88 OSTEOLOGY. 



is somewhat obscured, though even in their fully developed condition 

 sufficient evidence remains to demonstrate their compound nature. The 

 vertebrse so blended are termed the fixed or false vertebrae, whilst those between 

 which osseous union has not taken place are described as the movable or true 

 vertebrae. This fusion of the vertebral segments is met with at each extremity 

 of the vertebral column, more particularly the lower, where the column is modified 

 to adapt it for union with the girdle of the lower limb, and where also man's 

 degenerated caudal appendage is situated. But a partial union of the vertebral 

 segments also takes place at the upper end of the column, between the highest 

 two vertebrse, in association with the mechanism necessary to provide for the 

 movements of the head on the column. 



For descriptive purposes the vertebral column is subdivided according to the 

 regions through which it passes. Thus the vertebrae are described as cervical 

 (vertebrse cervicales), thoracic (vertebras thoracales), lumbar (vertebrse lurnbales), 

 sacral (vertebrse sacrales), and coccygeal (vertebrse caudales), according as they 

 lie in the regions of the neck, thorax, loins, pelvis, and tail. The number of 

 vertebrse met with in each region is fairly constant, though, as will be hereafter 

 pointed out, variations may occur in the number of the members of the different 

 series. The vertebrse in man are thus apportioned *7 cervical, 12 thoracic, 5 lumbar, 

 5 sacral, and 4 or 5 coccygeal ; the former three groups comprise the true or mov- 

 able vertebrse, the latter two the false or fixed vertebras. The vertebral formula 

 may be thus expressed : 



Movable or True Vertebras. Fixed or False Vertebrae. 



Cervical. Thoracic. Lumbar. Sacral. Coccygeal. 



7 12 5 5 4 =33. 



The vertebrae, though displaying great diversity of characters in the regions above 

 enumerated, yet preserve certain features in common. All possess a solid part, 

 corpus vertebrae or body (centrum); all have articular processes by which they 

 articulate with their fellows ; most have muscular processes developed in connexion 

 with them ; whilst the majority display a vertebral foramen formed by the union 

 of a bony arcus vertebrse (vertebral arch) with the body. These common characters 

 may best be studied by selecting for description an intermediate member of 

 the series. For this purpose one of the middle or lower thoracic vertebrse 

 may be chosen. 



A typical vertebra may be described as consisting of a body composed of 

 a mass of spongy bone, more or less cylindrical in form. The size and shape of 

 the body is liable to considerable variation according to the vertebra examined. 

 The superior and inferior surfaces of the body are very slightly concave dorso- 

 ventrally and from side to side, due to the thickening of the bone around its 

 margins. In the recent condition these surfaces afford attachment for the inter- 

 vertebral fibro-cartilages, which are placed like pads between the bodies of the 

 movable members of the series. The circumference of the body, formed as it is 

 of more compact bone than the interior, is usually slightly concave from above 

 downwards, though the dorsal surface becomes flat, where the body forms the 

 anterior boundary of the vertebral foramen, at which point it is usually slightly 

 concave from side to side. The vertical surfaces of the body are pierced here and 

 there by foramina for the passage of nutrient vessels, more particularly on the 

 dorsal surface, where a depression of considerable size receives the openings 

 of the canals through which some of the veins which drain the body of the bone 

 escape. Connected with the body posteriorly there is a bony vertebral arch, 

 which, by its union with the body, encloses a foramen of variable size, called the 

 vertebral foramen. When the vertebrae are placed on the top of each other these 

 foramina form, with the uniting ligaments, a continuous canal vertebral canal 

 in which the spinal medulla, with its coverings, is lodged. The vertebral arch, 

 which is formed by the union of the roots of the vertebral arches (pedicles) and 

 laminae, besides enclosing the vertebral foramen, also supports the spinous and trans- 

 verse processes, which may be regarded as a series of levers to which muscles are 



