io6 4 THE SENSES 



tion. The eye is turned towards the temple when the external 

 rectus alone contracts, towards the nose when the internal rectus 

 alone contracts. The common axis of the superior and inferior 

 recti, (3, lies in the horizontal visual plane in the primary position, 

 but makes an angle of about 20 with the transverse axis, its inner 

 end being tilted forwards. The consequence is that contraction 

 of the superior rectus turns the eye up, and contraction of the 

 inferior rectus turns it down, but both movements are also com- 

 bined 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 rota- 

 tion 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 (in ward) = none. 



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

 outward) = upward. 



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

 and out ward)= downward. 



SECTION II. 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 sensa- 

 tion 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 



