Dissection of Sheep's Head. 247 



the investigation of the Membrana Nictitans, and its accom- 

 panying gland, viz. the Harderian Gland. 



In tracing back the Inferior Oblique muscle to its origin 

 below the Lachrymal duct, a light pink gland will have been 

 noted in the interval between the Inferior and Internal Recti, 

 and the Inferior Oblique muscles : this is the Harderian gland, 

 by which a thick whitish humour is secreted. Associated 

 with this gland, in the angle of the eye, between the globe 

 and the side of the orbit, will be found the accessory eyelid 

 or Membrana Nictitans, a fibro-cartilaginous structure con- 

 tinuous posteriorly with the pad of fat which is insinuated 

 between all the muscles of the eye. When the eye is in its 

 natural position the margin of this membrane is only to be 

 seen, the rest being buried in the ocular sheath ; but when 

 by the contraction of the Hecti and Suspensorius muscles, the 

 globe of the eye is drawn back against the pad of fat, the 

 membrane protrudes across the eye. The secretion of the 

 Harderian gland is poured forth from two openings under the 

 Membrana Nictitans, in order to facilitate the motion of this 

 membrane over the eyeball. 



Nerves of the Orbit. 



Eyeball is supplied by Second Cerebral (sensory). 



Levator Palpebrae Third Cerebral (motor). 

 Rectus Superior 



Rectus Inferior 



Rectus Internus 



ObKquus Inferior 



{i.e. the Third Nerve supplies all but three of the muscles 

 of the eye) 



Obliquus Superior is supplied by Fourth Cerebral (motor). 

 Fore-part of the Head, Nose, \ Fifth Cerebral, Ophthalmic 



and Lachrymal Gland ) Division (sensory). 



Rectus Externus is supplied by Sixth Cerebral (motor). 

 Choanoid or Suspensorius 



