1090 



THE O ROANS OF SPECIAL SENSE 



hammock below the eyeball, being expanded in the centre, and narrow at its 

 extremities, which are attached to the malar and lacrimal bones respectively. 1 

 The anterior one-third of the globe is covered by the conjunctiva, or mucous 

 membrane, reflected from the inner surfaces of the lids (Fig. 797). A lateral view 

 of the globe shows that it is composed of segments of two spheres of different sizes 

 (Figs. 796 and 797). The anterior segment is one of a small sphere, and forms 

 about one-sixth of the eyeball. It is more prominent than the posterior seg- 

 ment, which is one of a much larger sphere, and forms about five-sixths of the globe. 

 Between the small, anterior or corneal segment and the large, posterior or scleral 

 segment is a shallow and narrow groove, the scleral sulcus (sulcus sclerae). The 

 anterior pole is the centre of the anterior portion of the cornea. The posterior 

 pole is the centre of the posterior portion of the sclera. A straight line joining 



OPTIC 

 AXIS 



POSTERIOR CHAMBER 

 OF EYE 



OCULAR 

 CONJUNCTIVA 



PARS CILIARIS 

 RETINAE 



ORA SERRATA 



PARS OPTICA 

 RETINA 



SCLEROTIC 



YELLOW 

 SPOT 



OPTIC 

 EXCAVATION 



EXTERNAL 

 AXIS 



FIG. 796. The right eye in horizontal section. (Toldt.) 



these two poles is the sagittal or optic axis (axis optica) (Fig. 796). A line drawn 

 around the eyeball equally distant at all points from the two poles is called the 

 equator (Fig. 796). The plane of the equator divides the globe in an anterior 

 and a posterior hemisphere. The visual axis (linea visus) (Fig. 796) passes in a 

 straight line from the first nodal point on the cornea to the fovea centralis of , the 

 yellow spot on the retina. A nodal point is the point of intersection of convergent 

 rays with the visual axis. The first nodal point is 6.9685 mm. behind the summit 

 of the cornea. The axes of the eyeballs are nearly parallel to the mesal plane, 

 and therefore do not correspond to the axes of the orbits, which are inclined to this 

 plane, forming with it an anterior angle of about 30 degrees. The optic nerves 



1 See a paper by C. B. Lockwood, Journal of Anatomy and Physiology, vol. xx, part i, p. 1. 



