•10 



THE SKELETON 



The Afetamorphosis of the Brdiuliinl Bars 



These rods of cartilivao arc named, from before backwards, the mandibular, hyoid, and 

 thyroid bars. Tliey may with care be easily dissected in tlie foetus between the third and fourth 

 months. Their metaiudrphosis is as Ibllows : — 



The two extremities of the mandibular bar ossify ; tlie distal end ultimately forms 

 that portion of the mandible adjacent to the symi)hysis; the proximal end ossifies as the 

 malleus. The intermediate portion disappears; the only vesti^^e is a band of fibrous tissue, 

 tlu' spheno-mandibular ligament, extending from the spine of the sphenoid to the spine of the 

 mandible. 



The hyoid bar I'usi's distally with the thyroid bar, and is represented by tlie hyoid bone. Its 

 ]troximal end becomes the incus, tympano-hyal, and styloid process; the intervening strip is 

 rei)resented in the adult by the stylo-hyoid ligament. 



The styloid process has two centres of ossification. A nucleus appears anterior to the stylo- 



FiG. 9G.— The CHoximo-CnAXirM. 



ETHMOID CARTILAGE 



OPTIC FORAMEN 



FORAMEN ROTUNOUM 



FORAMEN OVALE 



HIATUS FALLOPII 

 INTERNAL AUDITORY MEATUS 



JUGULAR FORAMEN 



ANTERIOR CONDYLOID FORAMEN _ 



CONDYLES — *^^ 



— NASAL CARTILAGES 



ORBITO-SPHENOID 

 ALI-SPHENOID 

 PERIOTIC CAPSULE 



, ___^, Nfe t ^v lU AQUEOUCTUS VESTIBULI 



h fl, ' _^ ] }''A FLOCCULAR FOSSA 



A / n ^'/^__L_ MASTOID FORAMEN 



! » .^ OCCIPITAL CARTILAGE 



mastoid foramen, and is known as the tympano-hyal. At birth the true styloid process is carti- 

 laginous. In \\w. second year it ossifies, and subsequently becomes firmly ankylosed to the 

 tympano-hyal. Occasionally the hyoid bar may ossify throughout. When this is the case the 

 styloid jtrocess reaches to the hyoid and replaces the stylo-hyoid ligament. 



The stapes originates in a piece of cartilage which is traversed by the temi)orary stapedial 

 artery. Its centres of ossification require investigation. The internal pterygoid ])late arises in 

 a i)iece of cartilage which represents the palato-quadrate of lower vertebrates. The internal 

 pterygoid i)lat(' is an ai)pendicular element engrafted on to the sphenoid. In many mammals it 

 IS a sei)arate bone throughout life. In the same way the styloid process is an appendicular 

 element ankylosed to the petrosal and the tympanic plate ; in many mammals it remains separate 

 throughout life. 



The SkuU at Birth 



The .skull at birth presents, when compared with the adult skull, several important and 

 interesting features. Its peculiarities may be considered under three headings : — The peculi- 

 arities of the foetal skull as a whole; the condition of individual bones; the remnants of the 

 chondral skull. 



The general characters of the fcEtal skull. — The most striking features of the skull at birth 

 are, its relatively large size in comjiarison with the body, and the jiredominauce of the cerebral 

 over the facial portion of the skull ; the latter is, in fact, very small. 



The frontal and parietal eminences are large and conspicuous; the sutures are absent ; the 

 adjacent margins of the bones of the vault are se])arated by septa of fibrous tissue contiiuious 

 with the (hira mater internally and the pericranium externally; hence it is difficult to separate 

 the roof bones from the underlying dura inater, each bone being lodged as it were in a dense 

 membranous sac. The bones of the vault con.sist of a single layer without any dijiloe, and their 

 cranial surfaces present no digital impressions. Six memlmmous spaces exist, nameil fontanelles : 

 two are median, named anterior and posterior; and two exist on each side, termed anterior and 

 j)osterior lateral fontanelles. Kuch angle ol" the parietal bones is in relation with a Ibntanelle. 



