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, ze. 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 fossee ; 
it is innervated by nerves arising from the sphenopalatine 
ganglion, and contracting, under their action, causes the eye to 
protrude." 
The occasional presence of laterally and medially diverted 
offshoots of the levator palpebree 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 
hes at the median angle of the 
eye, and is known as the plica 
semilunaris (pl., Fig. 91). This 
corresponds with the third eye- 
hd, the so-called  nictitating 
membrane, of the lower animals. 
In Birds, Anurous Amphibians 
[some Sharks], and in many 
Reptiles it is highly developed, 
Fic. 91.—Human Eve. and, by means of a special mus- 
cdacarmenla ehegnais;2tsmi® 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. 
