506 ORGAN OF HEARING. 



wards to the lachrymal sac, where they terminate beneath a valvular 

 semilunar fold of the lining membrane of the sac. The superior duct 

 at first ascends, and then turns suddenly inwards towards the sac, 

 forming an abrupt angle. The inferior duct forms the same kind of 

 angle, by descending at first, and then turning abruptly inwards. 

 They are dense and elastic in structure, and remain constantly open, 

 so that they act like capillary tubes in absorbing the tears from the 

 surface of the eye. The two fasciculi of the tensor tarsi muscle are 

 inserted into these ducts, and serve to draw them inwards. 



The Lachrymal sac is the upper extremity of the nasal duct, and is 

 scarcely more dilated than the rest of the canal. It is lodged in the 

 <-] groove of the lachrymal bone, and is often distinguished internally 

 from the nasal duct by a semilujjar or circular valve. The sac consists 

 of mucous membrane, but is covered in and retained in its place 

 by a fibrous expansion, derived from the tendon of the orbicularis, 

 which is inserted into the ridge on the lachrymal bone ; it is also 

 covered by the tensor tarsi muscle, which arises from the same ridge, 

 and in its action upon the lachrymal canals may serve to compress the 

 lachrymal sac. 



The Nasal duct is a short canal about three quarters of an inch in 

 length, directed downwards, backwards, and a little outwards to the 

 inferior meatus of the nose, where it terminates by an expanded orifice. 

 It is lined by mucous membrane, which is continuous with the con- 

 junctiva above, and with the pituitary membrane of the nose be- 

 low. Obstruction from inflammation and suppuration of this duct 

 constitutes the disease called fistula lachryrnalis. 



Vessels and nerves. The lachrymal gland is supplied with blood 

 by the lachrymal branch of the ophthalmic artery, and with nerves by 

 the lachrymal branch of the ophthalmic and orbital branch of the 

 superior maxillary. 



THE ORGAN OF HEARING. 



The apparatus of hearing is composed of three parts ; the external 

 ear, middle ear or tympanum, and internal ear or labyrinth. 



The EXTERNAL EAR consists of two portions, the pinna and meatus; 

 the former representing a kind of funnel which collects the vibrations 

 of the atmosphere, called sounds, and the latter a tube which conveys 

 the vibrations to the tympanum. 



The PINNA presents a number of folds and hollows upon its surface, 

 which have different names assigned to them. Thus the external 

 folded margin is called the helix (1*.%, a fold). The elevation parallel 

 to and in front of the helix is called antihelix (av<r<\ opposite). The 

 pointed process, projecting like a valve over the opening of the ear 

 from the face, is called the tragus (ra.yo?, a goat), probably from 

 being sometimes covered with bristly hair like that of a goat ; and a 

 tubercle opposite to this is the antitragus. The lower dependent and 



