EXOSKELETON 9 



usually a cavity between the two elements filled with a (synovial) 

 fluid which lubricates the articulating surfaces. Where there is 

 httle or no motion the connecting substance is less in amount, and 

 special names are given to the different conditions. Connexion by 

 ligament is syndesmosis, by cartilage, synchondrosis. In the skull 

 many of the bones are very close together, the opposing edges inter- 

 digitating by saw-toothed margins, cases of suture. When two 

 bones are so closely united in the adult that no trace of suture or 

 other indication of primitive distinctness persists, it is called 

 sjmostosis. 



THE SKELETON 



For convenience of treatment the skeleton is divided into dermal 

 or exoskeleton, and internal or endoskeleton. The first is developed 

 exclusively in the skin, especially in the corium. The endoskeleton 

 belongs in the deeper mesenchymatous tissues; much of it has a 

 cartilage stage, more or less cartilage persisting through life. Most 

 of the membrane bones of the endoskeleton, and especially those of 

 the skull, are clearly of dermal origin in the lower Vertebrates, but 

 they are so closely related to the deeper structures that they are 

 dealt with along with the endoskeleton. 



The endoskeleton readily falls into axial and appendicular 

 portions. The axial is restricted to the head, body proper, and tail, 

 and includes the vertebral column, skull, ribs and breast bone, all 

 lying in or near the body axis. The appendicular skeleton includes 

 the parts in the appendages (median and paired fins, arms and legs) 

 and the skeletal arches or girdles in the trunk, which support the 

 paired appendages. In the following pages the order of this outhne 

 will be followed, both in the general accounts and in the description 

 of parts in the separate classes and orders. 



EXOSKELETON 



The exoskeleton consists of calcified structures (primitively 

 scales) which arise as direct ossifications in the corium, no cartilage 

 appearing in their history. To the mesenchymatous parts there 

 may be added other calcified portions secreted by the basal layer of 

 the epidermis as described above (p. 5). This dermal skeleton is 

 best developed in fishes and reptiles, is very scanty in mammals 

 and recent Amphibians, and no traces of it appear in Cyclostomes or 

 birds. 



