THE TEMPOEAL BONES. 



131 



within the bone called the tympanum and forming the tegmen tympani. Laterally 

 the line of fusion of the petrous with the squamous part is often indicated by a faint 

 and irregular petro-squamous fissure. 



Posterior Surface. The most conspicuous object on the posterior surface of the 

 petrous part of the bone is the meatus acusticus interims (internal acoustic meatus), about 

 8 mm. deep in the adult. This has an oblique oval aperture, and leads laterally 

 and slightly downwards into the substance of the bone, giving passage to the acoustic 

 and facial nerves, together with the nervus intermedius and the auditory branch of 

 the basilar artery. The canal appears to end blindly ; but if it is large, or still better, 

 if part of it is cut away, its fundus will be seen to be crossed by a horizontal ridge, the 

 falciform crest, which divides it into two fossae, the floors of which (laminae, cribrosse) 

 are pierced by numerous small foramina for the branches of the acoustic nerve and 

 the vessels passing to the membranous labyrinth, whilst in the anterior and upper 

 part of the higher fossa the orifice of the canalis facialis, through which the 

 facial nerve passes, is seen leading in the direction of the hiatus canalis facialis 

 (vide supra). Lateral to the internal acoustic meatus and above it, close to the 

 superior border, an irregular depression, often faintly marked, with one or two small 

 foramina opening into it, is to be noticed. This is the fossa subarcuata, best seen 



in young bones (see Fig. 143 C), where 

 it forms a distinct recess, which is 

 bounded above by the bulging caused by 

 the superior semicircular canal, within 

 the concavity of which it is placed ; it 

 lodges a process of the dura mater. 

 Below and lateral to this, separated 

 from it by a smooth, elevated curved 

 ridge, is the opening of the apertura 

 externa aquaeductus vestibuli (aqueduct 

 of the vestibule), often concealed in a 

 narrow curved fissure overhung by a 

 sharp scale of bone. In this is lodged 

 the saccus endolymphaticus, 

 internal developed as an evagi nation 

 from the otocyst, together 

 with a small vein. The ridge 

 above it corresponds to the 

 upper half of the posterior 



External acoustic 

 meatus 



Osseous part of the 

 auditory tube 



acoustic 

 meatus 



Vestibule 

 Canalis facialis 



Fenestra vestibuli cut across i i 



nestra cochin cut arross semicircular canal. 



Superior opening of the canal for the 

 tympanic branch of glosso-pharyngeal 



FIG. 140. VERTICAL TRANSVERSE SECTION THROUGH THE LEFT 

 TEMPORAL BONE (Anterior Half of Section). 



Connexions. The temporal 

 bone articulates with the zygomatic, 

 sphenoid, parietal, and occipital 

 bones, and by a movable joint with 

 the mandible. Occasionally the temporal articulates with the frontal, as happens normally in 

 the anthropoid apes ; although the region of the pterioii is characterised by an X-like form, 

 in the lower races of man there is no evidence that the occurrence of a fronto-squamosal 

 suture is more frequent in the lower than the higher races, its occurrence being due to the 

 inanner of fusion of the so-called epipteric ossicles with the surrounding bones. 



Ossification. The temporal bone of man represents the fused periotic, squamosal, 

 and tympanic elements ; the two latter are membrane or investing bones, whilst the 

 former is developed in cartilage around the auditory capsule. The cartilages of the 

 I. and II. visceral arches are also intimately associated with its development, as will be 

 ftpewhere explained (Appendix E). The human temporal bone is characterised by the 

 large proportionate size of the squamosal, the comparatively small size of the tympanic, 

 the absence of an auditory bulla, and the exceptional development of the mastoid process. 



Ossification commences in the ear capsule in the fifth month, and proceeds so rapidly 

 that by the end of the sixth month the individual centres aTe more or less fused. Of 

 these, one, the Pro-otic (Huxley), which appears in the vicinity of the eminentia arcuata, 

 s the most definite in position and form ; from this a lamina of bone of spiral form 

 is developed, which covers in the medial limb of the superior semicircular canal, and 

 forms the roof of the internal acoustic meatus, together with the commencement of the 



9a 



