208 THE ORGANS OF SPECIAL SENSE 



by conditions which interfere with the tension of the labyrin- 

 thine fluid. Meniere's disease (p. 90) may be caused by an 

 increase in pressure, possibly owing to over-secretion of endo- 

 lymph, or it may also be caused by haemorrhage into the 

 interior of the semicircular ducts. Both of these factors 

 produce their results by direct mechanical stimulation of the 

 terminal branches of the vestibular nerve. 



The cochlea is the essential part of the auditory apparatus, 

 while the semicircular ducts are concerned with equilibration. 



THE EYE 



i. THE LACRIMAL APPARATUS. The Lacrimal Gland lies 

 in contact with a depression in the antero-lateral part of the 

 roof of the orbit. Inferiorly, it rests on the eyeball, behind, 

 and on the superior fornix of the conjunctiva, in front, so that 

 when the upper eyelid is everted, the outline of the anterior 

 part of the gland can be made out. It is from the anterior 

 part of the lacrimal gland that its numerous ducts pass, and 

 they open directly into the conjunctival sac. The tears are 

 carried downwards and medially across the anterior surface of 

 the eyeball by the movements of the eyelids. The lacrimal 

 secretion forms a thin film over the eyeball, and the surface 

 tension of -the fluid maintains the sheet intact. 



At the medial end of the border of each eyelid, there is a 

 small papilla, in the centre of which a fine opening, termed the 

 lacrimal punctum, leads into the lacrimal duct. These ducts 

 carry away the excess of fluid from the medial corner of the 

 conjunctival sac. At first, for about 2 mm., they are directed 

 at right angles to the border of the lids, but they then turn, 

 almost at right angles, and run medially to open into the 

 lacrimal sac (Fig. 81). 



The Lacrimal Sac is placed in a groove in the anterior 

 part of the medial wall of the orbit. Its upper extremity is 

 blind, and, after being joined by the lacrimal ducts, the sac 

 narrows inferiorly and becomes continuous with the naso- 



