2 THE SKELKTOX 



other hand, tlu-rf an- nunifrous instances in the skull, of IxHies the ossilieatinn of 

 whicli begins in, and is j)erfeeted 1)V, the internienil)ranons method. OssiHeation in 

 a few instances conniiences in membrane, but later invades tracts of cartilage; 

 occasionally the process l)egins in perichondrium and remains restricted to it, never 

 invading the underlying cartilage, which gradually disappears as the result of 

 continui'd jiressure exerted upon it by the growing bone. The vomer and nasal 

 bones are the best examples of this mode of develo})ment. 



A Classifikd List of the Bones to snow their INIode of Development 



1. All the limb-bones and those of the vertebral colunm are pre-formed in car- 

 tilage and perfected in membrane, with the exception of the clavicles. These begin 

 in membrane, proceed in cartilage, and are finally perfected m membrane. 



•2. The Skull. 



Membrane-Bones 



Parietals. Interparietal portion of occii)ital. 



Frontal. "Wormian l)ones and the epipterics. 



Scjuaniosals. Tympanies. 



Maxilke. Mandible (except part near the 



Malars. symphysis). 



Palates. Lachrymals. 



Nasals. Vomer. 



Cartilage Bones 



Sphenoid. Occipital (except inter})arietal i)()rtion). 



Petrosals. Ethmoid. 



Mallei. Incudes. 



Stapes. Styloid processes. 



Hyoid. Symphysial portion of the mandible. 



Inferior turbinals. Internal pterygoid plate. 



Many of the skull bones are composite, that is, they consist of two or more ele- 

 ments Avhich remain separate in other vertebrates. To this group belong: — 



The Occipital. Maxill*. Frontal. 



Temporals. Hyoid. INlaiars. 



Ethmoid. Sphenoid. j\ran(li])k'. 



The dt'tails of the development and ossification of each bone are added to the 

 description. 



The limb-bones differ in several important particulars from those of the skull. 

 Some of the long l)ones have many centres of ossification, but the centres are of 

 very different morphological value from those of the skull. Speaking generally, it 

 is only the ]irimary nuclei that have any especial value for the morphologist. The 

 primary nuc-leus of a long bone appears l)efore birth. In only three instances does 

 a secondary centre appear before birth; e.g. the condyles of the femur, the head of 

 the tibia, and occasionally in the head of the humerus. Many primary ossitic 

 nuclei api)ear after l)irth; for example, those for the cari)al bones, the cuneiform 

 and scaphoid (navicular) bones <^f the foot, the coracoid process of the sca]»ula, 

 and the third, fourth and fifth ])ieces of the sternum. 



When a bone ossifies from one nucleus only, this nucleus may a]t])ear before 

 or after birth. Examples: the astragalus at the seventh month of embryonic life, 

 and the trapezoid at the eighth year. When a bone possesses one or more secondary 

 centres, the primary nucleus, as a rule, appears early. Examples: the femur, 

 humerus, phalanges, and the calcaneum. 



Secondary centres which remain for a time distinct from the main portion of a 

 bone are termed epiphyses. An e))i])hysis may arise from a single nucleus, as is 

 the case with the lower end of the femur; or fn^m several, as at the u])i)er i-nd of 

 the humerus. Promincnees al)out the ends oi long l)ones may be cap])ed l)y sepa- 



