viii OCULAK MOVEMENTS 391 



fibro-cartilaginous ring (trochlea) attached to the fovea trochlearis 

 of the frontal bone ; it then bends outward and backward, and is 

 fixed below the inner margin of the superior rectus to the equator 

 of the eyeball. The inferior oblique arises from the inner wall of 

 the orbit just below the fossa sacci lacriinalis; it runs outward, 

 bends back and up between the inferior and external recti, and 

 is inserted on the outer and lower part of the eyeball, opposite to 

 and parallel with the insertion of the superior oblique, as a 

 flattened muscle without a tendon. To realise how these muscles 

 produce the ocular movements it is necessary to assume that all 

 the movements take place round a given fixed point, known as 

 the centre of rotation of the eyeball. 



The position of the centre of rotation (Drehpunkt) varies some- 

 what with the form of the eye. Junge, Donders, and Doijer 

 showed that in emmetropic eyes it lies 13*54 mm. behind the 

 summit of the cornea, in hypermetropic 12'32 mm., in myopic 

 eyes 15 '86 mm. The centre of rotation of the average emmetropic 

 eye lies about T3 mm. behind the middle point of the eye. 



The three principal axes of the eye pass through its centre of 

 rotation (Z>) : the sagittal axis (yy') coincides with the line of 

 vision ; the transverse axis (xx) coincides with a line that unites 

 the outer or temporal edges of the orbit ; the vertical axis (not 

 shown in figure) passes through the eye perpendicular to the hori- 

 zontal and transverse axes. These three axes constitute a system 

 of co-ordinates which intersect at a right angle in the centre of 

 rotation. . 



Three planes are also spoken of in the eye, their position being 

 always relative to the two axes : the horizontal plane, correspond- 

 ing to the sagittal and transverse axes, divides the eye into upper 

 and lower halves ; the vertical plane, corresponding to the vertical 

 and horizontal axes, divides the eye into inner and outer halves ; 

 the equatorial plane, corresponding to the vertical and transverse 

 axes, divides the eye into posterior and anterior halves. The 

 horizontal and vertical planes cut the fovea centralis of the retina, 

 and divide it into four quadrants. 



Helmholtz gave the name of line of sight or vision (Blicklinie) 

 to the vertical axis which unites the centre of rotation of the 

 eye to the fixation point ; the plane of sight or visual plane 

 (Blickebene) passes through the visual lines of both eyes, and is 

 halved by the medial sagittal plane of the body (S& of Fig. 183). 



The fixation point (JBlickpunkt) of the eye can be raised or 

 lowered, or turned from side to side. The field which can thus be 

 covered by the eye while the head remains motionless is the field 

 of vision (Blickfeld) ; it corresponds to a portion of a spherical 

 surface, with the rotation point of the eye as its centre. 



Given the primary position of the eyes (lines of sight parallel, 

 and plane of sight horizontal), the degree to which the visual 



2 c 2 



