ii6 HEREDITY AND EUGENICS 



rence also of pterygium.* It is believed that all 

 these conditions form one complex, and whether 

 all appear may depend on other factors present. 



Nettleship (191 2) has also described a family in 

 which night-blindness was sex-linked and was as- 

 sociated with myopia. It would, therefore, appear 

 that the same condition may arise through an altera- 

 tion in the sex chromosome or in another chromosome. 



The inheritance of manv other abnormalities of the 

 eye is known. For example, Folkar (1909) has traced 

 nodular opacity of the cornea through three genera- 

 tions, finding nine cases abnormal to nineteen normal 

 or unknown. 



A bibliography of hereditary eye defects has 

 recently been published b}^ Howe (1921). Among 

 hereditar}^ defects are included corneal degeneration, 

 coloboma (congenital fissure) of the iris, aniridia 

 (absence of iris), glaucoma (a disease of the eye wdth 

 intense intra-ocular pressure, resulting in hardness of 

 the eye and blindness), cataract, congenital disloca- 

 tion of the lens, retinitis pigmentosa (inflammation 

 of the retina, with sclerosis, pigmentation, and atrophy 

 of the retina), atrophy of the optic nerve, micro- 

 phthalmus (abnormally small e3^es), myopia (short- 

 sightedness), astigmatism (a defect in the lens of the 

 eye, in which not all light rays are brought to a focus 

 at the same point), nystagmus (continuous rolling 

 movement of the eyeball), and ptosis (drooping of the 

 eyelid owing to paralysis). 



Briggs (191 8) has studied the inheritance of ptosis 

 (paralytic drooping of the eyelid) through six genera- 

 tions from a w^oman having this ocular defect. The 

 lineage belongs to a family in the mountains of the 

 vSouthern States, and included 128 individuals, half 



* Congenital pterygium or epitarsus is an abnormality of the 

 eye in which there is a fold in the conjunctiva or membrane lining 

 the eyelid and covering the front of the eyeball. 



