ANATOMY OF VEETEBEATES. 



27 



In the arrangement of the parts of the dermo-, 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 habits and well-being of the creatures 

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

 and relative position of dermal, tendinal, and visceral bones seems 

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

 osseous system, and might be termed the e 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. Type segment or vertebra. — Each complete segment, 

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

 according to a type or general plan, exera- 17 



plified in fig. 17 ; in which they form a 

 hoop or arch above, and another beneath, 

 a central piece. The upper hoop, encircling 

 a segment of the nervous axis, is called the 

 neural l arch, n ; the lower hoop, encircling 

 a part of the vascular system, is called the 

 haemal 2 arch, h : their common centre is 

 termed the ' centrum.' 3 The neural arch 

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

 pophyses,' 4 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' 5 d, d. The hannal arch is formed by 

 a pair of bones, called 'pleurapophyses,' 6 pi; by a second pair, 

 called ' htemapophyses,' 7 h ; and by a bone, sometimes bifid, called 

 the ' haemal spine,' lis. It also sometimes includes parts, or 

 bones, called ' parapophyses.' 8 Bones, moreover, may be developed 

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



The parts of a vertebra which are developed from independent 

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

 from previously ossified parts are called ' exogenous : ' the autoge- 

 nous parts of a vertebra are its ' elements,' the exogenous parts 

 are its ' processes.' No part, however, is absolutely autogenous 



1 Gr. neuron, nerve. 



2 Gr. haima, blood. 



8 Gr. kentron, centre. 

 * Gr. for nerve, and apophusis, a pro- 

 jecting part. 



5 Gr. dia, across, and apophusis. 



6 Gr. pleuron, rib, and apophusis. 



7 Gr. for blood, and apophusis. 



8 Gr. para, transverse, and apophusis. 



