668 THE SKELETAL SYSTEM 



314F). The primary marrow spaces, established by the original invasion of 

 the perichondria! vascular buds, merge to form the secondary marrow areas 



of the developing bone. This merging process is effected by the dissolution 

 of previously formed bony spicules or trabeculae. 



c. Conversion of Cancellous Bone into Compact Bone 



Spongy or cancellous bone is converted into compact bone by the deposition 

 of layers of compact bone between the trabeculae or columns of spongy bone, 

 thus obliterating the marrow cavities around the trabeculae of the cancel- 

 lous bone and converting the intervening areas into Haversian systems 

 (fig. 314C, D). 



D. Development (Morphogenesis) of the Endoskeleton 



1. Definitions 



For pedagogical purposes, the hard, skeletal tissues may be divided into 

 the external skeleton or exoskeleton and the internal skeleton or endoskeleton. 

 The exoskeleton comprises all the hard, protective structures which are de- 

 rived from the mesenchyme of the dermis and from the epithelium of the 

 epidermis, described in Chapter 12. The exoskeleton as a whole will not be 

 described further. 



Excluding the exoskeleton and the softer, connective-tissue portion of the 

 skeletal tissues, we shall proceed with a description of the morphogenesis of 

 the main skeletal support of the vertebrate body, the endoskeleton. The endo- 

 skeleton is composed of the axial skeleton and the appendicular skeleton. 

 The axial skeleton is composed of the skeleton of the head, the skeleton of 

 the trunk, and the skeleton of the tail. The skeleton of the appendages is 

 made up of the pectoral and pelvic girdles and the bony supports for the 

 appendages. 



2. Morphogenesis of the Axial Skeleton 

 a. General Features of the Skeleton of the Head 

 The cranium or skeleton of the head comprises: 



( 1 ) the protective parts for the special sense organs and the brain, and 



(2) the skeleton of the oral area and anterior end of the digestive tract. 



That portion of the cranium which protects the brain and its associated, 

 special sense organs may be called the skull, cranium proper, or neurocranium 

 (fig. 315D), whereas that which surrounds the anterior portion of the digestive 

 tract and pharyngeal area is known as the visceral skeleton or splanchno- 

 cranium (fig. 315D). 



