1 24 PHYSIOLOGY. 



The eye has six muscles, four straight, the recti; and two oblique, 

 the obliqui. The first are supplied by the third pair of nerves, ex- 

 cept the rectus externus, which has the sixth. Of the latter, the 

 superior oblique receives the fourth pair ; the inferior, a branch of 

 the third. When the recti act together, they fix the eyeball ; when 

 singly, they turned it towards their respective sides. The obliqioe 

 antagonise the recti, and in addition, when acting together, draw the 

 globe inwards, and converge the axes of the eye. The superior ob- 

 lique turns the eye downwards and outwards, and the inferior oblique 

 upwards and inwards. There is, however, considerable uncertainty 

 as to their functions. 



The adaptation of the eye to distances, by which a perfectly dis- 

 tinct image is perceived, whether the object be far or near, is a phe- 

 nomenon as yet not explained. By some it is thought to be entirely 

 educational ; by others it is supposed to depend upon a varying 

 length of the focal distances, produced either by altering the shape 

 of the globe of the eye, so as to diminish or increase its convexity, 

 or by altering the position of the lens by muscular agency. It is 

 interesting to remark, that the adjusting power of the eye is lost or 

 greatly impaired by the extraction of the lens, or by paralysing the 

 ciliary and iridial muscles by belladonna. According to Dr. Clay 

 Wallace, the ciliary muscle advances the lens by compressing the 

 veins, and thus causing an erection or lengthening of the ciliary pro- 

 cesses. The contraction of the iris, which takes place when the 

 eyes converge, is supposed to depend on the third pair, which is dis- 

 tributed to it, and the internal recti. 



The forms of imperfect vision, known as myopia and presbyopia, 

 are to be attributed entirely to defects in the optical adaptation of the 

 eye. In myopia, or near-sightedness, the refractive power of the 

 eye is too great, and the rays of light are brought to a focus before 

 reaching the retina. This defect is to be corrected by a double con- 

 cave lens which shall disperse the rays, and thus overcome the too 

 great convergence, [n presbyopia, ox far-sightedness, there is the 

 opposite defect, there is not suflicient refracting power, and the rays 

 reach a focus behind the retina. This defect is to be obviated by 

 convex lenses, which increase the refracting power of the eye. 

 Myopia, which commonly occurs in young persons, is not always 

 corrected by age. The presbyopic eye is always present after the 

 operation for cataract. 



It is known that the rays of light from the opposite points of a 

 luminous object, by reason of the changes they undergo through the 

 successive refractions they experience, cross one another, and thus 

 the image on the retina appears inverted. The question then arises, 

 how do we see objects erect, with an inverted image on the retina ? 

 Volkmann has shown, that Sir D. Brewster's law of visible direc- 

 tion, which affirmed that every object is seen in the direction of a 



