142 HUMAN ANATOMY. 



It appears from the above that in the adult the neck is a little more than one- 

 fifth of the movable part of the spine and the loins a little less than one-third. In 

 the young embryo these proportions are reversed, but by the time of birth these two 

 parts are nearly equal. 



Movements of the Head. — Those between the occiput and atlas are almost 

 wholly limited to flexion and exte7isio7i, of which the latter is much the greater. 

 This is in part due to the reception of the posterior pointed extremities of the articu- 

 lar processes of the atlas into the inner parts of the posterior condyloid fossae. The 

 anterior occipito-atlantal ligament and the odontoid ligaments are tense in extreme 

 extension. \x\ flexion the tip of the odontoid is very close to, if it does not touch, 

 the basilar process. The range of both these motions is much increased by the 

 participation of the cervical region. There may be a little lateral motioti between 

 the atlas and head, and there is some slight rotation. The great variation of the 

 shape of the articular facets makes it clear that both the nature and extent of the 

 motions must vary considerably. 



The joint between the atlas and axis is devoted almost wholly to rotation. The 

 transverse ligament keeps the odontoid in place, and the very strong odontoid liga- 

 ments check rotation alternately. The head is highest when directed straight forward, 

 but the joints are in more perfect adaptation if one condyle be a little anterior to the 

 other, and if the adas be slightly rotated on the axis. This position, though entail- 

 ing a slight loss of height, is the one naturally chosen as that of greatest stability. 



Movements of the Spine. — The very extensive range of motion of the 

 whole spine is the sum of many small movements occurring at the intervertebral 

 disks. The whole column is a flexible rod, but this conception is modified by the 

 following peculiarities : ( i ) the motion is not equally distributed, owing to the vary- 

 ing distances between the disks and the diflferences of thickness of the disks them- 

 selves ; (2) the bodies, which form the essential part of the rod, are not circular, so 

 that motion is easier in one direction than in another ; (3) the rod is not straight 

 but curved ; (4) the kind of motion is influenced by the articular processes, and 

 varies in the different regions. Other modifying circumstances exist, but these suf- 

 fice to show that, while certain general principles may be laid down, an accurate 

 analysis of the spinal movements is absolutely impossible. 



The incompressible semifluid centre of each disk has been compared to a ball 

 on which the. rest of the disk plays. This would, therefore, be a universal joint 

 were there no restraining apparatus. The motions are flexion and extension, — 

 i.e., angular movements on a transverse axis ; lateral motion, — i.e., the same on 

 an antero-posterior axis, and rotation on a vertical axis. It is unlikely that any 

 single one of these motions ever occurs without some mingling of another. Flexion 

 and extension are greatest in the neck and loins. Extension is more free than 

 flexion in the neck, where it is limited by the locking of the laminae, which, when 

 the head is thrown as far back as possible, gives great rigidity to the neck. In the 

 loins and in the region of th? last two thoracic vertebrae flexion is the more exten- 

 sive. Before the spine is consolidated, slight flexion is possible throughout the back, 

 but extension is very quickly checked by the locking of the laminae and spines. 

 Lateral motion is greatest in the neck, considerable in the back and least in the 

 loins. Such motion is always associated with rotation, which is most free in the 

 neck, considerable in the back, and very slight, at most, in the loins. It is to be 

 remembered that motions both in the antero-posterior and in the transverse plane 

 are checked by the tension of the ligaments on the side of the body of the vertebra 

 opposite to the direction of the motion, and also by the resistance to compression 

 of that side of the intervertebral disk towards which the motion occurs. The liga- 

 menta subflava, being elastic, tend continually to bring the bones back into position 

 from the innumerable slight displacements to which they are subject. That this 

 replacement is effected by a purely physical property of the tissue instead of by 

 muscular action implies a great saving of energy. The amount of all motions, and 

 of rotation in particular, decreases throughout life and varies much in individuals. 

 According to Keen, the rotary motion between the atlas and the axis amounts to 

 twenty-five degrees, that in the rest of the neck to forty-five degrees, and that of the 

 thoracic and liunbar regions to about thirty degrees on each side. 



