694 ORGANS OF SENSE. 



nerves enter its posterior border, whilst its anterior margin is closely adherent 

 to the back part of the upper eyelid, and is covered, on its inner surface, by a 

 reflection of the conjunctiva. This margin is separated from the rest of the 

 gland by a slight depression, hence it is sometimes described as a separate lobe, 

 called the palpebral portion of the gland. In structure and general appearance 

 the lachrymal resembles the salivary glands. Its ducts, about seven in num- 

 ber, run obliquely beneath the mucous membrane for a short distance, and 

 separating from each other, open by a series of minute orifices on the upper 

 and outer half 'of the conjunctiva, near its reflection on to the globe. These 

 orifices are arranged in a row, so as to disperse the secretion over the surface 

 of the membrane. 



The lachrymal canals commence a't the minute orifices, puncta lachrymalia, 

 seen on the margin of the lids, at the outer extremity of the lacus lachrymalis. 

 They commence on the summit of a slightly elevated papilla, the papilla lach- 

 rymalis, and lead into minute canals, the canaliculi, which proceed inwards to 

 terminate in the lachrymal sac. The superior canal, the smaller and longer of 

 the two, at first ascends, and then bends at an acute angle, and passes inwards 

 and downwards to the lachrymal sac. The inferior canal at first descends, and 

 then abruptly changing its course, passes almost horizontally inwards. They 

 are dense and elastic in structure, and somewhat dilated at their angle. 



The lachrymal sac is the upper dilated extremity of the nasal duct, and is 

 lodged in a deep groove formed by the lachrymal bone and nasal process of 

 the superior maxillary. It is oval in form, its upper extremity being closed in 

 and rounded, whilst below it is continued into the nasal duct. It is covered by 

 the Tensor Tarsi muscle and by a fibrous expansion derived from the tendo 

 oculi, which is attached to the ridge on the lachrymal bone. In structure, it 

 consists of a fibrous elastic coat, lined internally by mucous membrane; the 

 latter being continuous, through the canaliculi, with the mucous lining of the 

 conjunctiva, and through the nasal duct with the pituitary membrane of the 

 nose. 



The nasal duct is a membranous canal, about three-quarters of an inch in 

 length, which extends from the lower part of the lachrymal sac to the inferior 

 meatus of the nose, where it terminates by a somewhat expanded orifice, pro- 

 vided with an imperfect valve formed by the mucous membrane. It is con- 

 tained in an osseous canal, formed by the superior maxillary, the lachrymal, 

 and the inferior turbinated bones, is narrower in the middle than at each 

 extremity, and takes a direction downwards, backwards, and a little outwards. 

 It is lined by mucous membrane, which is continuous below with the pituitary 

 lining of the nose. In the caualiculi, this^ membrane is provided with scaly 

 epithelium; but in the lachrymal sac and nasal duct, the epithelium is ciliated 

 as in the nose. 



THE EAR. 



The Organ of Hearing has three parts; the external ear, the middle ear or 

 tympanum, and the internal ear or labyrinth. 



THE EXTERNAL EAR. 



The External Ear consists of an expanded portion named pinna, or auricle, 

 and the auditory canal, or meatus. The former serves to collect the vibrations 

 of tlw air l>y which sound is produced, and the latter conducts those vibrations 

 to the tympanum. 



The jw'wna, or auricle (Fig. 376), is formed by a layer of cartilage, covered 

 with integument, and connected to the commencement of the auditory canal ; 

 it is of an ovoid form, its surface uneven, with its larger end directed upwards. 

 Its outer surface is irregularly concave, directed slightly forwards, and presents 



