82 A MANUAL OF ANATOMY 



squamo-zygomatic and tympanic, and (2) a petrosal, a plate of cartilage inter- 

 vening, and these unite in the course of the first year. At birth the bone is of 

 loose consistence, the mastoid portion is flat, the external auditory meatus is un- 

 developed, the tympanic annulus and membrana tympani are on a level with 

 the exterior of the bone, the glenoid and jugular fossae are shallow, the floccular 

 fossa is conspicuous, and the hiatus Fallopii is an open groove./~The tympanic 

 plate now becomes formed in fibrous tissue, by the extension of osseous matter 

 outwards from two tubercles on the anterior and posterior parts of the outer 

 aspect of the tympanic annulus superiorly. As these tubercles grow, they 

 meet and enclose an opening in the floor of the external auditory meatus, 

 which usually becomes closed before the period of puberty, but it may persist 

 throughout life as the foramen of Huschke. The mastoid antrum is present at 

 birth, and is of large proportionate size, its outer wall being very thin. The 

 mastoid process becomes developed in the course of the second year, and the 

 antrum becomes relatively smaller, its outer wall at the same time becoming 

 thicker. The mastoid cells do not appear until the approach of the period of 

 puberty. 



Styloid Process. — This process is developed separately from the upper end 

 of the cartilage of the second visceral arch. It has two centres of ossification, 

 one for the tympano-hyal or basal part appearing before birth, which soon 

 joins the rest of the bone, and the other for the stylo-hyal appearing in 

 the second year. The latter portion does not attain marked development 

 until after puberty, and its union with the tympano-hyal usually takes place 

 in adult life, but it sometimes persists in an independent condition. 



The Sphenoid Bone. 



The sphenoid bone is so named from the wedge-like position 

 which it occupies in the base of the skull, where it lies with its 

 long axis placed transversely. It enters into the formation of the 

 anterior, middle, and posterior, fossae of the base, the temporal 

 and nasal fossae, and the orbits. It consists of a central portion 

 or body, two great wings, two small wings, and two pterygoid 

 processes. 



The body presents six surfaces — superior, inferior, anterior, 

 posterior, and two lateral, one at either side. Within the body 

 are two large cavities, called the sphenoidal air sinuses, each of 

 which opens on the anterior surface by a small circular aperture. 



The superior surface presents at its centre a depression, called 

 the sella turcica or pituitary fossa, for the pituitary body 01 hypo- 

 physis cerebri, and in the foetus .t is pierced by the superior opening 

 of the cranio-pharyngeal canal. In front of the sella turcica is 

 the olivary eminence, which indicates the place of junction of the 

 presphenoid and postsphenoid portions, and anterior to this is 

 a transverse furrow, called the optic groove, both of which support 

 the optic commissure or chiasma. The groove leads at either side 

 to the optic foramen, by which the optic nerve leaves the cranial 

 cavity, and anteriorly it is limited by a transverse ridge, called 

 the limbus sphenoidalis. In front of the limbus (border) is a smooth 

 elevated platform, called the jugum sphenoidale, which is con- 

 tinuous laterally with the superior surface of the small wing, and 

 presents at either side the olfactory groove for the olfactory bulb. 

 The anterior border of the jugum is projected in the middle line 

 into the ethmoidal spine, which articulates with the posterior 



