8 VERTEBRATE SKELETON 



Ossification of both kinds of bone proceeds from one or more 

 centres in a single bone. In many cases each centre indicates a 

 separate ancestral bone, and where there are two or more centres a 

 bone complex is indicated. But this is not always the case, although 

 there is no rule definitely deciding the matter; comparisons with 

 other forms are often necessary to settle a question. 



Bone complexes are common. These may be unions of two or more mem- 

 brane bones, or of cartilage bones, or, less common, of cartilage and membrane 

 bones into a single bone in the adult. There are also supernumerary bones, of 

 little morphological significance, in various places. Such are the Wormian 

 (sutural) bones of the human skull, the patella and pisiform in the Hmbs, etc. 



The distinction between membrane and cartilage bones must be employed 

 with great care in tracing homologies. It is maintained with plausibiUty that 

 all ossifications had their beginnings with membrane bones and that these have 

 come from dermal scales. It is common in Teleosts to find bones of the two 

 types intimately associated, true cartilage bone being formed within the peri- 

 chondrium of a cartilage while a membrane bone develops on the outer surface 

 of the same membrane, the two bones fusing later to a single element. It is not 

 possible to determine by structure alone whether a bone be cartilaginous or 

 membranous in origin; this can only be settled by following the development. 



The question which is older, membrane bone or cartilage, has not been settled. 

 In the most primitive Vertebrates (Cyclostomes) cartilage occurs, but nothing 

 resembhng bone. In the development of the higher vertebrates cartilage always 

 appears before bone. On the other hand the oldest fossil fishes known have an 

 external skeleton of bony plates, and in Ostracoderms no internal skeleton has 

 been found. In higher Vertebrates membrane bone appears in ontogeny long 

 before there is any ossification of cartilage. 



The nomenclature of bones is based upon human anatomy, and so far as 

 possible the names of the parts in man are transferred to those of lower Verte- 

 brates. Sometimes this is easy, but again, more or less difficulty is encountered. 

 Some of the bones of the adult human skull are represented by several perfectly 

 distinct bones in the lower groups. Then there are cases where a bone present in 

 a lower form has been completely lost, no representative of it being found in man 

 or any mammal. So far as possible the names of the bones of the lower 

 groups are those of man or are based upon actual or supposed homologies, 

 but not all questions have been settled as to homologies, as will be apparent in 

 the following pages. 



The bones (and cartilages) of the skeleton are connected with 

 each other in different ways, some permitting more or less motion 

 while others result in fixed joints. The connecting substance may be 

 cartilage or hgament, and where there is great mobility a true joint 

 (diarthrosis) is formed. In the development of diarthroses there is 



