496 



THE EYE IN EVOLUTION 



1900). Its action is probably to pull back the eye, a function eminently 

 required in Herbivora which feed with the head lowered and also in 

 Sirenians which graze at the water's edge ; in man this action is taken 

 over by the tonicity of the recti themselves (Grimsdale, 1921). In the 

 rhinoceros and at least one species of the Ursidse {Melursus labiatus) a 

 simultaneous contraction of the retractor and lateral rectus muscles 

 flicks the eye quickly to the temporal side and at the same time retracts 

 it — a substitute for blinking movements of the lids. A similar move- 

 ment is seen occurring about once in eacli minute in the okapi, and as 



M. rectus 

 laleralU 



FiG. 654. — The Retractor Bulbi 

 Muscle of a Sheep (Bland-Sutton). 



Fig. 655. — The Scleral Insertion 

 OF the Orbital Muscles of the 

 Dog. 



View from behind. 1-4, the inser- 

 tions of the 4 heads of the retractor 

 muscle which alternate with and are 

 closer to the posterior pole than the 

 recti (after O. C. Bradley). 



the eye retracts the nictitating membrane, well developed as in most 

 Ungulates, sweeps across the globe (Briickner, 1950). As a secondary 

 action it helps to thrust out the nictitating membrane by pressure from 

 behind. Watrous and Olmsted (1941) reported that after excision of 

 all the other extrinsic muscles in the dog, the retractor bulbi was 

 eventually capable of moving the eyeball in all directions. 



In the higher Primates the retractor muscle is vestigial or absent. In 

 Macacus, the remnant lies above the lateral rectus, and in this region vestigial 

 muscular fibres have been found in man (Nussbaum, 1893 ; Ledouble, 1897 ; 

 Fleischer, 1907). Indeed, according to Lewitsky (1910), thei'e is always a well- 

 marked connective tissue strand in this position in man, running from the back 

 of the fascia bulbi to the apex of the orbit. Whitnall (1911) has reported a case 

 wherein a well-developed muscle of four strands existed (Fig. 656). 



The orbital muscle of Miiller is found in many Vertebrates 

 (An; ' ihians. Reptiles, Birds) as a well-developed striated muscle mass ; 

 in Mc iuals it retrogresses and its fibres become plain. According to 



