256 
ANATOMY OF REGIONS. 
these is called the posterior straight. Its presence is related to 
that of the membrana nictitans, as it is missing in animals where 
this organ is absent, especially in monkeys and man; it is princi¬ 
pally the contraction of this muscle which draws the globe towards 
the bottom of the orbit and produces the projections of the 
membrana nictitans. A thick adipous tissue, united to this organ, 
rests between these two muscular layers and also between the 
deep layer and the optic nerve. 
Let us also mention the oblique muscles which allow the motions 
of pivoting of the eye when the head is inclined laterally ; the 
superior oblique, or great oblique, which, from the bottom of the 
ocular sheath, reflexes itself upon a small tendon attached on the 
internal side of the orbit, hence to the superior face of the eye. 
The internal oblique rises from the internal face of the orbit and 
goes to the superior face of the eye. These muscles are antago¬ 
nistic of each other when examined on the same eye, and on the 
contrary antagonistic of that of the same name in the opposite 
eye, when the parellelism of the axis must be maintained as in 
the case of lateral inclination of the head. 
The elevator of the superior eyelid is also lodged in the ocular 
sheath, altogether on the internal face above the superior straight. 
Numerous blood vessels are distributed to these muscles or 
run through them so as to reach the globe of the eye. 
Amongst them we will mention the ophthalmic artery, an orbi- 
3 tal brand), the supraciliary and central arteries of the retina. 
A large vein, the alveolar, runs through the ocular sheath to' 
empty into the cavernous sinus; it establishes communication 
between this and the glosse-facial, with which it unites on the an¬ 
terior border of the masseter near its superior insertion. 
Nerves are numerous. We have the nerve of first pair, or 
the optic, functional nerve of the eye ; that of the third pair, com¬ 
mon ocular motor, going to the superior, posterior, internal and 
inferior straight; the pathetic which goes to the external straight. 
And then the ophthalmic of Willis, which carries excessive sensi¬ 
bility to all the parts of the eye, in connection with the orbital 
branch of the maxillary nerve. 
{To be continued.) 
