114 OSTEOLOGY 



and eighth months ossification of the vertebral arches takes place; and about the same time the 

 costal centers for the lateral parts make their appearance. The junctions of the vertebral 

 arches with the bodies take place in the lower vertebrae as early as the second year, but are not 

 effected in the uppermost until the fifth or sixth year. About the sixteenth year the epiphysial 

 plates for the upper and under surfaces of the bodies are formed; and between the eighteenth and 

 twentieth years, those for the lateral surfaces make their appearance. The bodies of the sacral 

 vertebrae are, during early life, separated from each other by intervertebral fibrocartilages, but 

 about the eighteenth year the two lowest segments become united by bone, and the process of 

 bony union gradually extends upward, with the result that between the twenty-fifth and thirtieth 

 years of life all the segments are united. On examining a sagittal section of the sacrum, the situa- 

 tions of the intervertebral fibrocartilages are indicated by a series of oval cavities (Fig. 99). 



Coccyx. The coccyx is ossified from four centers, one for each segment. The ossific nuclei 

 make their appearance in the following order: in the first segment between the first and fourth 

 years; in the second between the fifth and tenth years; in the third between the tenth and fifteenth 

 years; in the fourth between the fourteenth and twentieth years. As age advances, the segments 

 unite with om another, the union between the first and second segments being frequently delayed 

 until after the age of twenty-five or thirty. At a late period of life, especially in females, the coccyx 

 often fuses with the sacrum. 



THE VERTEBRAL COLUMN AS A WHOLE. 



The vertebral column is situated in the median line, as the posterior part of the 

 trunk; its average length in the male is about 71 cm. Of this length the cervical 

 part measures 12.5 cm., the thoracic about 28 cm., the lumbar 18 cm., and the 

 sacrum and coccyx 12.5 cm. The female column is about 61 cm. in length. 



Curves. Viewed laterally (Fig. Ill), the vertebral column presents several 

 curves, which correspond to the different regions of the column, and are called 

 cervical, thoracic, lumbar, and pelvic. The cervical curve, convex forward, begins 

 at the apex of the odontoid process, and ends at the middle of the second thoracic 

 vertebra; it is the least marked of all the curves. The thoracic curve, concave for- 

 ward, begins at the middle of the second and ends at the middle of the twelfth 

 thoracic vertebra. Its most prominent point behind corresponds to the spinous 

 process of the seventh thoracic vertebra. The lumbar curve is more marked in 

 the female than in the male; it begins at the middle of the last thoracic vertebra, 

 and ends at the sacrovertebral angle. It is convex anteriorly, the convexity of 

 the lower three vertebrae being much greater than that of the upper two. The 

 pelvic curve begins at the sacrovertebral articulation, and ends at the point of the 

 coccyx; its concavity is directed downward and forward. The thoracic and pelvic 

 curves are termed primary curves, because they alone are present during fetal life. 

 The cervical and lumbar curves are compensatory or secondary, and are developed 

 after birth, the former when the child is able to hold up its head (at three or four 

 months), and to sit upright (at nine months), the latter at twelve or eighteen 

 months, when the child begins to walk. 



The vertebral column has also a slight lateral curvature, the convexity of which 

 is directed toward the right side. This may be produced by muscular action, 

 most persons using the right arm in preference to the left, especially in making 

 long-continued efforts, when the body is curved to the right side. In support of 

 this explanation it has been found that in one or two individuals who were left- 

 handed, the convexity was to the left side. By others this curvature is regarded as 

 being produced by the aortic arch and upper part of the descending thoracic 

 aorta- a view which is supported by the fact that in cases where the viscera are 

 transposed and the aorta is on the right side, the convexity of the curve is 

 directed to the left side. 



Surfaces. Anterior Surface. When viewed from in front, the width of the bodies 

 of the vertebrae is seen to increase from the second cervical to the first thoracic; 

 there is then a slight diminution in the next three vertebrae; below this there 

 is again a gradual and progressive increase in width as low as the sacrovertebral 

 angle. From this point there is a rapid diminution, to the apex of the coccyx. 





