132 HUMAN ANATOMY. 



may, by an excessive growth of these elements, become sacraHzed, articulating more or less per- 

 fectly with the ilium, and, conversely, the tirst sacral may have almost freed itself from those 

 below it. Thus we may hnd a partially sacralized vertebra, which may be either the twenty-fifth 

 or the twenty-fourth. It often happens, particularly in the latter case, that a vertebra appears to 

 be a first sacral on superficial examination, which is found to have little or nothing to do in form- 

 ing the articular surface, in which case it is not a true sacral, for the first sacral is the fulcralis 

 which has the largest surface for the joint with the ilium. A false promontory may coexist with 

 the normal one. This is probably most frequent when the twenty-fourth vertebra is partly 

 sacralized. Any of the preceding peculiarities may be unilateral, so that sometimes a vertebra 

 may seem from one side to belong surely to one region, and equally surely to the other region 

 when seen from the opposite side. 



There is, however, another set of variations in which the number of presacral vertebrae is 

 increased or diminished. There may be, for instance, one thoracic or one lumbar vertebra too 

 many or too few,' without any compensatory change in the next region. In these cases, more- 

 over, the terminal vertebrae of the region may be very nearly typical ones, and sometimes even 

 the size of the vertebrae will be modified so as to give the region its approximate relative length. 

 Similar changes may be found in the neck, but they are exceedingly rare. 



Variations of either kind are likely to have an effect on the column as a whole ; thus, il 

 there be a large cervical rib the last thoracic rib is likely to be small, or if the first rib is rudi- 

 mentary the last is apt to be large. It follows that the thorax seems to be in certain cases 

 moved upward or downward ; this change may occur on one side only. 



Rosenberg's theory, formerly much in vogue, is that there are opposite tendencies at the 

 two ends of the spine. At the upper there is a tendency for the cervical region to encroach on 

 the thoracic, and at the lower for each of the regions to encroach on the one above it. Such 

 changes he considers progressive. On the other hand, the opposite movement by which the 

 thorax encroaches on the neck or loins is considered reversive. Rosenberg has described a 

 spine which he considers archaic, in which there are two extra presacral vertebrae and fifteen 

 pairs of ribs, the first being cervical. There are two spines in the Warren Museum with a simi- 

 lar number of presacrals in which the last is sacralized on one side. As to the way in which 

 anomalies of the lower part of the spine come about, Rosenberg ^ thinks he has shown that in 

 the course of development the sacrum is composed of vertebras placed farther back than the 

 permanent ones, and that the ilium enters into connection with vertebrae more and more ante- 

 rior. As new ones join it above former ones become detached from it below. If it does not 

 make the usual progress the spine is archaic, having too many presacrals ; if it goes too far the 

 spine is of the future. Rosenberg's theory has been overthrown by Bardeen,' who has shown 

 that the original position of the ilium is opposite the superior part of the lumbar region and 

 that it travels tailwards. Having joined a vertebra at the fifth week, it never leaves it. At this 

 early time the thoracic vertebrae are differentiated. The author-^ and Fischel* believe that 

 numerical variation is the result of an error in segmentation. 



A want of development of the bodies, which may be only half the normal height, is found 

 almost exclusively in the lumbar region. We have seen (apparently congenital) fusion of the 

 lumbar bodies while all the arches were present, but three of them crowded together. The 

 separation of the pedicles of the fifth lumbar from the body is a very rare anomaly among 

 whites, but not among American aborigines. 



ARTICULATIONS OF THE VERTEBRAL COLUMN. 

 The ligaments connecting the segments of the spine may be divided, according 

 to the parts of the vertebree which they unite, into two groups : 



1. Those connecting the Bodies of the Vertebrae ; 



2. Those connecting the Laminae and the Processes. 



LIGAMENTS CONNECTING THE BODIES. 

 Intervertebral Disks ^ (Figs. 162, 163). — These form a series of fibro-carti- 

 lages interposed between the bodies of the vertebrae, forming about one-fourth of the 

 movable part of the spine and adding greatly to its strength. They are developed, 

 like the bodies, around the notochord, persisting parts of this structure forming a 

 central core to each disk. The outer part of the disks consists of oblique layers of 

 fibres, slanting alternately in opposite directions, some almost horizontal, which hold 

 the vertebral bodies firmly together ; the centre of the disks is occupied by a space 

 containing fiuid in the meshes of a yellowish pulp.** This central core is strongly 

 compressed, so as practically to be a resistant ball within the more yielding fibro- 

 cartilaginous socket. The proportion of the disks to the vertebral bodies varies in 

 the different parts of the spine. They are absolutely largest in the lumbar region, 

 but relatively in the cervical. For many reasons it is difficult to reckon the per- 



^Morph. Jahrbuch, Bd. i. and xxvii. *Anatomische Hefte, No. 95, 15(06. 



' Anat. Anzeiger, Bd. xxv., 1904, and American Journal of Anatomy, vol. iv., 1905. 



^Dwight : Memoirs Boston Society of Nat. Hist, vol. v., 1901. 



^ Fibrocartilagines intervertebrales. * Nucleus pulposus. 



