THE NERVOUS SYSTEM. 221 
The point on either side where the two eyelids meet is 
termed the canthus or angle. At the medial or inner canthus 
are two minute apertures, the puncta lachrymalia, leading 
into two short canals which unite to form the nasal duct. 
The lachrymal canal, which forms the channel for this duct, 
is clearly visible in the lachrymal bone of the dried skull. 
At the medial canthus there is a prominent fold of mucous 
membrane, the membrana nictitans, or plica semilunaris, 
which is a rudimentary structure in the cat, but is found 
well developed in birds, which have the power of sweeping 
it rapidly across the eyeball, thereby removing dust. 
On the inner surface of each lid are the Meibomian 
glands. The lachrymal gland lies in the dorso-lateral 
region of the orbit, and its ducts open on the ventral sur- 
face of the upper lid, whence the tears flow over the con- 
junctiva ventrad to the puncta lachrymalia. The Harderian 
gland is the very small gland at the medial canthus. 
The muscles controlling the movements of the eyeball 
are seven in number: four are recti muscles, two are oblique, 
and one is a retractor. The recti muscles (Fig. 93) origi- 
nate on the bone around the optic foramen, and are inserted 
on the sclerotic coat, caudad of the equatorial ring. The 
external rectus is inserted on the lateral aspect; the internal 
rectus, on the medial aspect; the superior rectus, on the 
dorsal aspect; and the inferior rectus on the ventral aspect. 
The superior oblique muscle arises from the sphenoid bone 
medial of the optic foramen, extends along the medial wall 
of the orbital cavity to its dorso-medial margin, where it 
passes through a tendinous loop fastened to the frontal bone 
and then turns laterad to its insertion in the sclerotic beneath 
the superior rectus. The inferior oblique arises from the 
lachrymal bone and is inserted on the sclerotic between the 
external and inferior recti muscles. The above muscles 
may be demonstrated by cutting away the lateral and dorsal 
