COCCYX VERTEBRAL COLUMN AS A WHOLE. 13 



outwards, usually forming with the sacrum the notch for the fifth sacral 

 nerve, and in some instances uniting with the sacrum so 

 as to form a fifth sacral foramen. 



Fig. 10. FOUR COCCTGEAL VERTEBRA, SEEN FRO^T BEFORE. FROM A 

 MALE SUBJECT OF MIDDLE AGE. ^ 



The upper piece is separate from the second ; the three lower are 

 united together in one piece, and separated only by grooves. 1 is 

 placed above the middle or body of the first coccygeal vertebra ; 1' is 

 below the fourth piece ; 6 indicates the transverse portion ; 7, the 

 superior articulating tubercle. J T 



The remaining three coccygeal vertebrae are much smaller than the first, 

 and correspond solely to vertebral bodies. When separate, the second piece 

 presents an upper and lower flattened surface. The third and fourth pieces 

 are mere rounded nodules. In middle life, the first piece is usually sepa- 

 rate, while the three lower pieces are most frequently united into one, the 

 original separation being indicated only by transverse grooves. 



In advanced life, the coccygaal vertebrae, having been previously joined 

 into one bone, become also united to the sacrum. This union occurs at 

 an earlier age and more frequently in the male than in the female, but 

 it is subject to much variation. 



THE VERTEBRAL COLUMN AS A WHOLE. 



The average, length of the vertebral column is about twenty-eight inches. 

 Its length varies to a considerable extent in different persons, but not so 

 much as might be anticipated from a comparison of their stature ; the rela- 

 tive height of individuals depending more frequently on a difference in the 

 length of their lower limbs than of the vertebral column. 



CURVES. The vertebral column presents four curves, directed back- 

 wards and forwards. In the neck and loins the convexity is forwards ; 

 in the back and pelvis it is in the opposite direction. The lumbar 

 convexity is much greater than the cervical ; and the sacral concavity 

 is greater than the thoracic. In the dorsal region, there is also very 

 frequently a slight degree of lateral curvature, the convexity of which in 

 the great majority of cases is directed towards the right side. 



In connection with the thoracic concavity of the column, the bodies of the dorsal 

 vertebrae are somewhat thinner in front than behind. The pelvic concavity is chiefly 

 dependent on the diminished vertical diameter of the bodies of the sacrum in front, 

 and the bending forward of the coccyx. The cervical and lumbar convexities are 

 attended with a slightly greater thickness of the anterior than of the posterior parts 

 of the intervertebral discs, and they are in part maintained by the elastic tension of 

 the ligamenta subflava acting upon the most flexible portions of the column. (W. & 

 E. Weber. " Mechanik der Mensch. Gewerkzeuge," p. 91 ; and Henle, " Handbuch 

 der Syst. Anat. des Menschen," vol. i., p. 32.) These curves are connected with the 

 maintenance of the erect posture. They are absent in infants and increase towards 

 adult age. They confer upon the column the advantages of a spring, giving it 

 greater strength and elasticity, and at the same time assist in preserving the 

 equilibrium of the body. 



The lateral curvature in the dorsal region has been supposed by some anatomists 

 to be connected with the position of the aorta on the left side of the column. 

 Cruveilhier mentions, in support of this opinion, three cases of transposition of the 

 aorta, in which the convexity of the lateral curve was directed to the left side. 

 (Cruveilhier, "Traite d'Anatomie," 4th edit. p. 65.) By a majority of writers this 

 curve is imputed to the greater muscular action on the right side than the left, 

 occasioned by the preference usually given to the right arm. This explanation 

 originated with Bichat. Otto describes a case of right aorta, in which the curve of 



