THE HEAD AND NECK. 133 



the medium of a periosteal membrane ; (3) the promontory, formed by a turn 

 of the cochlea, and covered by the tympanic plexus ; the promontory is below 

 and in front of the fenestra ovalis ; (4) the fenestra rotunda, covered by the 

 membrana secundaria, and communicating with the scala tympani of the 

 cochlea ; (5) the pyramid, from whose summit emerges the tendon of the sta- 

 pedius. A special branch of the seventh nerve pierces the pyramid for the 

 supply of the stapedius muscle. 



5. The posterior ivall commu\\\ca.tQS with the mastoid antrum. As indicated in 

 a previous paragraph, the mastoid antrum is the backward continuation of the 

 upper part of the tympanic cavity, which is called the attic. 



6. The anterior wall of the tympanum presents but one thing for examination 

 the canalis musculo-tubarius. This canal is divided into an upper and a lower 

 compartment by a horizontal lamina of bone, called the processus cochleari- 

 formis. The upper compartment lodges the tensor tympani muscle ; the lower 



FIG. 89. EAR AND TYMPANUM. 



I. Pinna, or auricle. 2. Concha. 3. External auditory canal. 4. Membrana tympani. 5. Incus. 

 6. Malleus. 7. Manubrium mallei. 8. Tensor tympani. 9. Tympanic cavity. 10. Eustachian 

 tube. II. Superior semicircular canal. 12. Posterior semicircular canal. 13. External semi- 

 circular canal. 14. Cochlea. 15. Internal auditory canal. 16. Facial nerve. 17. Large petrosal 

 nerve. 18. Vestibular branch of auditory nerve. 19. Cochlear branch. 



is the osseous part of the Eustachian tube. These structures can be readily seen 

 in dissection. A common broom straw or hairpin may be thrust through the 

 lower part of the canalis musculo-tubarius as a guide. 



Before you attempt to dissect the seventh nerve in its tortuous canal through 

 the petrous portion of the temporal bone, study well the following branches this 

 nerve gives off between the internal auditory meatus and the stylo-mastoid fora- 

 men. In the auditory canal a short communicating branch, rather large and 

 fatty, extends from the seventh to the eighth nerve. This is called the portio 

 inter duram et mollem. The bony canal occupied by the seventh nerve, from the 

 time it leaves the eighth nerve until it emerges from the stylo-mastoid foramen, is 

 called the aqneductus Fallopii, or the facial canal. In this canal the following 

 branches are given off, as you may see by figure 90. 



I. From the geniculate ganglion, the great superficial petrosal nerve ; this 

 passes through the cartilage in the foramen lacerum medium, and is joined by 



