VII. 



THE ENDOSKELETON: THE COMPARATIVE ANATOMY OF 

 THE GIRDLES, THE STERNUM, AND THE PAIRED 

 APPENDAGES 



lateral fin folds 



median fin fold 



A. GENERAL CONSIDERATIONS 



i. Definitions. — The girdles are crescent-shaped or arch-shaped portions of the endo- 

 skeleton which function for the support of the paired appendages. The center of the arch is 

 directed ventraily, the points dorsally. The girdles are composed of cartilage in the lower 

 forms; this is partly or completely ossified in the higher ones. The pectoral girdle supports the 

 anterior appendages; the pelvic girdle the posterior appendages. 



The sternum or breastbone is an elongated structure lying in the median ventral line of the 

 anterior part of the trunk region. It is commonly composed of a chain of cartilages or bones, or 



both. The ribs and 

 the pectoral girdle us- 

 ually articulate with 

 the sternum. Such 

 arrangements streng- 

 then the anterior 

 part of the trunk in 

 relation to the air- 

 breathing habit and 

 the presence of lungs. 

 The paired append- 

 ages consist of fins in 

 the fishes and limbs in 

 all of the land verte- 

 brates. The anterior 

 or pectoral append- 

 ages articulate with 

 the pectoral girdle, 

 and their support is 

 transmitted to the 

 body by means of this 

 girdle. Similarly the 

 posterior or pelvic 

 appendages are articulated to the pelvic girdle and transmit their support through this girdle. 

 As the support of the posterior appendages is generally the more important, the pelvic 

 girdle is commonly stronger and more massive than the pectoral girdle. This is particularly 

 true of biped vertebrates. The appendages possess an internal skeleton. 



2. The origin of the paired appendages and the girdles. — The origin of these structures 

 in vertebrates is obscure. According to the most probable theory, the fin-fold theory, the 

 ancestral vertebrate possessed a pair of continuous folds , one running along each side of the trunk . 

 These fused behind the anus to a single median fin which extended around the tail and along 

 the median dorsal line (see Fig. 23 A). This hypothetical condition resembles that actually 

 ■occurring in Amphioxus, with its paired ventral metapleural folds and median caudal and dorsal 



78 



paired fins 



Fig. 23. — Diagrams to illustrate the theory of the origin of the median 

 and paired fins through the persistence of certain regions of originally con- 

 tinuous median and lateral fin folds. A, early stage showing the median 

 dorsal fin fold and the two lateral fin folds uniting at the anus. B, later 

 stage illustrating persistence of certain regions of the fin folds as the median 

 and paired fins, and disappearance of the remainder of the fin folds, as 

 indicated by dotted lines. (From Wilder's History of the Human Body, 

 courtesy of Henry Holt and Company.) 



