ANATOMY OF VERTEBRATES. 27 



In the arrangement of the parts of the clermo-, splanchno-, and 

 sclero-skeletons, no common pattern is recognisable. One can 

 but discern a purpose gained by such bony plates, cases, or rods, 

 in special relation to the halDits and well-being of the creatures 

 manifesting them ; but the diversity in the number, size, shape, 

 and relative position of dermal, tendinal, and visceral l^ones seems 

 interminable. The neuro-skeleton, which is the main part of the 

 osseous system, and might be termed the ' skeleton proper,' ex- 

 emplifies not only the principle of design and adaptation, but 

 that of unity of composition. Its parts are arranged in a series 

 of segments following and articulating with each other in the 

 direction of the axis of the body. 



§ 14. Tyjte scrpuent or vertebra. — Each complete segment, 

 called ' vertebra,' consists of a series of osseous pieces arranged 

 according to a type or general plan, exem- 17 



plified in fig. 17 ; in which they form a 

 hoop or arch aljove, and another beneath, 

 a central piece. The upper hoop, encircling 

 a segment of the nervous axis, is called the 

 neural ' arch, n ; the lower hoop, encircling 

 a part of the vascular system, is called the 

 hremal ^ arch, 11 : their common centre is 

 termed the 'centrum.''^ The neural arch 

 is formed by a pair of bones, called ' neura- 

 pophyses,''' n, n, and by a bone sometimes 

 cleft or bifid, called the ' neural spine,' 

 ns : it also sometimes includes a pair of 

 bones, called 'diapophyses'" d, d. The hasmal arch is formed by 

 a pair of bones, called ' pleurapophyscs,'^ /;/; by a second pair, 

 called ' hajmapophyscs,'' A ; and by a bone, sometimes bifid, called 

 the 'hajmal spine,' hs. It also sometimes includes parts, or 

 bones, called ' parapojihyses.'^ Bones, moreover, may be developed 

 which diverge as rays from one or more of the above parts. 



The parts of a vertel^ra which are developed from independent 

 centres of ossification are called ' autogenous ; ' those that grow 

 from prc^'iously ossified parts are called ' exogenous : ' the autoge- 

 nous parts of a vertel^ra are its ' elements,' the exogenous parts 

 are its ' processes.' ISTo part, however, is absolutely autogenous 



' Gr. neuron, ncrvo. ' Gr. dia, across, and apophusis. 



' Gr. haiina, blood. ^ Gr. pleuron, rib, and apophusis. 



' Gr. kentron, centre. ' Gr. for blood, and apoplnms. 



* Gr. for norve, and apophusis, a pro- ° Gr. jmra, transverse, and apophusis. 

 jectina; part. 



