Preliminary 3 



All the names just given appear to be used more or less indiscriminately, but in a 

 very general way the order in which they are placed above corresponds with the 

 diminishing size of the different structural characters to which they refer. 



A projecting articular process on a bone is frequently referred to as the head, its 

 narrowed attachment to the rest of the bone as the neck, and the remainder constitutes 

 the body, or, in a long bone, the shaft. A condyle is a protruding mass carrying an 

 articular surface, and a ramus is a broad arm or process of bone projecting from the 

 main bodv. 



All the terms given above, with numerous others of more special usage, will be 

 better understood both in their meaning and application by the student after a short 

 time spent in the study of the individual bones, and no more need be said about them 

 now. 



Bones do not take shape as such ab initio, but are preformed in the embryo as 

 condensations of mesenchyme, which in some cases become cartilaginous before 

 ossification commences in them, but in others remain unchondrified : in the former 

 case the bones are said to be formed in cartilage, and in the latter they are formed in 

 membrane. These terms simply mean that the bones have replaced cartilage or 

 non-cartilaginous " membrane " as the case may be. The process of ossification is 

 essentially similar in both varieties of formation, save that in chondral ossification 

 the cartilage is calcified first and then absorbed and replaced by the true bony formation. 

 Certain large cells, called osleoblasts, have the power of depositing or forming bone 

 round themselves : they exist in the covering tissues of the developing bone (periosteum 

 or perichondrium), and, in the case of chondral ossification, grow into the cartilage 

 and occupy the spaces made in that structure by the confluence of the cell-spaces 

 that goes with calcification. The early bone thus formed is removed by the action 

 of other cells known as osteoclasts, and in this way a medullary cavity is provided in 

 long bones while additional bone is being laid down on the surface under the perios- 

 teum, so that the bone increases in thickness. Thus there is no direct ossification 

 of cartilage, but a replacement of it by bone that is made in the same way as in 

 membrane bones. 



The greater part of the skeleton is composed of cartilage bones, including that of the limbs, 

 trunk, and base of skull ; the bones of the face and vault of the skull are formed in membrane. 

 The chondral skeleton is the modified endoskeleton, and the bones formed in membrane are usually 

 considered to represent dermal bones, structure formed originally as part of the exoskeleton, which 

 in the course of evolution have sunk to a deeper position and joined the other skeletal structures- 



The invasion of cartilage by osteoblasts leads to an ossification commencing in 

 the cartilage and termed endochondral, to distinguish it from that taking place under 

 the periosteum and known as ectochondral : it is evident that ectochondral ossifica- 

 tion, as indeed is the endochondral process really, is simply ossification in membrane 

 occurring on cartilage. 



The ossification commences constantly at one spot and at a fairly constant time 

 in each individual bone, making a centre of ossification ; the process extends from 

 this centre. But a bone may have more than one centre in fact, most bones have 

 more than one and some have several. Such centres can be divided into primary 

 and secondary, and as a rule the primary centres appear before birth and the secondary 

 ones after that event. The smaller secondary centres form what are termed 

 cpiphyses: an epiphysis is therefore a part of a bone which is developed from a 

 secondary centre, is at first separated from the main bone by a connecting area c 

 unossified cartilage, and joins the main ossification at a later date to make the adult 



I 2 



