THE SENSES 953 



but both movements are also combined with a slight inward 

 rotation. The common axis of the oblique muscles, a, makes 

 an angle of 60 with the transverse axis, the outer end of it 

 being the most anterior. The direction of traction of the 

 superior oblique is, of course, given not by the line joining its 

 bony origin and its insertion, but by the direction of the portion 

 reflected over the pulley. When the superior oblique contracts 

 alone, the eyeball is rotated outwards and downwards ; the 

 inferior oblique causes an outward and upward rotation. None 

 of the common axes of rotation of the pairs of muscles, except 

 that of the external and internal recti, lies in Listing's plane. 

 Now, as we have seen that every movement which the eye, 

 supposed to be originally in the primary position, can execute 

 may be considered as a rotation round an axis in this plane, it 

 is clear that every movement, except truly transverse rotation, 

 must be brought about by more than one pair of muscles. For 

 vertical rotation, the inward pull of the superior rectus is 

 antagonized by a simultaneous outward pull of the inferior 

 oblique ; for downward rotation, the inferior rectus and superior 

 oblique act together. In oblique movements, a muscle of each 

 of the three pairs is concerned. The effect on the eyeball of 

 simultaneous contraction of certain pairs of muscles may be 

 summarized thus : 



External rectus (outward) + internal rectus (inward) =none. 



Superior rectus (upward and inward) + inferior oblique (upward 

 and outward) = up ward. 



Inferior rectus (downward and inward) + superior oblique (down- 

 ward and outward) = down ward. 



HEARING. 



The transverse vibrations of the ether fall upon all parts of the 

 surface of the body, but only find nerve-endings capable of giving 

 the sensation of light in the little 'discs, which we call the retinae. 

 So the much longer and slower longitudinal waves of condensation 

 and rarefaction which are being constantly originated in the air 

 or imparted to it by solid or liquid bodies that have been themselves 

 set vibrating fall upon all parts of the surface, but only produce the 

 sensation of sound when they strike upon the tiny mechanism of the 

 internal ear. 



But just as the ethereal vibrations, and especially those of greater 

 wave-length, are able to excite certain end-organs in the skin which 

 have to do with the sensation of temperature, so the sound-waves, 

 when sufficiently large, are also capable of stimulating certain 

 cutaneous nerves and of giving rise to a sensation of intermittent 

 pressure or thrill. This is readily perceived when the finger is 

 immersed in a vessel of water into which dips a tube connected with 

 a source of sound, or when a vibrating bell or tuning-fork is touched. 



