126 



ESSENTIALS OF PHYSIOLOGY. 



supplied by the third cerebral nerve. Definite groups of nerve cells 

 can be localised in the nucleus of the third nerve for each muscle 

 supplied by it, stimulation of particular areas in the nucleus being 

 followed by contraction of particular muscles. The groups of cells for 

 the ciliary muscle, sphincter pupillse, and internal recti lie close 

 together in the anterior part of the nucleus ; and the centre for the 

 internal rectus on each side is connected with that of the sixth nerve 

 of the same side, so that when the internal rectus contracts there is 

 reciprocal relaxation of the external rectus of the same eye. 



AMETROPIA. 



The condition of the normal or standard eye is called emmetropia, 

 and any departure from the standard is known as ametropia. When 



the antero-posterior diameter of the 

 eyeball is too short, so that parallel 

 rays are focussed behind the retina, 

 the condition is called hypermetro- 

 pia ; when the antero-posterior dia- 

 meter is unduly long, so that parallel 

 rays are focussed in front of the 

 retina, the term myopia is used to 

 indicate the defect (fig. 38) ; when 

 the rays of light entering the eye in 

 one meridian are refracted to a 

 greater or less degree than those 

 which enter in another meridian, the 

 condition is spoken of as astigmatism 

 (fig. 39). 



A moderate degree of hyper- 

 tnetropia does not necessarily involve 

 FIG. 38 --Diagrams of course taken by defective yision because the defect 

 parallel rays on entering the eye. 

 (Starling's Principles of Physiology.} is usually compensated by increased 



A, an emmetropic; B, a hypermetropic ; and p OW er of the ciliary muscle, SO that 

 C. a myopic eye. 



rays of light are focussed on the 



retina by an increased use of the accommodative mechanism already 

 described. The extra strain involved in this compensatory effort, 

 however, often leads to unpleasant symptoms, and it is advisable 

 in many cases of hypermetropia to correct the error by the use of 

 convex spectacles. 



Myopia cannot be overcome by any accommodative act, and 

 distinct vision of distant objects can only be obtained in a myopic eye 

 by the use of concave spectacles, which cause the rays of light entering 



