148 THE STRUCTURE OF MAN 



the processus falciformis and the pecten which are permanently 

 retained, but in Man the corresponding structure undergoes com- 

 plete degeneration before birth. 



We meet with indications of atavism in connection with the 

 accessory parts of the eye. In the fissura orbitalis inferior, for 

 instance, there is an accumulation of smooth muscle, which is 

 the last vestige of the well-developed musculus orbitalis of lower 

 Mammals. In these animals the orbital fossa is usually in open 

 communication with the temporal, i.e. the two are not separated 

 by a bony septum (cf. ante, p. 58). This sheet -like muscle 

 forms the boundary between the temporal and the orbital fossae ; 

 it is innervated by nerves arising from the sphenopalatine 

 ganglion, and contracting, under their action, causes the eye to 

 protrude. 1 



The occasional presence of laterally and medially diverted 

 offshoots of the levator palpebrse superioris muscle suggests that 

 it may once have been more extensive than at present. It may 

 be regarded as the vestige of the much more strongly developed 

 palpebralis muscle of certain lower Mammals ; further investiga- 

 tion of this subject, however, is required. 



Great interest attaches to the fold of the conjunctiva which 

 lies at the median angle of the 

 eye, and is known as the plica 

 semilunaris (pi., Fig. 91). This 

 corresponds with the third eye- 

 lid, the so - called nictitating 

 membrane, of the lower animals. 

 In Birds, Anurous Amphibians 

 [some Sharks], and in many 



pi. Eeptiles it is highly developed, 



FIG. 91. HUMAN EYE. and, by means of a special mus- 



cular apparatus, can be drawn 

 across the eyeball. It serves not 

 only to cover, but to keep clean the surface of the eye, the 

 upper lid [which in Man performs that function] being im- 

 movable, and the lower slightly movable or but little developed. 

 In Man, as in the Apes, in association with the absence of a 

 retractor bulbi muscle, this third eyelid has undergone great 

 degeneration, but it may still enclose (more frequently in Negroes 

 than in Caucasians) a cartilaginous support. Among sixteen 



1 Nussbaum has recently announced the discovery in a human orbit of a muscle 

 homologous with the retractor bulbi of lower vertebrata. This awaits confirmation. 



