THE CRYSTALLINE LENS. 



1471 



FIG. 1228. 



If the lesion affect the chiasm, as from tumors of the pituitary body, periostitis 

 of the body of the sphenoid bone, tuberculous or syphilitic exudate, causing pressure 

 on the mesial portion of the chiasm involving the decussating fibres, the nasal half 

 of each eye supplied by these fibres will be blind (heteronymous hemianopsia). 

 Since the nasal half of each eye perceives the temporal half of the visual field, this 

 variety of half-blindness is called bitemporal hemianopsia. 



If the optic fibres of one side in front of the chiasm are involved, the disturbance 

 of vision will affect one eye only, so that the occurrence of absolute blindness of one 

 eye, without other known cause, with good sight in the other, would suggest a lesion 

 in front of the chiasm. 



Inflammation of the intraocular end of the optic nerve that is, of the optic disc, 

 or papilla gives rise to the condition to which the name optic neuritis, or papillitis, 

 is applied, which is then recognizable with the ophthalmoscope. If in addition to 

 or independently of the signs of inflammation there are marked engorgement, oedema, 

 and the evidence of mechanical compression, so that the swollen nerve-head protrudes 

 into the vitreous beyond y 2 to ^ mm., the phenomena of "choked disc" are pre- 

 sent. This variety of papillitis, as well as more moderate grades of optic neuritis, 

 constitutes one of the important symptoms of brain tumor, occurring in fully 80 per 

 cent, of the cases. The development of the papillitis does not necessarily depend 

 upon the size of the growth, nor upon its situation, except that tumors of the 

 medulla are less apt to originate optic neuritis than those in other parts of the brain. 

 Usually a bilateral condition, it is sometimes unilateral, and under such circum- 

 stances it suggests that the cerebrum is the seat of the growth, and is, on the whole, 

 in favor of the tumor being on the same side as the neuritis. With this exception, 

 however, optic neuritis, although an important symptom of brain tumor, has no 

 localizing significance. Other intracranial causes of optic neuritis are the various 

 types of meningitis (when the ophthalmoscopic picture often appears in the form of 

 the so-called "descending neuritis"), abscess and soft- 

 ening of the brain, cerebritis, hydrocephalus and aneu- 

 rism. In addition to the intracranial causes of papillitis, 

 this phenomenon may arise from a general infection 

 for example, influenza, syphilis, rheumatism, small-pox, 

 etc. and is then known as infectious optic neuritis. 

 It is also caused by various toxic agents, by anaemia, by 

 menstrual disturbances, nephritis, and other constitu- 

 tional disorders (de Schweinitz). 



Injuries of the optic nerve are most frequently the 

 result of fractures of the base of the skull at the optic 

 foramen, the nerve being injured by the fragments. 

 It may be wounded by foreign bodies entering the orbit, 

 with or without injury of the eyeball. 



THE CRYSTALLINE LENS. 



The lens, the most important part of the refractive 

 apparatus of the eye, is a biconvex body situated on a 

 level with the anterior plane of the ciliary body, from 

 which it is suspended by the suspensory ligament, or 

 zonule of Zinn. Its anterior surface supports the pu- 

 pillary margin of the iris, and its posterior surface rests 

 in a depression, the patellar fossa, on the anterior sur- 

 face of the vitreous body. It is completely transparent and enclosed in a transparent 

 elastic membrane, the lens capsule. Together with the capsule, the lens measures 

 from 9-10 mm. in its transverse diameter, and about 4 mm. in thickness from pole 

 to pole. The convexity of its two surfaces is not the same, that of the posterior 

 being greater than that of the anterior. Neither are these convexities constant, since 

 they are continually changing with the variations in lens-power incident to viewing 

 distant or near objects. The radius of curvature of the anterior surface is approxi- 

 mately 9 mm. and that of the posterior surface 6 mm. when the eye is accommodated 



Meridional section of human lens 

 and its capsule ; anterior epithelium 

 and transitional zone are seen. X 7- 

 (Babuchin.) 



