

THE ORGAN OF HEARING 



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from the medial palpebral ligament, and its deep surface is crossed by the lacrimal 

 part of the Orbicularis oculi (page 380) , which is attached to the crest on the 

 lacrimal bone. 



Structure. The lacrimal sac consists of a fibrous elastic coat, lined internally by mucous 

 membrane: the latter is continuous, through the lacrimal ducts, with the conjunctiva, and 

 through the nasolacrimal duct with the mucous membrane of the nasal cavity. 



The Nasolacrimal Duct (ductus nasolacrimalis; nasal duct). The nasolacrimal 

 duct is a membranous canal, about 18 mm. in length, which extends from the lower 



part of the lacrimal sac to the inferior 

 meatus of the nose, where it ends by a 

 somewhat expanded orifice, provided 

 with an imperfect valve, the plica 

 lacrimalis ( Hasneri) , formed by a fold 

 of the mucous membrane. It is con- 

 tained in an osseous canal, formed by 



Puncta lacrimalia . 



FIG. 896. The lacrimal apparatus. Right side. 



FIG. 897. Alveoli of lacrimal gland. 



the maxilla, the lacrimal bone, and the inferior nasal concha; it is narrower in 

 the middle than at either end, and is directed downward, backward, and a little 

 lateralward. The mucous lining of the lacrimal sac and nasolacrimal duct is 

 covered with columnar epithelium, which in places is ciliated. 



THE ORGAN OF HEARING (ORGANON AUDITUS; THE EAR). 



The ear, or organ of hearing, is divisible into three parts: the external ear, the 

 middle ear or tympanic cavity, and the internal ear or labyrinth. 



Cavity of hind-brain 



Hind-brain 



Auditory pit 



Ectoderm 



Notochord 



FIG. 898. Section through the head of a human 

 embryo, about twelve days old, in the region of the 

 hind-btain. (KolLrnann.) 



Auditory 

 vesicle 



FIG. 899. Section through hind-brain and audi- 

 tory vesicles of an embryo more advanced than that 

 of Fig. 898. (After Hi?.) 



The Development of the Ear. The first rudiment of the internal ear appears 

 shortly after that of the eye, in the form of a patch of thickened ectoderm, the 

 auditory plate, over the region of the hind-brain. The auditory plate becomes 

 depressed and converted into the auditory pit (Fig. 898). The mouth of the pit is 





