ii.] . THE SKELETON. 9 



The various forms of joints may be arranged under two 



principal heads. In one, the contiguous surfaces of the 



bones are connected by interposed fibrous tissue, passing 



directly from one to the other, filling up the space between 



them, and allowing of only a limited amount of motion, as 



the case with the bodies of the vertebrae. 



The other and more frequent and more perfect form of 



)int is that in which the contiguous extremities of the bones 



re covered by a thin layer of very smooth cartilage, and 



irrounded by a capsular ligament, attached only round the 



Iges of the articular surfaces, and which is lined by a 



wvial membrane, so called from its secreting a viscid 



jricating fluid termed synovia. The amount of motion 



jrmitted in these " synovial joints " varies according to the 



of the opposed articular surfaces and the arrangement 



the ligaments which hold them together. When the two 



irfaces are nearly flat, and the bones firmly bound by strong 



lort ligaments, as in those which compose the carpus and 



irsus, the motion is reduced to an extremely slight gliding 



)f one on the other. Joints in the form of a hinge, as at 



the elbow, allow of a free motion in one plane only. Ball 



and socket joints, as at the shoulder and hip, allow of the 



greatest variety of movements. 



The Endoskeleton is divided into an axial portion, be- 

 longing to the head and trunk, and an appendicular portion, 

 belonging to the limbs. There are also certain bones called 

 splanchnic, being developed within the substance of some of 

 the viscera. Such are the os cordis and os penis found in 

 some Mammals. These, however, are more appropriately 

 treated of with the anatomy of the organ of which they form 

 a part. 



The Axial Skeleton consists of the vertebral column, the 

 skull, the sternum, and the ribs. 



