THE BONES OF THE HEAD 79 



is inwards, slightly fonvards, and finally downwards, its length being 

 rather more than | inch (14 millimetres). It leads to the t}Tnpanum.. 

 and its deep end, which is nearly circular, is closed by the membrana 

 tympani. This membrane is placed obliquely, and forms an acute 

 angle with the lower wall and an obtuse angle with the upper, 

 so that the floor of the meatus is longer than the roof, the anterior 

 wall being also longer than the posterior. Its floor presents a 

 slight elevation at the centre, where the passage is narrower than 

 elsewhere, this portion being called the isthmus. 



The tympanic plate is situated behind the fissure of Glaser, and 

 is quadrilateral. It presents two surfaces and four borders. The 

 external surface forms the posterior part of the glenoid fossa, and 

 lodges the deep portion of the parotid gland. The internal surface 

 forms the anterior, inferior, and part of the posterior, walls of the 

 external auditory meatus, and the anterior and inferior walls of 

 the t\Tnpanum, and at its inner or deep end it presents a groove, 

 deficient above, for the membrana tvTiipani, caUed the sulcus tym- 

 panicus. The outer border forms the external auditory process, and 

 is curved and rough for the cartilage of the pinna. The inne,- border 

 is situated immediately outside the bony part of the Eustachian 

 tube, and is short and irregular. The upper border bounds the 

 fissure of Glaser posteriorly, and the lower b&rder forms at its back 

 part the vaginal process, which ensheathes the base of the styloid 

 process externally. The tympanic plate sometimes presents a small 

 opening at its centre, called the foramen of Buschke. 



The styloid process, which is cylindrical and tapering, starts 

 ■from a point immediately in front of the stylo-mastoid foramen, 

 and is directed downwards and inwards. The muscular and 

 ligamentous relations of the process are as follows: The stylo- 

 phar^mgeus muscle arises from the iimer aspect of the base; the 

 stylo-hyoid muscle from the posterior and outer aspect of the 

 process near its base; the stylo-glossus muscle from the front of 

 the process near its tip; the stylo-mandibular ligament is attached 

 to it just below the stylo-glossus; and the stylo-hyoid ligament is 

 attached exactly to the tip. 



The blood - supply of the bone is chiefly derived from the 

 following sources: The squamous portion receives externally 

 branches from the anterior and posterior deep temporal arteries 

 of the internal maxillary, and internally branches of the middle 

 meningeal. Other arterial twigs enter the bone at definite points, 

 as follows : internal auditory from the basilar, through the internal 

 auditory meatus ; petrosal from the middle meningeal, through the 

 hiatus Fallopii ; stylo-mastoid from the posterior auricular, through 

 the stylo-mastoid foramen ; t\-mpanic from the internal maxillary, 

 through the Glaserian fissure; tj-mpanic from the ascending 

 pharyngeal, through the t>'mpanic canaliculus; t}Tnpanic from 

 the internal carotid, through the foramen on the posterior wall 

 of the vertical portion of the carotid canal; the mastoid from the 

 occipital, through the arterial fissure on the outer surface of the 



