4 VERTEBRAL COLUMN. 



and arranged. Superiorly, it supports the skull ; laterally, it has attached 

 to it the ribs, through which it receives the weight of the upper limbs ; and 

 near its inferior extremity it rests upon the pelvic bones, which communicate 

 the weight of the body to the lower limbs. Besides being a pillar of support 

 to the rest of the skeleton, it furnishes protection to the spinal cord by 

 enclosing it in an osseous canal. It is composed of a series of bones, called 

 vertebrce, the most of which are united together by joints and elastic sub- 

 stance, and a few by bony connection, in such a manner that, although the 

 amount of motion allowed between each pair is slight, the aggregate of that 

 in the whole is sufficient to give the column very considerable flexibility. 



The twenty-four upper vertebra) remain separate in the adult, and retain 

 their mobility : they increase in size from above downwards. They are suc- 

 ceeded by five others, which rapidly diminish in size from above downwards, 

 and which are united into one mass, called the sacrum ; beyond the sacrum 

 are four dwindled terminal members of the series, which as age advances 

 become likewise united, and form the coccyx. Thus the column may be said 

 to consist of two irregular pyramids, the common base of which is at the 

 superior extremity of the sacrum. 



THE MOVEABLE VERTEBRAE. 



GENERAL CHARACTERS. 



The general characters of the vertebrae are best exhibited in those which 

 are placed near the middle of the column. Those at the extremities present 

 a greater number of distinctive peculiarities. The following description is 

 applicable to the great majority of naoveable vertebrae. 



Each vertebra has more or less the form of a ring, and presents for con- 

 sideration a body, arch, and processes. 



The body, the large mass in the anterior part of the vertebra, is a short 

 column, which, when united by elastic intervertebral plates with the others 

 of the series, contributes to form a firm but flexible pillar of support. 

 Anteriorly, it is convex forwards, and slightly hollowed from above down- 

 wards. Posteriorly, it forms part of the ring, and is slightly concave from 

 side to side. Its superior and inferior surfaces are nearly flat, excepting in 

 the cervical region, and give attachment to the intervertebral platea. The 

 vertical surfaces are pierced by numerous foramina for blood-vessels, princi- 

 pally veins : one or more of these, situated near the middle of the posterior 

 surface, exceeds the other greatly in size. 



The arch consists of two symmetrical halves which spring from the body, 

 towards its back part, and meet in the middle line behind. The anterior 

 part of each lateral half, rounded and narrow, is called the pedicle ; the 

 posterior part is broad and flat, and is called the lamina or plate. 



The spinous process or spine projects backwards from the arch in the 

 middle line. The appearance presented by the linear series of spinous pro- 

 cesses has led to the application of the name spine to the whole column. 

 The transverse processes, placed one at each side, project outwards from the 

 arch. The articulating processes, two superior and two inferior, project 

 upwards and downwards from the laminae. They are furnished with articular 

 surfaces, coated with cartilage, which in the superior processes look back- 

 wards, and in the inferior look forwards, so that the former face the latter 

 in adjoining vertebrae. 



The concavities on the upper and lower borders of the pedicles are named 

 notches, and constitute by the apposition of those of contiguous vertebrae the 



