26 THE BONES. 



Annular Portion (neural arch or neurop&physes). — This is formed by an 

 osseous plate that curves suddenly downwards, in the shape of an arch, the two 

 extremities of which approach each other, enclose the body, and become united 

 to it. It offers for study: 1. An internal and an external surface. 2. An 

 anterior and a posterior border. 



Surfaces. — The internal surface, concave and smooth, forms, with the superior 

 face of the body, the spinal foramen. The external, convex and irregular, pre- 

 sents : 1 . A single prominence, raised in the middle of the superior portion, and 

 named the spinous process {neural spine). 2. The transverse jrrocesses (diapo- 

 physes) are a double pair of eminences, one on each side, and projected trans- 

 versely outwards. 



Borders. — The anterior border has two articular facets looking upwards : 

 these are the anterior articular processes {prezygopophyses'), right and left. In 

 each is a notch which, when placed in opposition to a similar excavation in the 

 preceding vertebra, forms the intervertebral foramen. The posterior border pre- 

 sents the same peculiarities, with this difference, that the articular faces of the 

 posterior articular processes {postzyyopophyses) are inclined downwards, to corre- 

 spond with the anterior facets of the succeeding vertebra.^ 



Structure of the vertebr(e,. — The compact substance, which is abundant in the 

 spinous portion, forms in the body an extremely thin layer, inclosing a volumi- 

 nous nucleus of spongy tissue. The latter is traversed by numerous venous 

 canals, which open on the surface of the bone. 



Development. — It has been already shown that the body and spinous 

 portion of a vertebra constitute, in young animals, two distinct pieces. Each 

 was primarily formed from two lateral centres, which met on the median line. 

 In the body, the fusion of these centres is so prompt, that it is generally believed, 

 perhaps justly, that the development of this part of the vertebra proceeds from 

 a single centre of ossification. The union of the two centres in the annular 

 portion, usually designated the vertebral lamince {p)arapophyses), is slower. It 

 commences in the most anterior vertebrfe, and is latest in the sacral and coccy- 

 geal regions. To the three principal pieces of the vertebra in process of 

 ossification, are added, at a subsequent period, complementary centres of ossifi- 

 cation, variable in number according to the regions and species of animal ; 

 there is always one for each of the anterior and posterior surfaces of the 

 vertebral bodies ; while others, much less constant, concur to form the spinous 

 and transverse processes. 



Characters proper to the Vertebrae of each Region. 



A casual inspection of a vertebra might suffice, strictly speaking, to dis- 

 tinguish the region of the spine to which it belonged. For instance, a cervical 

 vertebra is recognized by its volume, the absence of a spinous process, and the 

 foramen which traverses the base of its transverse processes. The dorsal 

 vertebra is conspicuous by its tubercular transverse processes, and by being 

 furnished, outwardly, with an articular surface, as well as by the depressions on 

 its body destined to receive the heads of the ribs. The lumbar vertebra has its 

 long flattened transverse processes ; while the coccygeal vertebra offers rudi- 



(' Vertebrae which have centra concave at both ends, are designated amphiraslous. Those 

 distinguished by a concavity in front and a convexity behind, are known as procxlous ; but if 

 the cavity is behind and tlie convexity before, they are then named opisthocxlous. A vertebra 

 of the above description belongs therefore, tu the opidhoceelous class.) 



