1448 



HUMAN ANATOMY. 



the surface of the cornea. The axes of the eyeballs are practically parallel, when 

 fixed on a distant object, but the optic nerves converge considerably, so that they 

 enter the eyeball from 2-3 mm. to the nasal side of the posterior pole of the eye. 

 The general form of the eyeball is that of a sphere, but in sagittal section it is found 

 to be composed of the segments of two spheres, an anterior smaller segment, corre- 

 sponding to the transparent cornea, which has a radius of from 7-8 mm. and a pos- 

 terior opaque segment, corresponding to the sclera, with a radius of 12 mm. The 

 junction between the two segments is marked externally by a broad, shallow groove, 

 the sulcus sclerce, which is filled by the scleral conjunctiva. 



The diameters of the eyeball measure approximately as follows : the antero-pos- 

 terior, 24.2 mm. ; the vertical, 23.2 mm. ; and the transverse, 23.6 mm. Its shape 

 is, therefore, that of a spheroid somewhat flattened from above downward, and from 



Fig. 1202. 



Lens 



Suspensory' ligament of lens 

 Canal of Scblemm 

 Ciliary process 



Conjunctiva 



Cornea 



Anterior chamber 



Iris 



Posterior chamber 



Sclerocorneal juncture 



Tendon of in- 

 ternal rectus- 

 muscle 



Vena vorticosa 



Tendon of 

 -external rectus 

 muscle 



\'itreous 



Sclera 



Ciliary nerve' 



Posterior ciliary vessels 



Hyaloid canal 



Optic nerve 



Central retinal vessels 



Choroid 



Retina 

 Fovea centralis 



Optic papilla 



Diagrammatic horizontal section of right eye. X zVz. 



side to side. The diameters are slightly greater in the male than in the female, and 

 vary according to the refractive power, being longer in nearsighted or myopic, and 

 shorter in oversighted or hyperopic eyes. 



The eyeball consists of three concentric coats or tunics : ( i ) the external or 

 fibrous tunic, composed of the sclerotic and the cornea ; (2) the middle or vascular 

 tunic, which is pigmented and partly muscular, and is composed, from behind for- 

 ward, of the choroid, the ciliary body, and the iris ; and (3) the inner ox nervous 

 tunic, the retina, an expansion of the brain, which contains beside the nerve-cells 

 and the nerve-fibres the specialized neuroepithelium for the reception of visual stimuli. 



Within these tunics are enclosed the refracting media, the crystalline lens, the 

 aqueous humor and the vitreous body. 



Practical Considerations. — Congenital anomalies may affect the whole eye. 

 the appendages, or the individual structures of the eye. 



The eye may be congenitally absent, on one or both sides (anophthalmos). In 

 some cases of apparent absence the eyeball has been found to be exceedingly small 



