234 ELEMENTS OF MAMMALIAN ANATOMY. 



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 

 surface of the upper lid, whence the tears flow over 

 the conjunctiva ventrad to the puncta lachrymalia. 

 The Harderian gland is the very small gland at the mesal 

 canthus. 



The muscles controlling the movements of the eye- 

 ball are seven in number: four are recti muscles, two 

 are oblique, and one is a retractor. The recti muscles 

 (Fig. 90) originate on the bone around the optic for- 

 amen, 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 mesal 

 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 mesad 

 of the optic foramen, extends along the mesal wall of 

 the orbital cavity to its dorso-mesal 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 walls of the orbital cavity. The 

 retractor oculi originates on the boundary of the optic 

 foramen and is inserted into the sclerotic around the 

 entrance of the optic nerve. This muscle is completely 

 hidden by the recti muscles. 



