264 ESSENTIALS OF ZOOLOGY 



The temporal bone completes the brain case laterally, and 

 it consists of a number of "elements. The squamosal is applied 

 to the parietals and alisphenoid, and bears a prominent curved 

 process, the zygoma, which furnishes an articular surface for 

 the lower jaw and articulates distally with the jugal to form 

 the zygomatic arch. It is a membrane bone ; so is the tympanic, 

 which forms a widely distended bulla protecting and supporting 

 the drum of the ear. The bulla and the periotic bone bound 

 and form the bony walls of the middle ear, into which the 

 Eustachian tube opens. The outer part of the tympanic 

 supports the beginning of the external passage of the ear. 

 The middle ear is traversed by a series of small bones, the 

 auditory ossicles, which intervene between the drum and the 

 fenestra ovalis of the periotic, and which serve to transmit the 

 sound waves. These have already been mentioned as deriva- 

 tives of the mandibular and hyoid arches. The malleus is 

 attached by its long process, the manubrium, to the tympanum, 

 and it is articulated to the tympanic bone and to the incus. 

 The incus is articulated to the periotic and the stapes. The 

 stapes is articulated to the incus through the intervention of 

 a small disc, the os orbiculare, and is applied by a broad base 

 to the membrane of the fenestra ovalis (fig. 140). It is per- 

 forated by the stapedial artery, a small branch of the carotid. 

 Joints occur between the ossicles, and the ossicles are sup- 

 ported by ligaments. They are cartilage bones. The periotic 

 is a cartilage bone derived from the ossification of the cartilage 

 of the auditory capsule. It is seen only externally, wedged 

 between the squamosal and the exoccipital, but it forms an 

 important element of the inner wall, the so-called petrous 

 portion of the temporal. The inner surface of the bone presents 

 a wedge-shaped ridge for the attachment of the tentorium 

 cerebelli. Below the ridge are the floccular fossa for the 

 floccular lobe of the cerebellum, and the internal auditory 

 foramen for the nerves VII, VIII. The auditory nerve ends 

 in the auditory organ which is lodged in the bone. The facial 

 nerve passes through the periotic, emerges on the inner face 

 of the middle ear, descends close to the stapes and gives off the 

 chorda tympani to the lower jaw, and then comes to the surface 

 of the skull through the stylomastoid foramen between the 



