3 62 NORMAL HISTOLOGY. 



approximately uniform calibre of .5 mm. in its horizontal portion. The 

 canaliculus possesses a lining of stratified squamous epithelium, which rests 

 upon a delicate tunica propria rich in elastic fibres, muscular fibres from the 



orbicularis palpebrarum affording additional sup- 

 port. The muscle-bundles run parallel to the 

 horizontal portion of the canaliculus, but are 

 arranged as a circular sphincter about the vertical 

 portion. 



The lachrymal sac may be regarded as 

 the upper dilated portion of the naso-lachrymal 

 duct, the lower part of which passes through a 

 bony canal and opens into the inferior nasal 

 meatus. The sac is about 15 mm. long, and 

 6 mm. in diameter when distended. The wall 

 of the sac is composed of a fibre-elastic tunica 

 FIG. 411. Alveoli of lachrymal gland p rO pria of Ivmphoid character and is loosely 



more highly magnified. X 235. . . J . \ . . i 



connected with the periosteum by a stratum rich 



in veins. A few small branched tubular glands are usually present. It is 

 lined with a double layer of columnar epithelial cells, which in part are 

 provided with cilia. 



The naso-lachrymal duct, the lower portion of the tear-passage, 

 varies from 1 2-24 mm. in length, and is from 3-4 mm. in diameter. The 

 mucous membrane lining the duct is clothed with columnar epithelium 

 and may contain small glands in the lower portion. It is separated from 

 the periosteum by areolar tissue and a venous plexus. 



THE EAR. 



The auditory organ is conventionally described as the external, middle 

 and internal ear structures lodged entirely or in part within the tem- 

 poral bone. The external ear includes the auricle and the external auditory 

 canal; the middle ear the tympanum, the Eustachian tube and the mastoid 

 cells; and the internal ear the labyrinth, with the peripheral ramifications of 

 the auditory nerve. Such division, moreover, is justified by the develop- 

 mental history of the organ, since the internal ear is developed essentially 

 from the highly differentiated otic vesicle which gives rise to the complicated 

 membranous labyrinth; the middle ear largely from the first pharyngeal 

 pouch; whilst the external ear represents the deepened and modified boun- 

 daries of the first external visceral furrow. 



THE EXTERNAL EAR. 



The external ear, the outermost subdivision of the auditory organ, in- 

 cludes ( i ) the auricle, the funnel-shaped appendage attached to the side of 

 the head for the collection of the sound-waves, and (2) the external auditory 

 canal, which conveys these stimuli to the tympanic membrane, the flexible 

 partition closing the canal and separating it from the middle portion of the 

 ear. 



The Auricle. The outwardly directed external surface of the auricle 

 is irregularly concave and presents several well-marked depressions and ele- 

 vations, which depend, for the most part, upon the corresponding modelling 

 of the underlying cartilage. 



