576 ANATOMY IX A NUTSHELL. 



The mastoid cells are no1 present at birth but at the age of puberty there 

 are a few and they occupy the greater portion of the mastoid process. They 

 are secondary organs of hearing. 



The Eustachian tube is a passage from the uaso-pharynx to the tympanum. 

 It passes inward, downward, and forward from the tympanum. It is between 

 an inch and a half and two inches long and from one-twelfth to one-fifth of an 

 inch in diameter. It is made up of a bony part which is smaller than the car- 

 tilaginous portion and situated in the temporal bone, and of a cartilaginous 

 portion which is somewhat trumpet-shaped ending hi the pharynx. Its pur- 

 pose is to equalize the air pressure within the tympanum with that of the ex- 

 ternal car. At the point where the bony portion joins the cartilaginous portion 

 i« the isthmus tuba-. The opening of the Eustachian tube into the tympanum 



PLATE CCXC. 



A Lymphatic Node (After Gerrish.) 



is on the anterior wall, while its pharyngeal opening is on the lateral wall of 

 the naso-pharynx behind the posterior nares. This tube is closed except dur 

 ing swallowing, when it is opened by the following muscles: Tensor palati 

 Levator palati. Salpingo-pharyngeus, and pari of the Palato-pharyngeus. 



lessox re. 



The Internal Lai;. (Labyrinth). 

 This car consists of a bony labyrinth within which is the membranous 

 labyrinth. 



Tin' bony labyrinth is made up of the vestibule, the cochlea, and the 



SEMK li;< i LAB CAN ^LS. 



The membranous labyrinth is smaller than the bony labyrinth of which it 

 is a casl and the space between the two is lined with endothelium which con- 

 tain- parilymph. The parts which make the membranous labyrinth are, the 

 rjTRK i.i which is a membranous sac in the vestibule, saccule which is also a 

 membranous sac in the vestibule, membranous semicirculab canals which 

 are in 'he osseous canals, and the membranous cochlea which is aspiral tube 

 inclosed in the osseous cochlea. 





