Extensive morphometric measurements were made on the vestibular system of the rabbit ( Oryctulagus cuniculus), the gerbil (Meriones unguiculatus), the chinchilla (Chinchilla laniger ), and the squirrel monkey (Saimiri sciureus) from serial sections of temporal bones. Additionally, a more limited set of measurements were also completed on the owl monkey (Aotus trivirgatus), the Capuchin monkey (Cebus sp.), the harp seal ( Pagophilus groenlandicus Erxleben , 1777), and the two-toed sloth ( Choloepus sp.). The following measurements were made: 1) radius of curvature (R) of each membranous semicircular canal (herein called semicircular duct-see nomenclature in Nomina Anatomica (1968) ), 2) cross-sectional diameter of the ducts and the osseous semicircular canals, and 3) some pertinent morphometrics of the cristae ampullares and the utricle. In all species studied 1) the radii of curvature of the three semicircular ducts are dissimilar, with that of the lateral duct being as small as, or smaller than, those of the anterior and posterior ducts; 2) R for the anterior duct is largest in the harp seal and the rabbit; 3) the canal and duct dimensions are largest in the Capuchin and squirrel monkeys, the two-toed sloth, and the harp seal, and smallest in the gerbil; 4) the proportion of otic fluid "space" that is occupied by endolymph shows a ranking of gerbil greater than rabbit greater than two-toed sloth greater than chinchilla = owl monkey greater than squirrel monkey greater than Capuchin monkey greater than harp seal; and 5) the gross ampullary and utricular dimensions are largest in the harp seal and smallest in the gerbil. These measurements were used for determining the time constants describing semicircular-canal dynamics in the Steinhausen (1931, 1933) and Oman -Marcus (1980) equations.

A detailed morphometric study of the basilar membrane was made from serial sections and graphic reconstructions of the cochlea of three little brown bats. Four distinct morphometric changes were observed within the basilar membrane. First, between 0-1.4 mm from the basal end of the cochlea, there is a rapid increase in width and cross-sectional area of the basilar membrane. Secondly, between 1.4-2.5 mm, there is little change in width of the basilar membrane (its cross-sectional area is at its greatest in this region). Thirdly, between 2.7-3.1 mm, there is a sudden decrease in cross-sectional area concomitant with an increase in the width of the basilar membrane. Finally, between 3.1 mm and the apex, there is a gradual decrease in cross-sectional area concomitant with an increase in the width of the basilar membrane. The magnitudes of the cross-sectional areas of the scalae media and vestibuli decrease from base to apex, but this is not true for the scala tympani. The cross-sectional area of the scala tympani appears to decrease from the base to 0.7 mm, then it increases up to 1.4 mm, and then it decreases to the apex. These morphometric changes in the basilar membrane of the little brown bat are compared to those in other echolocating and non-echolocating mammals. The significance of these changes is discussed in relation to the range of hearing in the little brown bat.

Surface preparations of the organ of Corti of four harp seals were used to study the effect of prolonged ingestion of methyl mercury on the sensory cell population.A low level of damage to the sensory hair cells occurred throughout the length of the cochlea. Damage was confined to the three outer rows of sensory hair cells especially the third outermost row. At each location along the length of the cochlea, sensory hair cell damage in the seals on a daily dose of 25.0 mg/kg of methyl mercury exceeded the damage to the cochlea of the seals fed on a daily dose of 0.25 mg/kg of methyl mercury. Greatest damage in all the mercury-treated seals occurred in the middle coil of the cochlea. Seals on the higher mercury diet showed a 20–24% sensory cell damage at the upper middle coil, about 19–26 mm from the base, whereas only 4–5% damage was found within same region in the cochlea of the seals on the lower mercury diet.This lack of specificity and low level of damage to the sensory hair cells seems characteristic of mercury and is a direct contrast to other known ototoxic agents.

Neuromorphometric parameters of the vestibular system were determined from serial sections of temporal bones from four little brown bats. Well-developed eminentiae cruciatae project from the cristae ampullares of the anterior and posterior membranous ampullae. A total of 4,500 bipolar ganglion cells were enumerated within the vestibular ganglion. The widths of the cell somas varied from 2.5 to 20 micrometers, with 70% of them having widths between 5.0 and 12.5 micrometers. Two maxima were observed in a curve of ganglion cell density as a function of the length of the ganglion. The first maximum indicated a density of 4,800 cells per mm2 at a length 0.20 from the apex of the ganglion (in the pars dorsalis); the second, a density of 4,750 cells per mm2 at 0.38 mm (in the pars ventralis). The morphometric parameters studied were the radii of curvature of the semicircular ducts, the cross-sectional diameters of the semicircular canals and ducts, the dimensions of the cristae ampullares and their membranous ampullae, and dimensions pertaining to the statoconial organs. Surface areas (measured from graphic projections) were determined as 0.098 mm2 and 0.016 mm2 and hair cell count 500 and 1,300 cells for the saccular and utricular maculae, respectively. The radii of curvature of the three semicircular ducts, R, were dissimilar, with the anterior duct having the largest radius (R = 0.91 mm) and the posterior duct the smallest one (R = 0.69 mm). The average cross-sectional diameters of the anterior, lateral, and posterior ducts were measured as 0.11 mm, 0.14 mm, and 0.13 mm, respectively. Some of the morphological parameters were used to ascertain information regarding the dynamics of semicircular--canal function. In particular, the coefficients theta and II in the torsion pendulum model (Steinhausen, '31; Egmond et al., '49), and the time constants xi L congruent to II/delta and xi S congruent to theta/II of the torsion pendulum model were estimated for the little brown bat from these parameters. Where appropriate, comparisons were made to time constants obtained for other species.

The population and density of the bipolar ganglion cells and sensory hair cells were determined from serial sections and graphic reconstructions of the cochlea and spiral ganglion of four little brown bats. The spiral ganglion lacks a bony canal and is a one and one-half turn spiral of almost uniform thickness which ends in a broad flat nodule a t its apex. The average length of the nerve cell body is 7.4 p (range: 5-15 p ) and it is 6.4 p wide (5-10 p). The average number of ganglion cells enumerated was 63.2 x lo3 (range: 60.6 x 103-65.2 x lo3) and following correction for "split cell error," the total number was 55.3 x lo3 ganglion cells. The ganglion cell density curve increases gradually to a maximum value of 11.7 x lo3 cells/mm2 at 2.4-3.2 mm from the basal end. The two and onehalf turn cochlea is 6.9 mm long (6.8-7.1 mm) with a n estimated total of 700 inner and 2,800 outer sensory hair cells. The densities of outer and inner hair cells increase from the base to the apex by 21% and 32%, respectively. However, the density of sensory cells per unit of sensory epithelium decreases from base to apex. The ratio increases to a maximum value of 22: 1 a t 2.4 mm and then declines toward the apex. The significance of these findings is discussed and compared to similar studies on other high-frequency hearing mammals.Most anatomical studies of the mammalian ear include inferences about the function of that organ. Nowhere is this practice more prevalent than in anatomical studies on the ear of mammals having high-frequency hearing. Structural variations found within the ear of these mammals have been interpreted as adaptations for high-frequency hearing as in Cetacea (Reysenbach de Haan, '57; Wever e t al., '71a1, Phocida (Ramprashad e t al., '72; Ramprashad, '751, and Chiroptera (Pye, '66a,b, '67). More precisely, these variations are structural adaptations which augment stimulation of the sensory cells of the cochlea.Neuroanatomical studies of the cochlea of the Pacific white-sided dolphin (Lagenorhynchus obliquidens), bottlenose dolphin (Tursiops truncatus) and the harp seal (Pagophilus groenlandicus) have indicated that the total number of ganglion cells, and the ratio of ganglion cells to the sensory cells exceed those of the human (Wever e t al., '71b, '72; Ramprashad, '76). Since these aquatic mammals are associated with high-frequency hearing, it