THE JOINTS. 51 



The unions of the bones of the cranium are called sutures, from 

 the Latin word signifying to sew, because they seem as if stitched 

 together, the fibres of the one .bone forming prolongations which 

 pass into the notches, or spaces left by the similar prolongations of 

 the other, as is seen in the figure of the skull already given. Be- 

 tween these a thin layer of cartilage runs. These sutures run in 

 determinate lines over the head, as seen in the drawing of the 

 cranium, but in a work of this character it would be out of place 

 to give a more detailed description of them. 



The bones of the spine are united by thick layers of a peculiar 

 cartilage mixed with ligaments, placed between them, admitting of 

 but little motion between any pair of bones, but allowing consider- 

 able curvatures to take place in the whole length of the spine. 

 Strong ligaments also pass down the spine in front and behind, 

 binding its different pieces together. The different parts of the 

 pelvis are united by means of cartilage, each bone having its arti- 

 culating surface covered with cartilage, and these laid together, 

 with or without the intervention of a third layer, and are bound 

 firmly by ligaments passing over them. Such are the joints between 

 the two haunch-bones, and between them and the ischium. 



The joints of the extremities are of a more complicated nature. 

 The ends of the bones entering into the joints are covered with car- 

 tilage, then the bones are tied together by ligaments, and in addition 

 a membrane called synovial is spread over the ends of the bones 

 and lines the ligaments, forming a shut bag, whose inner surfaces 

 are everywhere in contact, and to obviate friction are moistened 

 with a bland mucilaginous fluid called synovia. This fluid, which 

 serves the purpose of oil to the joints, does not in reality contain 

 any, yet it has very much the feeling of oil when rubbed be- 

 tween the fingers. When it increases too much in quantity, it pro- 

 duces dropsy of the joints. 



A Finger-joint is represented. B. B. are the two bones. L. L. the two ligaments ; 

 the cartilages are seen covering the ends of the bones, and inside of this the synovial 

 rAembrane is seen lining them and the ligaments. A space is represented between the 

 bones merely for the sake of making it plainer ; but in reality the surfaces are in con- 

 tact. 



