i8 



VERTEBRATE SKELETON 



Elasmobranchs and Chondrostei excepted) is transformed into or 

 covered by bone long before the adult stage is reached. The part 

 of the skull surrounding the ahmentary tract — the visceral skeleton 

 — is also laid down in cartilage. Both chondrocranium and visceral 

 skeleton are reinforced by numerous membrane bones, most of which 

 are shown by comparative anatomy to be derived directly from the 

 skin and are comparable to the dermal skeleton just considered. 



Vertebral Column 



In its fullest development the vertebral column (spina dorsalis 

 or 'backbone') consists of a series of metameric bodies, the verte- 

 brae, which alternate with the muscle segments and are usually some- 

 what moveably articulated with each other, so that the column is 

 flexible. Each vertebra surrounds the notochord and spinal cord, 

 and in the tail, the caudal blood vessels as well. 

 In the lower Vertebrates each vertebra consists 

 of Httle more than its portion of the chorda and 

 the arches surrounding the spinal cord and blood 

 vessels. In the higher groups these parts may 

 fuse, the notochord may be lost and the whole 

 may ossify in the adult. For convenience of 

 reference a typical vertebra is described first. 



Each typical vertebra (fig. 15) consists of a 

 body or centrum, developed around the notochord, 

 and bearing two arches, a neural arch above, 

 surrounding the spinal cord; a similar haemal arch 

 on the ventral side, around the ventral blood vessels. 

 Each arch consists of a pair of plates (neurapo- 

 physes above, haemapophyses below), each arch 

 P».ryg"p„?hy:4f;:: teing completed by a keystone, the neural or 

 prezygapophysis; sc, haemal Spine (spinous processes). In the trunk 



spinal cord; v, vein. , , , , . t,- i i ^ 



the ha'mal arch is modified or absent. 

 Other parts may be connected with these. There are usually 

 pre- and post-zygapophyses (articular processes of human anatomy) 

 on the anterior and posterior faces of the neurapophyses, which 

 strengthen the articulation of the vertebrae. (There may be addi- 

 tional zygapophyses on the ha-mal arches of fishes.) The sides of 

 the vertebra may have 'transverse processes' for the attachment of 

 muscles and the articulation of ribs. Of these transverse processes 



Fig. 15. — Sche- 

 matic caudal verte- 

 brae, a, artery; c, 

 centrum; hp, haemapo- 

 physis; hs, haemal 

 spine; I, ligaments; 

 np, neurapophysis; 



