Blind Spot ; Yellow Spot 93 



attempts at ophthalmoscopic examination. With a little practice, 

 however, the optic disc, the entrance of the optic nerve, is discovered, 

 whitish in appearance, on account of the absence of the choroid at the 

 point of its perforation, there being nothing behind that part of the 

 retina but the lamina cribrosa (p. 83). The ascending and descend- 

 ing divisions of the central artery and vein are then made out. 



As the fibres cf the nerve radiate on to the front of the choroid 

 they leave a central depression in the disc, called the cup, and the 

 margin of the lamina cribrosa the sclerotic ring may often be seen 

 around the cup. 



The optic disc is the ' blind spot' of the retina ; and, for that 

 reason, it is placed out of the way of the visual axis. The exact 

 centre is the most delicate and useful part of the field, and is known 

 as the yellow spot. 



The nervous part of the retina extends to the ciliary processes, 

 where it ends as the ora serrata (ora, margin), but the retina is con- 

 tinued a little farther forwards by delicate fibrous tissue, even to the 

 capsule of the lens. 



The retina depends entirely on the central artery for its blood- 

 supply, and on a few twigs from the choroid which enter at the optic 

 disc. If in disease of the aortic valve a small vegetation be detached, 

 and carried into (embolus) and plug the arteria centralis, the eye, or 

 part of it, becomes suddenly blind, for the retina is completely deprived 

 of its nutrition. Examination then shows the arteries and veins of the 

 fundus shrunken, whilst broken thrombi may be seen in the artery. 

 In Bright's disease haemorrhages are very apt to occur in the inflamed 

 and swollen retina (albuminuric retinitis). 



The delicacy of the connections of the retina renders it liable to 

 detachment by injury and disease. 



THE EAR 



The external ear consists of the expanded pinna, composed of 

 yellow fibro-cartilage, and the auditory meatus. 



The deepest part of the pinna is the concha (cockle-shell), at the 

 front of which is a cartilaginous plate which acts as a shield to the 

 meatus, on which grows a tuft of hair like the beard of a goat (tragus). 

 Behind this shield is another plate, the anti-tragus, below which is 

 the fibrous lobule. (A keloid tumour sometimes springs from the scar 

 which necessarily results from piercing the lobule for an ear-ring.) 

 The tragus and anti-tragus are separated by a deep incisura. The 

 margin of the pinna is the helix (eXi, a spiral), and the groove be- 

 neath its incurved border is the fossa of the helix. Rather nearer 

 to the meatus is a thicker ridge, the anti-helix, which bifurcates above 

 to inclose \\\t fossa of the anti-helix. 



