OF THE ATLAS WITH THE AXIS. 227 



ceases below. In the lumbar region there is free flexion, extension, and lateral 

 movement, but no rotation. 



As Sir George Humphry has pointed out, the movements permitted are mainly 

 due to the shape and position of the articulating processes. In the loins the 

 inferior articulating processes are turned outward and embraced by the superior ; 

 this renders rotation in this region of the spine impossible, while there is nothing 

 to prevent a sliding upward and downward of the surfaces on each other, so as to 

 allow of flexion and extension. In the dorsal region, on the other hand, the 

 articulating processes, by their direction and mutual adaptation, especially at the 

 upper part of the series, permit of rotation, but prevent extension and flexion, 

 while in the cervical region the greater obliquity and lateral slant of the articular 

 processes allow not only flexion and extension, but also rotation. 



The principal muscles which produce flexion are the Sterno-mastoid, Rectus 

 capitis anticus major, and Longus colli ; the Scaleni ; the abdominal muscles and 

 the Psoas magnus. Extension is produced by the fourth layer of the muscles of 

 the back, assisted in the neck by the Splenius, Semispinalis dorsi et colli, and 

 the Multifidus spinse. Lateral motion is produced by the fourth layer of the 

 muscles of the back, by the Splenius and the Scaleni, the muscles of one side only 

 acting ; and rotation by the action of the following muscles of one side only viz. 

 the Sterno-mastoid, the Rectus capitis anticus major, the Scaleni, the Multifidus 

 epinse, the Complexus, and the abdominal muscles. 



II. Articulation of the Atlas with the Axis. 



The articulation of the Atlas with the Axis is of a complicated nature, comprising 

 no fewer than four distinct joints. There is a pivot articulation between the odontoid 

 process of the axis and the ring formed between the anterior arch of the atlas and 

 the transverse ligament (see Fig. 155). Here there are two joints : one in front 

 between the posterior surface of the anterior arch of the atlas and the front of the 

 odontoid process (the atlanto-odontoid joint of Cruveilhier] ; the other between the 

 anterior surface of the transverse ligament and the back of the process (the 

 syndesmo-odontoid joint). Between the articular processes of the two bones 

 there is a double arthrodia or gliding joint. The ligaments which connect these 

 bones are the 



Anterior Atlanto-axial. Transverse. 



Posterior Atlanto-axial. Two Capsular. 



The Anterior Atlanto-axial Ligament (Fig. 153) is a strong, membranous layer, 

 attached, above, to the lower border of the anterior arch of the atlas ; below, to the 

 base of the odontoid process and to the front of the body of the axis. It is strength- 

 ened in the middle line by a rounded cord, which is attached, above, to the tubercle 

 on the anterior arch of the atlas, and below to the body of the axis, being a con- 

 tinuation upward of the anterior common ligament of the spine. These ligaments 

 are in relation, in front, with the Recti antici majores. 



The Posterior Atlanto-axial Ligament (Fig. 154) is a broad and thin membra- 

 nous layer, attached, above, to the lower border of the posterior arch of the atlas ; 

 below, to the upper edge of the laminse of the axis. This ligament supplies the 

 place of the ligamenta subflava, and is in relation, behind, with the Inferior oblique 

 muscles. 



The Transverse Ligament 1 (Figs. 155, 156) is a thick, strong band, which arches 

 across the ring of the atlas, and serves to retain the odontoid process in firm con- 

 nection with its anterior arch. This ligament is flattened from before backward, 

 broader and thicker in the middle than at either extremity, and firmly attached on 



1 It has been found necessary to describe the transverse ligament with those of the atlas and axis ; 

 but the student must remember that it is really a portion of the mechanism by which the movements 

 of the head on the spine are regulated ; so that the connections between the atlas and axis ought 

 always to be studied together with those between the latter bones and the skull. 



