654 THE SKELETAL SYSTEM 



2. Morphogenesis of the axial skeleton 



a. General features of the skeleton of the head 



1) Neurocranium or cranium proper 



2) Visceral skeleton or splanchnocranium 



3) Development of the skull or neurocranium 



4) Vicissitudes of the splanchnocranium 



b. Ossification centers and the development of bony skulls 



c. Development of the axial skeleton 



1) Axial skeleton of the trunk 



a) Notochord 



b) Vertebrae 



c) Divisions of the vertebral column 



d) Ribs 



e) Sternum 



2) Axial skeleton of the tail 



d. Development of the appendicular skeleton of the paiied appendages 



1 ) General features 



2) Development of the skeleton of the free appendage 



3) Formation of the girdles 



e. Growth of bone 



f. Formation of joints 



1) Definitions 



2) Ankylosis (synosteosis) and synarthrosis 



3) Diarthroses 



4) Amphiarthroses 



g. Dermal bones 



A. Introduction 



1. Definition 



The word skeleton is used coinmonly to denote the hard, supporting frame- 

 work of the body, composed of bone and cartilage. In this restricted sense it 

 is employed to refer particularly to the internal or endoskeleton (see p. 668). 

 The word has a broader meaning, however, for the skeletal system includes 

 not only the bony and cartilaginous materials of the deeper-lying, internal 

 skeleton but also the softer, pliable connective tissues as well. Thus, the 

 skeletal tissues in a comprehensive sense may be divided as follows: 



(1) the soft skeleton, composed of pliable connective tissues which bind 

 together and support the various organs of the body and 



(2) the hard or firm skeleton, formed of bone, cartilage, and other struc- 

 tures which protect and sustain, and give rigidity to the body as a 

 whole. The exoskeletal structures described in Chapter 12 in reality 

 are a part of the hard, protective skeleton of the vertebrate body. 



(Note: Blood and lymph are often classified as a part of the connective 

 tissues. See Maximow and Bloom, '42, p. 39.) 



