I 



80 THE SKELETON 



subsequently joined to the process by a narrow neck. Its free surface articulates with the head 

 of the stapes. 



The stapes is the innermost ossicle. It has a head directed horizontally outward, capped 

 at its outer extremity by a disc resembling the head of the radius. The cup-shaped depression 

 receives the lenticular process of the incus. The base occupies the fenestra vestibuli, and like 

 this opening, the inferior border is straight, and the superior curved. The base is connected 

 with the head by means of two crura, and a narrow piece of bone called the neck. Of the two 

 crura, the anterior is the shorter and straighter. The crura with the base form a stirrup-shaped 

 arch, of which the inner margin presents a groove for the reception of the membrane stretched 

 across the hollow of the stapes. In the early embryo this hollow is traversed by the stapedial 

 arterj'. The neck is very short, and receives on its posterior border the tendon of the stapedius 

 muscle. 



Development. — The tympanic cavity represents the upper extremity of the first endodermal 

 branchial groove, which becomes converted into a blind pouch, the communication of which 

 with the pharyngeal cavity is the tuba auditiva (Eustachian tube). The thin membrane which 

 separates the endodermal from the ectodermal groove becomes the tympanic membrane, and 

 it is from the upper extremities of the axial skeletons of the first and second branchial arches, 

 which boimd the groove anteriorly and posteriorly, that the auditory ossicles are formed, the 

 malleus and incus belonging to the first arch and the stapes to the second (Reichert). The 

 ossicles consequently lie originally in the walls of the cavity, but they are surrounded by a loose 

 spongy tissue, which, on the entrance of air into the cavity, becomes compressed, allowing the 

 cavity to enfold the ossicles. These therefore are enclosed within an epithelium which is con- 

 tinuous medially with that lining the posterior tympanic wall, and laterally with that lining 

 the internal surface of the tympanic membrane. 



The mastoid cells are outgrowths of the cavity into the adjacent bone, and are therefore 

 lined with an epitheUum continuous with that of the cavity. 



THE OSSEOUS LABYRINTH 



The osseous labyrinth [labyrinthus osseus] (fig. 100) is a complex cavity 

 hollowed out of the petrous portion of the temporal bone and containing the 

 membranous labyrinth, the essential part of the organ of hearing. It is in- 

 eompleteh^ divided into three parts, named the vestibule, the semicircular canals, 

 and the cochlea. 



Fig. 100. — The Left Osseous Labyrinth. (After Henle. From a cast.) 



Superior semicircular canal 



Lateral semicircular canal 



Posterior semicircular canal 

 Fenestra cochleas Fenestra vestibuli 



The vestibule. — This is an oval chamber situated between the base of the internal auditory 

 meatus and the medial wall of the tympanum, with which it communicates by way of the 

 fenestra vestibuh. Anteriorly, the vestibule leads into the cochlea, and posteriorly it receive? 

 the extremities of the semicircular canals. It measures about 3 mm. transversely, and is some- 

 what longer antero-posteriorly. 



Its medial wall presents at the anterior part a circular depression, the spherical recess 

 (fovea hemispherica), which is perforated for the passage of nerve-twigs. This recess is sepa- 

 rated by a vertical ridge (the crista vestibuli) from the vestibular orifice of the aquaeductus 



Fig. 101. — The Cochlea in Sagittal Section. (After Ilenle.) 



Internal auditory mcati-.s 

 The spiral canal 



vestibuli, which passes obliquely backward to open on the posterior surface of the petrosal. 

 The roof contains an oval depression — the elliptical recess (fovea hemielUptica). 



The semicircular canals arc three in number. Arranged in different planes, each forms 

 about two-thirds of a circle. One extremity of each canal is dilated to form an ampulla. _ 



The superior canal lies transversely to the long axis of the petrosal, and is nearly vertical; 



