LIGAMENTS OF THE TRUNK. 117 



T-^O of an inch in diameter ; when isolated they are globular in form, 

 but are hexagonal or polyhedral when compressed. They are per- 

 fectly transparent, the cell-membrane being structureless and their 

 nucleus disappearing as they attain their full size. 



SVNOVIAL MEMBRANE is a thin membranous layer, which invests 

 the articular cartilages of the bones, and is thence reflected upon the 

 surfaces of the ligaments which surround and enter into the composi- 

 tion of a joint. It resembles the serous membranes in being a shut 

 sac, and secretes a transparent and viscous fluid, which is named 

 synovia. Synovia is an alkaline secretion, containing albumen, which 

 is coagulable at a boiling temperature. The continuation of this mem- 

 brane over the surface of the articular cartilage, a much agitated 

 question, has been decided by the interesting discoveries of Henle, 

 who has ascertained the existence of an epithelium upon cartilage 

 identical with that produced by the reflected portion of the membrane. 

 In some of the joints the synovial membrane is pressed into the 

 articular cavity by a cushion of fat : this mass was called by Havers 

 the synovial gland, from an incorrect supposition that it was the 

 source of the synovia ; it is found in the hip and in the knee-joint. 

 In the knee-joint, moreover, the synovial membrane forms folds, which 

 are most improperly named ligaments, as the mucous and alar liga- 

 ments, the two latter being an appendage to the cushion of fat. 

 Besides the synovial membranes entering into the composition of 

 joints, there are numerous smaller sacs of a similar kind interposed 

 between surfaces which move upon each other so as to cause friction ; 

 they are often associated with the articulations. These are the bursce 

 macosae ; they are shut sacs, analogous in structure to synovial mem- 

 branes, and secreting a similar synovial fluid. 



The epithelium of synovial membranes is of the kind termed 

 tesselated ; it is developed in the same manner with the epithelium 

 of other free surfaces, and is continually reproduced from beneath, 

 while the superficial layers are being rubbed off and lost. 



ARTICULATIONS. 



The joints may be arranged, according to a natural division, into 

 those of the trunk, those of the upper extremity, and those of the 

 lower extremity. 



LIGAMENTS OF THE TRUNK. The articulations of the trunk are 

 divisible into ten groups, viz. 



1. Of the vertebral column. 



2. Of the atlas, with the occipital bone. 



3. Of the axis, with the occipital bone. 



4. Of the atlas, with the axis. 



5. Of the lower jaw. 



