276 OSTEOLOGY. 



due to differences in the number of the coccygeal vertebrae ; these may vary from four 

 which may be regarded as the normal number to six. The number of presacral or movable 

 vertebrae is normally 24 (7 C, 12 Th, and 5 L) ; in which case the 25th vertebra forms the 

 first sacral segment (vertebra fulcralis of Welcker). The number of presacral vertebrae may 

 be increased by the intercalation of a segment either in the thoracic or lumbar region without 

 any alteration in the number of the sacral or coccygeal elements : thus we may have 7 C, 13 Th, 

 and 5 L, or 7 C, 12 Th, and 6 L, or it may be reduced by the disappearance of a vertebral segment 

 thus, 7 C, 12 Th, and 4 L. Such an arrangement presupposes developmental errors either 

 of excess or default in the segmentation of the column. On the other hand, the total number of 

 vertebral segments remaining the same (24 or 25), we may have variations in the number of those 

 assigned to different regions due to the addition of a vertebral segment to one, and its consequent 

 subtraction from another region. Thus, in the 24 presacral vertebrae, in cases of the occurrence of 

 cervical ribs the formula is rearranged thus 6 C, 13 Th, and 5 L, or, in the case of a lumbar 

 rib being present, the formula would be 7 C, 13 Th, 4 L, as happens normally in the gorilla and 

 chimpanzee. Similarly, the number of the presacral vertebrae (24) may be increased by the 

 withdrawal of a segment from the sacral region 7 C, 12 Th, 6 L, and 4 S or diminished by an 

 increase in the number of the sacral vertebrae, as in the formula 7 C, 12 Th, 4 L, and 6 S. .In- 

 crease in the number of sacral segments may be due to fusion with a lumbar vertebra, or by the 

 addition of a coccygeal element : the latter is more frequently the case. This variability in the 

 constitution of the sacrum is necessarily correlated with a shifting tailwards and headwards of 

 the pelvic girdle along the vertebral column. Rosenburg considers that the 26th, 27th, and 28th 

 vertebrae are the primitive sacral segments, and that the sacral characters of the 25th vertebrae 

 (the first sacral segment in the normal adult column) are only secondarily acquired. He thus 

 supposes that during development there is a Ijeadward shifting of the sacrum and pelvic girdle, 

 with a consequent reduction in the length of the presacral portion of the column. This view is 

 opposed by Paterson (Roy. Dublin Soc. Scientific Trans, vol. v. Ser. II.), who found that ossification 

 took place in the alae of the 25th vertebra (first adult sacral segment) before it made its appear- 

 ance in the alae of the 26th vertebra. He thus assumes that the alae of the 25th vertebra may 

 be regarded as the main and primary attachment with the ilium. His conclusions, based on a 

 large number of observations, are at variance with Rosenburg's views, for, according to his opinion, 

 liberation of the first sacral segment is more common than assimilation with the fifth lumbar 

 vertebra, and assimilation of the first coccygeal vertebra with the sacrum is more common than 

 liberation of the fifth sacral, thus leading to the inference that the sacrum tends to shift tail- 

 wards more often than headwards. Dwight (Anat. Anz. Jena, vol. xxviii. p. 33), after a study of 

 this question, whilst admitting that some of these variations may be reversive, denies that there 

 is any evidence that they are progressive, and further states that after the occurrence of the 

 original error in development, there is a tendency for the vertebral column to assume as nearly 

 as possible its normal disposition and proportions. 



Sternum. The sternum is liable to considerable individual variations affecting its length 

 and direction. The majority of bones are asymmetrical, displaying irregularities in the levels of 

 the clavicular facets. The higher costal facets may be closer together on one, usually the right 

 side, than the other, whilst the synchondrosis sternalis is often oblique, sloping somewhat to the 

 right. According to Birmingham, these are the result of the strain thrown on the shoulder by 

 pressure either directly applied or through the pull of a weight carried in the hand. 



Sometimes the sternum articulates with eight rib cartilages. This may happen on one or 

 both sides, but when unilateral, much more frequently on the right side a condition by some 

 associated with right-handedness. It is, however, more probably a persistence of the primitive 

 condition of the cartilaginous sternum, in which each half is connected with the anterior 

 extremities of the first eight costal arches. In some rare cases only six pairs of ribs articulate 

 by means of their costal cartilages with the sternum. Recently Lickley has brought forward 

 evidence to show that the seventh rib is undergoing regressive changes. (Anat. Anz. vol. xxiv. 

 p. 326.) 



Occasionally the presternum supports the first three ribs ; in other words, the manubrium 

 has absorbed the highest segment of the body. Keith has pointed out that this is the condition 

 most commonly met with in the gibbon, and regards its occurrence in man as a reversion to the 

 simian type. As far as is at present known, its occurrence seems more common in the lower 

 races. Through errors of development the sternum may be fissured throughout, due to failure 

 of fusion of the cartilaginous hemisterna. The two ossified halves are usually widely separated 

 above, but united together below by an arthrodial joint. The heart and pericardium are thus 

 uncovered by the bone. Occasionally this condition is associated with ectopia cordis, under 

 which circumstances life is rendered impossible. Through defects in ossification the body of the 

 sternum may be pierced by a hole, usually in its lower part, or through failure of fusion of the 

 collateral centres one or more of the segments of the body may be divided longitudinally. 



Sometimes small ossicles are found in the ligaments of the sterno-clavicular articulation. 

 These are the so-called episternal bones, 'the morphological significance of which, however, has 

 not yet been satisfactorily determined. They are by some regarded as the homologues of the 

 interclavicle or episternal bone of monotremata, whilst by others they are considered to represent 

 persistent and detached portions of the pre-coracoids. 



Ribs. The number of ribs may be increased or diminished. Increase may occur by the addition 

 of a cervical rib due to the independent development of the costal element in the transverse 

 process of the seventh cervical vertebra. This may happen on one or both sides. The range of 

 development of these cervical ribs varies ; they may unite in front with the sternum, or they may 



