly high value for the former; d. that in some forms the crista horizontalis is larger than the average of the two vertical cristae, possibly associated with the mode of locomotion; e. that the macula sacculi, throughout vertebrates, is of considerable size, as is the macula lagenae (where present); f. that there is a great actual and relative variation in the size of the papilla basilaris (organ of Corti), particularly among reptiles; g. that it is the size of the papilla amphibiorum which confers on amphibians such a high proportion of cochlear type sensory area. 2. Consideration of the ganglion cells of the octavus ganglia illustrates that they are in part intracranial and in part intraotic in position among all the vertebrates examined, and that: a. the intraotic portion is always in relation with the papilla basilaris and papilla amphibiorum, but also frequently in relation with the macula lagenae, crista posterior (and papilla neglecta), and the most inferior and posterior portion of the macula sacculi; b. the intracranial portion is practically always in relation with the crista anterior, crista externa, macula utriculi, and most of the macula sacculi; usually in relation with the crista posterior (and papilla neglecta); sometimes in relation with the macula lagenae; c. the ganglion cells related to perilymphatic or cochlear type sensory areas tend to lie consistently nearer such areas than do the ganglion cells related to the more endolymphatic type areas; d. the ganglion cell size generally varies noticeably from large to small cells; the large cells seemingly related to the cristae; cells intermediate in size more related to the maculae; the smallest cells related to the papilla basilaris (organ of Corti); e. the sensory areas are projected upon the ganglion cells so that the crista anterior is most rostrodorsal, followed in turn (progressing from rostrodorsal to caudoventral) by the crista externa, the macula utriculi, the macula sacculi, the crista posterior (and papilla neglecta), the macula lagenae, and the papilla basilaris (organ of Corti); the positions of the first three of the above areas within the ganglia is relatively constant — the others are frequently subject to considerable variation. 3. While it was not possible to determine a precise central projection of the inner ear sensory areas, nevertheless, based on the condition in Sphenodon, it was observed that: a. the ganglion cells related to the papilla basilaris and the macula lagenae sent their neuraxes into the most dorsal portion of the medulla, apparently into the area cochlearis; b. these were followed in order, from dorsal to ventral, by the fibers coming from the macula sacculi ganglion cells (running largely, if not entirely, into the so-called area vestibularis); those from the crista posterior ganglion cells; those from the crista anterior ganglion cells; those from the crista externa ganglion cells; and, most interiorly in the area vestibularis, those from the macula utriculi ganglion cells.