HARDWICKE'S SCIENCE-GOSSIP. 



269 



The posterior canal is the prolongation of the 

 hinder of the two short horizontal canals alluded to. 

 It lies in a vertical plane, inclined at an angle of about 

 55° with the mesial plane. Running at first outwards 

 and backwards, it curves downwards and then in- 

 wards and upwards, ending in an ampulla close below 

 the floor of the vestibule. 



The superior canal springs from the horizontal 

 commencement of the posterior canal, near the apex 

 of the vestibule. Rising at first vertically upwards, it 

 curves over in a vertical plane at right angles to that 

 of the posterior canal, bending forwards and outwards 

 and then downwards, till it ends in an ampulla close 

 above that of the horizontal canal. 



Since the ampullae are completely filled by the 

 membranous sacs, the perilymph space ends there. 

 Vibrations transmitted through the perilymph are 

 reflected down these curved tubes, passing easily 

 through the thin membranes floating in fluid but 

 reflected from the hard bony walls. 



But I believe that some vibrations reach one 

 ampulla with less loss of intensity than others, and 

 are thus discriminated. 



For when a ray of light, heat, or sound is reflected 

 from a surface, there is more loss of intensity in pro- 

 portion as the angle of incidence is small, and less 

 as it becomes more obtuse. 



The rays proceeding along the auditory axis im- 

 pinge at a very obtuse angle on the inner wall of the 

 vestibule, and are reflected with but little loss into 

 the posterior canal, coinciding in direction with the 

 axis of its first straight part. 



My theory is, that those molecular vibrations in the 

 ray of sound which take place along the line of the 

 reflected auditory axis, find their way most easily, 

 with the least loss of intensity, because with the 

 fewest reflections (as shown by the dotted line in the 

 diagram) into the ampulla of the posterior canal. 



Since the axis of the first part of the superior canal 

 is at right angles to the preceding in a vertical plane, 

 I believe that those molecular vibrations which take 

 place at right angles to the auditory axis in a vertical 

 plane, are transmitted with the least loss of intensity 

 to the ampulla of the superior canal. 



Similarly, I think that vibrations at right angles to 

 the auditory axis in a horizontal plane, are transmitted 

 with the least loss to the ampulla of the horizontal 

 canal. 



If this be so, we have in the semicircular canals 

 an apparatus by means of which we perceive, by the 

 relative intensity of the nerve-excitement telegraphed 

 from the ampullae, in what plane or planes the 

 molecules of the perilymph are oscillating ; and are 

 hence able to differentiate sounds to an almost infinite 

 extent ; for it is obvious that the possible combina- 

 tions of these ampullary signals is innumerable. 



Besides these ampullary messages, we have 

 messages from the heaps of auditory epithelium in 

 the utricle and saccule (upon which the rays of 



sound impinge directly without reflection) ; and also 

 from the organ of Corti, representing the pitch of its 

 component tones and over- tones. All these, combined 

 with the messages representing the actual tension of 

 the intrinsic muscles of the tympanum, in their 

 adjustment to the intensity of the sound, unite in the 

 auditory centres of the brain with those from the 

 opposite ear to form one perception of a single sound. 

 Here is room indeed for the variety that we actually 

 find in our conscious perception of the sounds of the 

 external world. 



THE NATURAL HISTORY OF DIATOMS, 

 By Professor J. Brun. 



{Continued from page 243,] 



r ~n*HEIR movement. — Every one knows by this 

 ■*■ time, that the spores of all the algae are 

 endowed with the power of moving in the water, that 

 is before they become fixed, and commence the re- 

 production of new individuals ; but amongst the 

 diatoms it is the individual itself, or rather the 

 frustule, which moves. This movement takes place 

 in a straight line in the direction of the length of the 

 valves ; there is an alternate advance and return. 

 Amongst the naviculae, this movement is caused by 

 an external current which is set up in the central 

 nucleus of one of the poles, and then changes 

 suddenly and always passes the central nucleus of 

 the other pole. The current reacts against the 

 surrounding water. This current can be rendered 

 very plain k by diluting the water with carmine or 

 indigo-blue. We then see the fine granules of these 

 colours running along the valve with the current. I 

 have seen this curious phenomenon in the Stanroneis 

 Phcenicenteron. One thing is certain, that the 

 endochrome does not co-operate to produce this 

 movement, and that the living and mobile valves have 

 no external organs, cilia, or plates assisting in loco- 

 motion. I have proved that the appendices which 

 we sometimes perceive on the surface of the valves 

 and which many naturalists have taken for swimming 

 organs, are only parasites. 



Their Parasites. — There is scarcely any living being 

 which has not its parasites. Diatoms, small though 

 they are, have theirs. So true is it that amongst the 

 infinitely small beings one finds " the battle for life," 

 and " the struggle for existence," and if the great in 

 general feed on the small, it is but natural that, the 

 small reuniting should destroy the great. Amongst 

 the diatoms, these parasites are always other kinds 

 of algae. Five common diatoms, Nitzschia linearis 

 and sigmoidea, Synedra splendens and Cymbella macu- 

 lata and cymbiformis are found sometimes in this 

 country covered all over with a filamentous parasite, 

 having the appearance of thick transparent hairs, 

 straight, rigid, and of a very pale greenish-yellow. 



