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HARDWICKE'S SCIENCE-GOSSIP. 



THE SEMICIRCULAR CANALS OF THE 

 INTERNAL EAR; THEIR PROBABLE 

 FUNCTION. 



By H. J. Benham, M.D. Lond. 



THE internal ear, or labyrinth, consists, as is 

 well known, of an ovoid chamber, called the 

 Vestibule, from which open the three semicircular 

 canals behind, and the cochlea in front. The laby- 

 rinth is filled with a watery fluid, the Perilymph; 

 in which float a series of membranous sacs and tubes. 

 These are filled with a similar fluid— the Endolymph, 

 and are attached by a portion of their circumference 

 to the bony walls of the labyrinth (Rudinger*) 

 whence they derive their blood-vessels and nerves. 

 The labyrinth is bounded by hard bony walls in 

 every direction except towards the tympanum, where 

 its walls are pierced by two windows. The upper 

 one, called the Fenestra ovalis, contains the base of 

 the stapes, attached to its margins by an elastic 

 ligament. As the membrana tympani vibrates, 

 the base of the stapes moves backwards and forwards 

 like a piston, and communicates its motion to the 

 perilymph. The other window, the Fenestra rotunda, 

 is simply closed by a flat membrane, which bulges 

 out when the base of the stapes is pushed inwards, 

 and vice versA, thus permitting the movement of 

 the perilymph. 



The complex structures in the spiral cochlear duct, 

 known collectively as the "organ of Corti," are 

 generally admitted to be the "lute of 3000 strings" 

 (Tyndall) by means of which we analyse sounds into 

 their constituent tones and over-tones ; but the 

 membranous semicircular canals have not yet been 

 assigned any definite function. 



Whilst preparing a paper recently for the Ipswich 

 Scientific Society on "The Mechanism of Hearing," 

 I was examining the base of a skull in which I had 

 laid open the various parts of the labyrinth from 

 above. It then struck me that since the three semi- 

 circular canals certainly lie in three planes at right 

 angles to one another, possibly their function might 

 be to analyse sounds into three sets, according to the 

 plane in which the molecules of the perilymph are 

 vibrating. If we consider the motion of one mole- 

 cule, it is evident that it may oscillate to and fro 

 along a given horizontal line, to and fro in a vertical 

 plane at right angles to this line, or to and fro in a 

 horizontal plane, at right angles to the given line. 

 Furthermore, all oblique motions of the molecule may 

 be considered as the resultant of movements in two 

 or three of these planes simultaneously ; or, in other 

 words, all possible oscillations of the molecules may 

 be expressed in terms of movements in one, two or all 

 three of these planes. 



The stapes moves to and fro in a nearly horizontal 



* Strieker's " Manual of Human and Comparative Histology." 

 New Sydenham Sociey. 



plane, in a line which forms an angle of about 35 

 with the vertical mesial plane of the body. This 

 line may conveniently be termed the auditory axis, 

 it is approximately the axial line of the external 

 auditory meatus, and is the line in which sounds are 

 heard with the greatest distinctness by one ear. 



The vestibule is somewhat conical, or rather pear- 

 shaped, the stalk of the pear representing the 

 commencement of the posterior semicircular canal. 

 Hence vibrations, transmitted from the stapes in the 

 direction of the auditory axis, are concentrated upon 

 this point, the inner and hinder portion of the 

 vestibule. From this point spring two cylindrical 

 canals, at first for a short distance both horizontal. 

 The most anterior continues to run in a plane almost 

 horizontal. After a short straight course outwards, it 

 curves forwards and then inwards, describing rather 

 more than half a circle, and then expands into an 

 ovoid chamber twice the diameter of the tube. This 

 is called the ampulla, and lies close to the wall of 

 the vestibule behind and external to the fenestra 

 ovalis. 



Through a very short canal, the ampulla is supplied 



SUPERIOR CANAL 



AUDITOR/ AXIS 



HOfUZONTtL 

 CANJL 



POSTERIOR- 

 CANAL 



COCHLEA 



\ AUDITORY NERVE 

 \ 

 \ 



'vestibule 

 pig. 157.— Section of the Labyrinth of Human Ear. 



with blood-vassels, and a branch of the auditory nerve. 

 It is filled by a membranous sac, forming the end of the 

 membranous canal, which however, occupies only one 

 third of the lumen of the rest of the bony canal and 

 is attached along its convex border (Rudinger). The 

 other end of each membranous canal is connected 

 with a large ovoid sac, called the utricle, lying in 

 the hinder part of the vestibule. In front lies a 

 smaller sac, the saccule, from which springs the 

 cochlear duct. Where the utricle and saccule are 

 attached to the inner vestibular wall, and at the 

 fundus of each ampulla, is a conical heap of highly- 

 specialised epithelium, in which the fibrils of one 

 branch of the auditory nerve end as " auditory hairs." 

 These are the organs by means of which the 

 sonorous vibrations are (so to speak) translated into 

 neurility, and telegraphed to the auditory nucleus in 

 the medulla, where the messages are combined into 

 auditory perceptions. The canal thus described is 

 the horizontal one. The other two are precisely 

 similar in structure and arrangement, and only differ 

 in that they lie in planes at right angles to this and 

 to each other. 



