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 semllunaris, 

 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 



