BOWMAN LECTURE. LXXTX 



marked varieties, and will probably be found true for all 

 as opportunities for investigation occur. When cataract 

 occurs at birth, or early in life, in brothers and sisters, both 

 parents being free and no history obtainable of ancestral 

 or collateral cases, when it is, in fact, what is cfi\]ed familial 

 without proof of heredity, there may be grounds for attri- 

 buting it to some defect of intra-uterine nutrition. But this 

 explanation, unlikely even when the mother is affected, is 

 impossible when the father, not the mother, suffers, for in 

 this case there must be a germinal cause. That the germ- 

 cell, whether male or female, should be able to transmit 

 a well-defined and often almost identical imperfection 

 limited to so small a part of the body as the lens, and 

 often to only a small portion even of it, shows how incon- 

 ceivably minute the morbid germinal representation may 

 be, and this whether we think of the lens itself or the 

 parts upon which it depends for nourishment at different 

 stages of its growth. From Priestley Smith's researches* 

 we may take it that the weight of the normal human lens 

 at between 20 and 80 years of age is about 175 mgrm. 

 or roughly three millionths of the ordinary body-weight 

 at that time of life.t Yet even this is too much. The 

 opacity in a typical case of discoid (or "Coppock") 

 cataract occupies only a small fraction of the entire lens, 

 possibly one twentieth or even less. The malign germinal 

 influence, whatever it is, presumably acts upon the lens 

 only at its earliest stage, possibly even before the closure 

 o the lens cup, and even then is so limited in its range 

 as to damage no other part of the epiblast ; or if another 

 interpretation be preferred, affects no other part of the 

 mesoblast than the minute portion concerned in the nutri- 

 tion of the rudimentary lens. 



In hereditary lamellar cataract the dimensions of the 

 opacity are not so extremely minute, but it also, like the 



* Priestley Smith, " On the Growth of the Crystalline Lens/' T.O.S., 

 Hi, 1883, p. 79. 



t Average body-weight of $ + V at 20 to 25 about 130 lb., or say 59 

 kilogrammes = 59,000,000 millegrammes -4- 175 = 337,154, or, say, one 

 third of a million. 



