282 AN INTRODUCTION TO ZOOLOGY 



elements, the pro-otic, the epi-otic and the opisth-otic and is divisible 

 into two secondary regions, a very dense hard petrous portion enclosing 

 the membranous labyrinth and a more cancellous mastoid portion 

 which is visible on the outside of the skull and bounded posteriorly 

 by the exoccipital bone. The surface of the periotic facing the 

 inside of the cranium has a deep pit-like depression, the floccular 

 fossa, in which is contained a projecting portion of the cerebellum 

 termed the floccular lobe. Below this are two openings side by side ; 

 the hinder one is the meatus auditorius internus through which the 

 auditory nerve passes from the brain to the inner ear, and the other 

 foramen is the aquaeductus Fallopii, transmitting the facial nerve. 

 The outer surface of the petrous portion is perforated by two holes 

 which, however, cannot be seen until the tympanic bone has been 

 removed. The anterior one is the fenestra ovalis and the posterior 

 one is the fenestra rotunda ; both lead to the internal ear from the 

 tympanic cavity. The hollow tympanic bone has the shape of a 

 short-necked flask. Its expanded part, termed the bulla, contains 

 the tympanic cavity, while the neck is the meatus auditorius externus 

 or external ear aperture. Across the base of the meatus in life lies the 

 tympanic membrane, which is tightly stretched over an incomplete 

 bony ring. The Eustachian tube leaves the tympanic cavity by an 

 aperture at the an tero- ventral corner of the tympanic bulla and 

 opens into the posterior part of the narial passage. Just next to 

 this, between the tympanic bone and the alisphenoid, is the foramen 

 lacerum medium for the transmission of the mandibular branch of 

 the trigeminal nerve. On the postero- ventral aspect of the bulla 

 is a round foramen through which the internal carotid artery passes 

 into the cranial cavity. Between the postero-lateral border of the 

 bulla and the mastoid portion of the periotic is the stylomastoid 

 foramen, through which runs the main branches of the facial nerve. 

 Across the tympanic cavity stretches a string of bones termed collec- 

 tively the auditory ossicles. The outermost of these, the malleus (c), 

 is a hammer-shaped bone with its blade-like handle attached to the 

 tympanic membrane and also articulating with the second bone, 

 the incus (c) . This is a pyrif orm bone produced behind into a short 

 process and below into a short stalk bent inwards. To the end of 

 this is attached a small disc of bone, the OS orbiculare (c) , which in 

 turn articulates with the arch of a stirrup-shaped bone, the stapes (c) . 

 The basal oval-shaped part of this bone is attached to the membrane 

 closing the fenestra ovalis. By means of this chain of bones the 

 vibrations of. the tympanic membrane are transmitted to the peri- 

 lymph surrounding the membranous labyrinth. 



The posterior end of the cranial region is composed of a ring of 

 four bones, termed the occipital segment. The dorsal member is 



