176 THE EYE. 



The eyes of man are placed as far as possible in safety in bony 

 sockets or orbits. In examining a skull, we cannot fail to have 

 noticed the thickening of the bones at their margins, and the 

 strength of the arches there formed. In many pre-historic races 

 they were much thicker than, with rare exceptions, they are 

 now, and resembled the great ridges and crests which protect 

 the eyes of the male Gorilla. 



In some blind mammals, with only rudimentary eyeballs, such 

 as the Italian and the Cape Moles, no eyelids exist, and in these 

 animals the skull shows no trace of an orbit, and the malar bone 

 (which generally forms and protects a large part of the orbit) is, 

 as a rule, either entirely wanting, or very rudimentary. Just as 

 with those deep-sea crustaceans, whose cephalo-thorax shows no 

 sign of an eye or trace of eye-stalk, and whose downward career 

 can be traced step by step from their light-appreciating ancestors : 

 much the same thing has happened in the great mammoth caves 

 in America to the spiders, beetles, etc., living there. 



In man relatively is the orbit best developed, and from its 

 position and character you might predict that he would not as a 

 rule depend on flight for safety, but would turn and face the 

 danger, unlike most ruminants and timid rodents, whose promi- 

 nent and laterally-placed eye can glance behind as easily as in 

 front, without a turn of the head. The eyebrows divert the 

 otherwise irritating stream of perspiration which toil or heat wring 

 from the forehead; the eyelashes help to guard against insects and 

 other small bodies, and the eyelids close accurately and swiftly by 

 involuntary reflex action at the near approach of a foreign body. 

 Often, indeed, no effort of the will, however strong, or determina- 

 tion well grounded and reasonable, is sufficient to restrain this 

 action. Dr. Darwin mentions how, when watching a poisonous 

 snake in its cage, which had a thick plate-glass front, he never 

 could restrain a blink and a wince when the angry reptile shot 

 forward its head to strike him, not realising the intervening glass 

 against which his face was pressed. However, should any foreign 

 body reach the exposed surface, the epithelium of the eye or 

 conjunctiva is so abundantly supplied with sensitive nerves, that 

 their irritation speedily produces a copious secretion of tears from 

 the lachrymal gland, in order that the foreign body may be 



