g I2 COMPARATIVE. HISTORICAL. 



According to Urbantschitsch analogous relations exist between all of the 

 sensory organs : Shading the eyes usually weakens the hearing; subjective auditory 

 sensations are usually increased by light; gustatory sensations are frequently 

 strengthened by red and green, etc. Color-blind, individuals exhibit also typical 

 defects of musical sense ; those that are green-blind confuse different tones that 

 they hear or repeat in a way that is different from those that are red-blind. 



It is often observed that the auditory impulse conveyed to one ear strengthens 

 the function of the other ear, as a result of stimulation of the auditory centers 

 of both sides. 



The auditory apparatus may be excited not only by sound-vibrations, but 

 also by other heterologous stimuli. It is mechanically excited by a sudden blow 

 or shock to the ear. If the fingers are placed tightly in the canal, and a trembling 

 motion is made, a singing, ringing sound is caused by the condensation and rare- 

 faction of the air in the canal. Stimulation of the auditory nerve by electricity 

 is discussed on p. 699 and pathological conditions of irritation on p. 700. 



COMPARATIVE. HISTORICAL. 



The lowest forms of fishes, the cyclostomata (lampreys) possess only a saccule, 

 provided with auditory hairs and otoliths, communicating with two semicircular 

 canals; the myxinoids have only one semicircular canal. Most of the other fishes 

 have a utricle, with three semicircular canals typically developed. The osseous 

 fishes have in the cysticula of Brechet (Fig. 325, V, C) the first indication of the 

 cochlear canal leading from the saccule. In the carp and the shad posterior 

 prolongations and diverticula of the labyrinth are connected with the air-bladder 

 by means of a chain of three auditory ossicles. In several of the herrings and 

 perches, bladder-like processes of the air-bladder are either in immediate contact 

 with the labyrinth, or in close proximity to it. According to Kreidl the carps, 

 and according to Beer the crustaceans, do not react at all through the auditory 

 apparatus to auditory stimuli, and the fishes only through their highly developed 

 cutaneous sense, which is set into activity by the sound-waves. The organs 

 of the "side line" in fishes are intended for the preservation of the equilibrium. 

 The amphibia are in general closely related to the fishes with respect to the con- 

 struction of the labyrinth, but the cochlea is not typically developed. Most 

 of them, except the frog, have no tympanum. The fenestra ovalis alone exists, 

 and not the fenestra rotunda, the former being connected in frogs with the exposed 

 tympanic membrane by means of three ossicles In reptiles the saccule, appended 

 to the cochlear canal, is quite prominent; in tortoises it is still a simple sac, but 

 in crocodiles it is longer and somewhat curved and dilated at its extremity. In 

 all reptiles the round window is found for the first time; through it the cochlea 

 communicates with the vestibule. The cochlea is divided into a scala tym- 

 pani and a scala vestibuli in crocodiles and birds. Snakes have no tym- 

 panic cavity. In birds the saccule and the utricle are fused (Fig. 325, IV, US). 

 The cochlear canal (UC), which is connected with the saccule by means of a 

 fine tube (C), is already longer. It exhibits indications of a spiral arrangement, 

 and it possesses a flask-like, blind end, the lagena (L), which is present likewise 

 in crocodiles. The auditory ossicles in reptiles and birds are reduced to one, 

 which is columnar in shape, and corresponds to the stapes; it is known as the 

 columella. The lowest mammals (echidna and duck-bill) are still more like the 

 birds in structure ; the higher mammals, however, exhibit the same type of auditory 

 apparatus as man (Fig. 325, III). In whales the Eustachian tube is always 

 open. According to G. Retzius all vertebrates possess so-called hair-cells as end- 

 organs of the auditory nerves. 



Among invertebrates the ear is found in a simple form in several of the medusas, 

 annelids, and molluscs. It is a round vesicle, filled with fluid, on the wall of which 

 are the auditory nerves with ganglionic enlargements. The inner wall of the 

 vesicle bears cells provided with cilia (auditory cells) , which contain either only 

 one otolith composed of concentric layers, or numerous crystalline movable 

 otoliths. The otoliths consist of an organic base, which is impregnated with 

 lime-salts. In the medusae the auditory vesicles lie in the margin of the bell 

 (marginal bodies). According to more recent views, however, the otoliths regulate 

 the equilibrium of the animal, by pressing harder in one direction on the surface 

 beneath them, with every change of position. Verworn proposes, therefore, to 

 call them statohths. Extirpation of the saccules containing the otoliths disturbs 

 the equilibrium of the animals. 



