CHAPTER VI. 



THE PROTECTIVE MECHANISMS OF THE 



EYEBALL. 



FOR the development of the conjunctiva it seems essential 

 that the membrane should for a time be converted into a 

 closed sac by the union of the margins of the eyelids 

 together in front of the eyeball. In mammals, like the 

 ruminants, who have to fend for themselves and run beside 

 their mothers directly they are born, the lid margins become 

 separated before birth, so that they are born into the world 

 with their eyes open. The young of nearly all the Carnivora, 

 of most rodents and Insectivora, who spend the first weeks 

 of life in some prepared lair or nest, are born blind, due to 

 the eyelids being still united in front of the eyes. The 

 same applies to the marsupials, whose young for some time 

 after birth are carried about in the maternal pouch. The 

 young of arboreal mammals, such as the lemurs, monkeys 

 and the higher apes, travel about with their mothers from 

 bough to bough directly after birth. The baby monkeys 

 are often carried by their mothers, while the baby lemurs 

 cling tightly to their ventral fur. When born into such an 

 environment it is obviously desirable to be endowed with 

 sight, and we find that these arboreal mammals, like the 

 ruminants, enter the world with their eyes open. Newborn 

 children are in many ways quite as helpless as the newborn 

 Carnivora, but, unlike them, the epithelial union of the 

 margins of their eyelids breaks through before birth; this 

 is doubtless an inheritance from their arboreal ancestors. 



