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 

 is the case with the bodies of the vertebrae. 



The other and more frequent and more perfect form of 

 joint is that in wdiichthe contiguous extremities of the bones 

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

 surrounded by a capsular ligament, attached only round the 

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

 synovial ineinbraiie, so called from its secreting a viscid 

 lubricating fluid termed syjwvia. The amount of motion 

 permitted in these " synovial joints " varies according to the 

 form of the opposed articular surfaces and the arrangement 

 of the ligaments which hold them together. When the two 

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

 short ligaments, as in those which compose the carpus and 

 tarsus, the motion is reduced to an extremely slight gliding 

 of 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 w^hich they form 

 a part. 



The Axial Skeleton consists of the vertebral column, the 

 skull, the sternum, and the ribs. 



