i84 HUMAN ANATOMY. 



tains large pneumatic cavities, sometimes diploe instead of air-cells, and, again, it 

 may be almost solid ; the latter condition is, however, probably always pathological. 

 According to Zuckerkandl's^ investigations of 250 temporal bones, the mastoid is 

 entirely pneumatic in 36.8 per cent, and wholly diploetic in 20 per cent. The re- 

 maining 43.2 per cent, were mixed, thje diploe being at the point of the mastoid and 

 the cells above. Neither size nor shape indicates its internal structure. The relation 

 of the cells to the lateral sinus has been already mentioned. 



The Facial Canal. — The course of the canal ^ for the facial nerve is important. 

 It runs outward from the superior fossa of the internal auditory meatus for some 

 three millimetres, until joined by the canal from the hiatus Fallopii. It then makes 

 a sharp turn (the genu ) backward, passing internal to the attic of the tympanum just 

 below the external semicircular canal, which almost always projects a little farther 

 outward. It then curves backward to descend to the stylo-mastoid foramen, passing 

 just above the fenestra ovalis. The descending portion is rarely strictly vertical. 

 Below the genu the facial canal may make a bend either outward or inward, but its 

 general line of descent usually inclines outward, sometimes very strongly. Rarely 

 the descent is tortuous. The lower part may incline forward. The genu is opposite 

 a point on the surface above the external meatus, and the subsequent course of the 

 canal can be indicated in general by a line following the posterior border of the 

 auditory opening. An instrument introduced straight into the front of the mastoid 

 will pass behind the facial canal.* The diameter of the latter is about one and one- 

 half millimetres. Just before its lower end a very minute canal, transmitting the 

 chorda tympani nerve, runs upward and forward from it to the cavity of the tympa- 

 num. From the front of the cavity this nerve escapes by the minute canal of Hugider, 

 which opens near the inner end of the fissure of Glaser, passing between the tym- 

 panic plate and the tegmen tympani. The facial canal has several other minute 

 openings. There are also minute canals for Jacobson' s nerve from the glosso- 

 pharyngeal, leading to the tympanum, and for Aryiold' s branch of the vagus, which 

 enters the jugular fossa and leaves by the fissure between the mastoid and tympanic* 

 portions. 



The carotid canal * is close to the front of the tympanum and just before the 

 cochlea of the internal ear. The internal auditory meatus is almost behind the canal, 

 and the Eustachian tube lies to the outer side of its horizontal portion. 



The temporal bone is porous in structure, except about the internal ear, where 

 it is very dense. A transverse section, either vertical or horizontal, through the 

 external and internal meatus (the middle and internal ears) shows how nearly the 

 entire bone is pierced (Fig. 202). The carotid canal and the jugular fossa, when 

 deep, are further sources of weakness. The fossa sometimes opens into the middle 

 ear by a small rent. 



Articulations. — The temporal bone joins the occipital by the petro-mastoid 

 portion. These two bones form the entire posterior fossa of the skull, except at the 

 extreme front, in the middle, where it extends along the back of the sphenoid, 

 and at the side, where a small portion of the lateral sinus is made by the posterior 

 inferior angle of the parietal. This latter bone articulates with the squamous and 

 the top of the mastoid. The great wing of the sphenoid fits into the angle between 

 the squamous and petrous portions, articulating at the side of the skull with the front 

 of the foramen. These two bones — the sphenoid and the temporal — form the entire 

 middle fossa. The malar bone joins the zygoma, completing the arch. The lower 

 jaw articulates with the glenoid fossa by a true joint. 



Development. — The sqtiamoiis portion is ossified in membrane from one 

 centre, appearing near the end of the second month of foetal life. In the course of 

 the third mcnth a centre appears in the lower part of the future tympanic ring. 

 The ossification of the petro-juastoid portion comes from several nuclei, the number 

 of which probably varies. The process begins towards the end of the fifth month 

 about the membranous labyrinth. The opisthotic nucleus lies at the inner side of 

 the tympanic cavity and spreads to the lower part of the bone. The pro'dtic is near 

 the superior semicircular canal. The epiotic, arising near the posterior canal, 



1 Monatsschrift fiir Ohrenheilkunde, Bd. xiii, 1S79. 



^ Joyce : Journal of Anatomy and Physiology, vol. xxxiv., 1900. 



^ Canalls facialis. ^ Canalls caroticus. 



