THE VERTEBRAL ARTICULATIONS. 



133 



the manner of a wheel rolling on the gi-onnd, this movement being usually 

 accompanied by a certain amount of gliding. 



Circumduction is the movement performed when the shaft of a 

 long bone or a part of a limb describes a cone, the apex of which is 

 placed in the joint at or near one extremity of the bone, while the sides 

 and base of the cone arc described by the rest of the moving part. 



Rotation signifies movement of a bone round its axis without any 

 great change of situation. 



Gliding is applied to that kind of movement in which the surfaces 

 of adjacent bones are displaced without am- accompanying angular 

 or rotatory motion, as in the movement of flat surfaces over eacii other 

 in some of the carpal and tarsal articulations, or in the movement of 

 advance and retreat of the lower jaw. 



In the various joints provided with sjTiovial cavities, the cartilaginous surfaces 

 of the Ijones are so formed as usually to be in close apposition or contact : but in 

 certain positions they are not entirely so. and there are even instances in which 

 the separation of the surfaces must be considerable, as in the case of the patella, 

 in some positions of the knee. In these cases the vacuity is filled up by folds 

 of the sjTiovial membrane, or by fatty processes connected with it. 



ARTICULATIONS OF THE TRUJSTK AND HEAD. 



ARTICULATIONS OF THE VERTEBRAL COLUMN. 



The moveable vertebra are connected together by elastic discs inter- 

 posed between the bodies ; by synovial joints between the articulating 

 processes ; and by ligaments. 



The intervertebral discs arc plates of composite structure placed 

 between the bodies of the vertebra) fi-om the axis to the sacrum. Each 

 is composed of a fibro-lamiuar part externally, and of a pulpy substance 

 in the centre. 



Fig. 116. — A Lumbar Vertebra, seen from Fig. 11 G. 



ABOVE, WITH PART OF THE INTERVERTEBRAL 



Disc adhering to tue bodv. h 



1,1, the concentric arrangement of the fibrous 

 lamina; ; 2, the central soft cartilaginous or 

 gelatinous substance. 



The laminar part forms more than 

 half of the mass, and consists of con- 

 centric lamina of fibro-cartilage and 

 fibrous tissue alternating one with 

 another. These lamince are not quite 

 vertical, for if a vertical section of a 

 disc be made, a certain number of the 

 layers nearest to the circumference of 

 the disc will be seen bulging outwards, 



while others situated more deeply and less closely compacted together 

 are convex towards the centre ; and when the spine is bent in any 

 direction, the curves of the different layers are augmented on the side 

 towards which the column is inclined. The individual layers consist 

 chiefly of fibres extending obliquely between the vertebras and firmly 



