THE VERTEBRAL COLUMN. 21 



proceeds from a single centre of ossification. The union of the two centres 

 in the annular portion, usually designated the vertebral lamince, is slower. 

 It commences in the most anterior vertebras, and is latest in the sacral and 

 coccygeal regions. To the two principal pieces of the vertebra in process of 

 ossification, is added, at a subsequent period, complementary points of ossifi- 

 cation, five or six in number : one or two for the spinous process, one for the 

 summit of each transverse process, another for the head, and the last for the 

 posterior cavity of the body. 



CHARACTERS PROPER TO THE VERTEBRJS 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 recognised 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 depression 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 rudimentary laminae and processes. There is no 

 necessity for noticing the sacrum, whose five pieces form one bone : a feature 

 which markedly distinguishes it from the other regions of the vertebral 

 column. But these few distinctive characteristics do not satisfy the require- 

 ments of descriptive anatomy ; so that it is necessary to undertake a more 

 extensive study of each of these regions. 



1. Cervical Vertebra. 



GENERAL CHARACTERS. These vertebrae, the longest and thickest in the 

 spine, present generally a cubic form. They are usually distinguished from 

 the vertebrae of the other regions by the following characters : The 

 inferior spine of the body is strongly marked, especially behind, where it 

 terminates in a small tubercle. The head is well detached from the re- 

 mainder of the bone, and describes a very short curve. The posterior cavity, 

 wide and deep, represents a veritable cotyloid depression, which is too 

 large to fit the head exactly ; the intermediate fibro- cartilage on these two 

 surfaces is also of a great thickness. The spinous process forms a simple 

 roughened, and but slightly prominent, ridge. The transverse processes, very 

 developed, are elongated in an antero-posterior direction, and inclined down- 

 wards. In this region they are designated the trachelian processes, because 

 of their relations with the trachea; a foramen that traverses them from 

 before to behind at their base has been, for the same reason, named the 

 trachelian foramen (vertebral foramen). The articular processes, large and 

 prominent, are inclined downwards and inwards. The notches are wide and 

 deep. 



SPECIFIC CHARACTERS. The seven cervical vertebras are reckoned from 

 before to behind, and receive numerical names indicating their place in the 

 region. 



First. The first vertebra of the neck, which has been named the atlas, 1 

 deserves a very careful description. At first sight there is recognised the 



1 So named from the mythological personage who was supposed to support the earth, 

 as the first vertebra (human) supports the head. (For this bone in the domesticated 

 animals the name is not appropriate.) 

 5 



