ARTICULATIONS OF THE BONES. 115 



this case is limited by the dimensions of the intervening 

 cartilage, sometimes by its nature ; for, where it serves to 

 connect the vertebrae of quadrupeds, it is much more flex- 

 ible and elastic than it is in the other parts. Where two 

 bones are united, by the intervention of ligament, or even 

 skin, the junction may be termed Syndesmosis, (jrwtwfMs li- 

 gamentum.) It is harmonic when the even edges of thin 

 bones are prevented from coming in contact, or from reced- 

 ing, by the intervening soft part. In this case, the extent 

 of motion is limited by the degree of flexion or extension 

 of which the connecting medium is susceptible. The shells 

 of the Bernacle (Lepas) afford good examples of this kind of 

 connection. In the squamoiis syndesmosis, the thin edgeofone 

 bone covers the surface of another, in such a manner as to 

 permit both bones to slide upon eacli other. The gill-flaps 

 of many fishes, some of the bones of the head of the km prey, 

 and the dorsal plates of the chitons exhibit this kind of arti- 

 culation. In the syndesmosis, by inclusion, the bones or hard 

 parts are in the form of tubes, united to each other at the 

 extremities by a common membrane passing from the one 

 to the other. When the common membrane is long, flexi- 

 ble, and elastic, almost every kind of motion may be per- 

 formed. When the tubes are nearly in contact, a lateral 

 movement only can be executed, by a part of the circumfe- 

 rence of the one tube penetrating the interior of the other. 

 When the one tube is smaller than the other, it may be 

 withdrawn into the cavity of the larger, or extended, at the 

 will of the animal. There are frequently processes and ca- 

 vities in the contiguous margins of the tubes, which regu- 

 late the direction and extent of the motion this kind of 

 junction is designed to perform. Numerous examples of 

 articulation by inclusion, occur in the limbs of insects and 

 crabs. 



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